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Application Note 105 December 2005 Current Sense Circuit Collection Making Sense of Current Tim Regan, Jon Munson Greg Zimmer, Michael Stokowski INTRODUCTION Sensing and/or controlling current flow is a fundamen- sensing, or negative supply

  1. 0.01µF 1/2 LTC6943 POSITIVE OR NEGATIVE RAIL 10 11 1µF R SHUNT 12 6943 • TA01b 14 15 1µF E E E R SHUNT Application Note 105 HIGH SIDE Measuring Bias Current Into an Avalanche Photo V CC R1 Diode (APD) Using an Instrumentation Amplifier 200Ω (Figures 12a and 12b) Rs – – 0.2Ω The upper circuit (a) uses an instrumentation amplifier Q1 1/2 LT1366 1/2 LT1366 (IA) powered by a separate rail (>1V above V IN ) to mea- TP0610L + + ( ) sure across the 1kΩ current shunt. The lower figure (b) I LOAD R2 V O = I LOAD • R S R1 is similar but derives its power supply from the APD bias R2 = I LOAD • 20Ω 20k LOAD line. The limitation of these circuits is the 35V maximum 1366 TA01 APD voltage, whereas some APDs may require 90V or more. In the single-supply configuration shown, there is Figure 10. Positive Supply Rail Current Sense also a dynamic range limitation due to V OL to consider. The advantage of this approach is the high accuracy that Precision Current Sensing in Supply Rails (Figure 11) is available in an IA. This is the same sampling architecture as used in the front end of the LTC2053 and LTC6800, but sans op amp 1k 1% V IN BIAS OUTPUT gain stage. This particular switch can handle up to 18V , so 10V TO 33V TO APD 35V the ultrahigh precision concept can be utilized at higher – voltages than the fully integrated ICs mentioned. This CURRENT MONITOR OUTPUT circuit simply commutates charge from the flying sense LT1789 0mA TO 1mA = 0V TO 1V + capacitor to the ground-referenced output capacitor so A = 1 that under DC input conditions the single-ended output AN92 F02a voltage is exactly the same as the differential across the Figure 12a sense resistor. A high precision buffer amplifier would typically follow this circuit (such as an LTC2054). The 1N4684 1k 3.3V commutation rate is user set by the capacitor connected 1% V IN BIAS OUTPUT 10V TO 35V TO APD to Pin 14. For negative supply monitoring, Pin 15 would 10M be tied to the negative rail rather than ground. – CURRENT MONITOR OUTPUT LT1789 0mA TO 1mA = 0V TO 1V + I A = 1 AN92 F02b Figure 12b Figure 12. Measuring Bias Current Into an Avalanche Photo Diode (APD) Using an Instrumentation Amplifier I = 9 6 7 Figure 11. Precision Current Sensing in Supply Rails an105fa AN105-5

  2. Application Note 105 HIGH SIDE Simple 500V Current Monitor (Figure 13) Bidirectional Battery-Current Monitor (Figure 14) Adding two external MOSFETs to hold off the voltage allows This circuit provides the capability of monitoring current the LTC6101 to connect to very high potentials and monitor in either direction through the sense resistor. To allow the current flow. The output current from the LTC6101, negative outputs to represent charging current, V EE is which is proportional to the sensed input voltage, flows connected to a small negative supply. In single-supply through M1 to create a ground referenced output voltage. operation (V EE at ground), the output range may be offset upwards by applying a positive reference level to V BIAS (1.25V for example). C3 may be used to form a filter in conjunction with the output resistance (R OUT ) of the part. This solution offers excellent precision (very low V OS ) and a fixed nominal gain of 8. DANGER! Lethal Potentials Present — Use Caution I SENSE V SENSE 500V – + R SENSE R IN 100Ω 4 3 L + DANGER!! – 2 5 O HIGH VOLTAGE!! A D 1 62V LTC6101 CMZ5944B M1 V OUT M2 R OUT M1 AND M2 ARE FQD3P50 TM 2M 4.99k R OUT V OUT = • V SENSE = 49.9 V SENSE R IN 6101 TA09 Figure 13. Simple 500V Current Monitor R SENSE TO CHARGER/ LOAD C1 15V 1µF 1 8 FIL – FIL + LT1787 V S– V S+ 2 7 V BIAS 3 6 DNC R OUT 4 5 V EE OUTPUT V OUT C2 C3* –5V 1µF 1000pF 1787 F02 *OPTIONAL Figure 14. Bidirectional Battery-Current Monitor an105fa AN105-6

  3. Application Note 105 HIGH SIDE LTC6101 Supply Current Included as Load in or an H-bridge. The circuit is programmable to produce up Measurement (Figure 15) to 1mA of full-scale output current into R OUT , yet draws a mere 250µA supply current when the load is off. This is the basic LTC6101 high side sensing supply-monitor configuration, where the supply current drawn by the IC is BATTERY BUS included in the readout signal. This configuration is use- ful when the IC current may not be negligible in terms of R IN overall current draw, such as in low power battery-powered R SENSE 100Ω 0.01Ω 3 4 applications. R SENSE should be selected to limit voltage drop to <500mV for best linearity. If it is desirable not to LOAD – + include the IC current in the readout, as in load monitor- 2 5 ing, Pin 5 may be connected directly to V + instead of the load. Gain accuracy of this circuit is limited only by the precision of the resistors selected by the user. 1 V OUT LT6101 4.99V = 10A R OUT 4.99k V + V OUT = I LOAD (R SENSE • R OUT /R IN ) DN374 F01 R IN R SENSE Figure 16. Simple High Side Current Sense Using the LTC6101 4 3 – + High Side Transimpedance Amplifier (Figure 17) LOAD 2 5 Current through a photodiode with a large reverse bias potential is converted to a ground referenced output volt- age directly through an LTC6101. The supply rail can be 1 as high as 70V . Gain of the I to V conversion, the trans- V OUT LTC6101 impedance, is set by the selection of resistor R L . R OUT 6101 F06 V S Figure 15. LTC6101 Supply Current Included as Load CMPZ4697* LASER MONITOR i PD (10V) PHOTODIODE in Measurement 4.75k 4.75k 10k Simple High Side Current Sense Using the LTC6101 (Figure 16) 4 3 This is a basic high side current monitor using the LTC6101. + – 2 5 The selection of R IN and R OUT establishes the desired gain of this circuit, powered directly from the battery bus. The current output of the LTC6101 allows it to be located re- 1 motely to R OUT . Thus, the amplifier can be placed directly LTC6101 V O at the shunt, while R OUT is placed near the monitoring R L electronics without ground drop errors. This circuit has V O = I PD • R L 6101 TA04 a fast 1µs response time that makes it ideal for providing *V Z SETS PHOTODIODE BIAS V Z + 4 ≤ V S ≤ V Z + 60 MOSFET load switch protection. The switch element may be the high side type connected between the sense resistor Figure 17. High Side Transimpedance Amplifier and the load, a low side type between the load and ground an105fa AN105-7

  4. Application Note 105 HIGH SIDE Intelligent High Side Switch (Figure 18) I SENSE V SENSE + – V S LOAD The LT1910 is a dedicated high side MOSFET driver with R SENSE R IN built in protection features. It provides the gate drive for a 3 4 power switch from standard logic voltage levels. It provides – + 5 2 V – shorted load protection by monitoring the current flow to through the switch. Adding an LTC6101 to the same circuit, sharing the same current sense resistor, provides a linear voltage signal proportional to the load current for LTC6101HV additional intelligent control. V LOGIC R OUT 48V Supply Current Monitor with Isolated Output and 105V Survivability (Figure 19) V OUT The HV version of the LTC6101 can operate with a total ANY OPTO-ISOLATOR supply voltage of 105V . Current flow in high supply voltage V – rails can be monitored directly or in an isolated fashion as shown in this circuit. The gain of the circuit and the N = OPTO-ISOLATOR CURRENT GAIN level of output current from the LTC6101 depends on the R SENSE V OUT = V LOGIC – I SENSE • • N • R OUT particular opto-isolator used. R IN 6101 TA08 Figure 19. 48V Supply Current Monitor with Isolated Output and 105V Survivability 10µF V LOGIC 63V 14V 47k 5 100Ω 3 1% FAULT 8 3 1 4 R S OFF ON LT1910 LTC6101 V O 6 4 2 100Ω 4.99k 1µF 2 1 5 SUB85N06-5 V O = 49.9 • R S • I L L O I L FOR R S = 5mΩ, A V O = 2.5V AT I L = 10A (FULL-SCALE) D 6101 TA07 Figure 18. Intelligent High Side Switch an105fa AN105-8

  5. Application Note 105 HIGH SIDE Precision, Wide Dynamic Range High Side Current Sensed Current Includes Monitor Circuit Supply Sensing (Figure 20) Current (Figure 21) The LTC6102 offers exceptionally high precision (V OS < To sense all current drawn from a battery power source 10µV) so that a low value sense resistor may be used. which is also powering the sensing circuitry requires the This reduces dissipation in the circuit and allows wider proper connection of the supply pin. Connecting the supply variations in current to be accurately measured. In this pin to the load side of the sense resistor adds the supply circuit, the components are scaled for a 10A measuring current to the load current. The sense amplifier operates properly with the inputs equal to the device V + supply. range, with the offset error corresponding to less than 10mA. This is effectively better than 10-bit dynamic range with dissipation under 100mW. + R IN V SENSE 20Ω 1mΩ – +IN –INS + – –INF L + V – V + O 5V TO A 105V 0.1µF D 5V 1µF V REG V OUT OUT LTC6102 LTC2433-1 TO µP R OUT 4.99k 6102 TA01 R OUT V OUT = • V SENSE = 249.5V SENSE *PROPER SHUNT SELECTION COULD ALLOW R IN MONITORING OF CURRENTS IN EXCESS OF 1000A Figure 20. Precision, Wide Dynamic Range High Side Current Sensing R SENSE R1 I SUPPLY 100 +IN –INS L –INF O + – V BATT I LOAD A V – V + D 0.1µF V REG OUT LTC6102 + R2 V OUT 4.99k – V OUT = 49.9 • R SENSE (I LOAD + I SUPPLY ) 6102 TA03 Figure 21. Sensed Current Includes Monitor Circuit Supply Current an105fa AN105-9

  6. Application Note 105 HIGH SIDE Wide Voltage Range Current Sensing (Figure 22) Smooth Current Monitor Output Signal by Simple Filtering (Figure 23) The LT6105 has a supply voltage that is independent from the potential at the current sense inputs. The input voltage The output impedance of the LT6105 amplifier is defined can extend below ground or exceed the sense amplifier by the value of the gain setting output resistor. Bypassing supply voltage. While the sensed current must flow in just this resistor with a single capacitor provides first order one direction, it can be sensed above the load, high side, filtering to smooth noisy current signals and spikes. or below the load, low side. Gain is programmed through resistor scaling and is set to 50 in the circuit shown. SOURCE LT6105 –0.3V TO 44V R IN2 V S+ 100Ω +IN + V OUT R IN1 0.02Ω V OUT = 1V/A 100Ω –IN – R OUT V S– 4.99k V + V – TO LOAD 2.85V TO 36V 6105 TA01 V OUT = V S + − V S ) • R OUT A V = R OUT ( – R IN1 = R IN2 = R IN ; ; R IN R IN Figure 22. Wide Voltage Range Current Sensing 2.85V TO 36V SOURCE 0V TO 44V V + LT6105 249Ω +IN V S+ + V OUT V OUT = 780mV/A 0.039Ω –IN – V S– 4.99k 0.22µF 249Ω V – 6105 TA02 TO LOAD Figure 23. Smooth Current Monitor Output Signal by Simple Filtering an105fa AN105-10

  7. Application Note 105 HIGH SIDE Power on Reset Pulse Using a TimerBlox Device up time delay interval, R7 and C1 create a falling edge to (Figure 24) trigger an LTC6993-3 one-shot programmed for 10µs. This pulse unlatches the comparators. R8 and Q2 will When power is first applied to a system the load current discharge C1 on loss of the supply to ensure that a full may require some time to rise to the normal operating delay interval occurs when power returns. level. This can trigger and latch the LT6109 comparator monitoring undercurrent conditions. After a known start- 5V 9 V + LT6109-1 V + R IN 100Ω 10 SENSEHI – R SENSE 1 SENSELO OUTA 8 + I LOAD V – R1 V + 8.06k INC2 7 R5 + 10k 3 OUTC2 – V – V + R4 400mV R2 V – 10k V + REFERENCE 1.5k 5V + 4 OUTC1 R8 CREATES A DELAYED 30.1k C1 10µs RESET PULSE INC1 6 Q1 – 0.1µF ON START-UP 2N2222 EN/ RST 2 R3 TRIG OUT 499Ω OPTIONAL: LTC6993-3 R7 DISCHARGES C1 V + 1M GND V – WHEN SUPPLY IS DISCONNECTED 610912 TA06 5 SET DIV R6 487k Figure 24. Power on Reset Pulse Using a TimerBlox Device an105fa AN105-11

  8. Application Note 105 HIGH SIDE Accurate Delayed Power on Reset Pulse Using LTC6994-1 delay timer is used to set an interval longer TimerBlox Devices (Figure 25) than the known time for the load current to settle (1 sec- ond in the example) then triggers an LTC6993-3 one-shot When power is first applied to a system the load current programmed for 10µs. This pulse unlatches the compara- may require some time to rise to the normal operating tors. The power-on delay time is resistor programmable level. This can trigger and latch the LT6109 comparator over a wide range. monitoring undercurrent conditions. In this circuit an 5V 9 V + LT6109-1 V + R IN 100Ω 10 SENSEHI – R SENSE 1 SENSELO OUTA 8 + I LOAD V – R1 V + 8.06k INC2 7 R5 + 10k 3 OUTC2 – V – V + R4 400mV R2 V – 10k V + REFERENCE 1.5k R8 + 100k 4 OUTC1 1 SECOND DELAY 10µs RESET PULSE INC1 6 – ON START-UP GENERATOR EN/ RST 2 R3 TRIG OUT TRIG OUT 499Ω C1 LTC6994-1 LTC6993-1 0.1µF V + V + GND GND V – C2 R6 0.1µF 5 610912 TA07 1M SET DIV SET DIV R7 R5 R4 191k 681k 487k Figure 25. Accurate Delayed Power on Reset Pulse Using TimerBlox Devices an105fa AN105-12

  9. Application Note 105 HIGH SIDE More High Side Circuits Are Shown in Other Chapters: FIGURE TITLE 40 Monitor Current in Positive or Negative Supply Lines 58 Bidirectional Precision Current Sensing 59 Differential Output Bidirectional 10A Current Sense 60 Absolute Value Output Bidirectional Current Sensing 93 High Voltage Current and Temperature Monitoring 104 Using Printed Circuit Sense Resistance 105 High Voltage, 5A High Side Current Sensing in Small Package 120 Bidirectional Current Sensing in H-Bridge Drivers 121 Single Output Provides 10A H-Bridge Current and Direction 123 Monitor Solenoid Current on the High Side 125 Large Input Voltage Range for Fused Solenoid Current Monitoring 126 Monitor both the ON Current and the Freewheeling Current Through a High Side Driven Solenoid 129 Simple DC Motor Torque Control 130 Small Motor Protection and Control 131 Large Motor Protection and Control 136 Coulomb Counting Battery Gas Gauge 142 Monitor Charge and Discharge Currents at One Output 143 Battery Stack Monitoring 145 High Voltage Battery Coulomb Counting 146 Low Voltage Battery Coulomb Counting 147 Single Cell Lithium-Ion Battery Coulomb Counter 148 Complete Single Cell Battery Protection 167 Monitor Current in an Isolated Supply Line 168 Monitoring a Fuse Protected Circuit 169 Circuit Fault Protection with Early Warning and Latching Load Disconnect 170 Use Comparator Output to Initialize Interrupt Routines 171 Current Sense with Over-current Latch and Power-On Reset with Loss of Supply 176 Directly Digitize Current with 16-Bit Resolution 177 Directly Digitizing Two Independent Currents 178 Digitize a Bidirectional Current Using a Single Sense Amplifier and ADC 179 Digitizing Charging and Loading Current in a Battery Monitor 180 Complete Digital Current Monitoring 181 Ampere-Hour Gauge 182 Power Sensing with Built In A-to-D Converter 183 Isolated Power Measurement 184 Fast Data Rate Isolated Power Measurement 185 Adding Temperature Measurement to Supply Power Measurement 186 Current, Voltage and Fuse Monitoring 187 Automotive Socket Power Monitoring an105fa AN105-13

  10. Application Note 105 HIGH SIDE More High Side Circuits Are Shown in Other Chapters: FIGURE TITLE 188 Power over Ethernet, PoE, Monitoring 189 Monitor Current, Voltage and Temperature 208 Remote Current Sensing with Minimal Wiring 209 Use Kelvin Connections to Maintain High Current Accuracy 210 Crystal/Reference Oven Controller 211 Power Intensive Circuit Board Monitoring 212 Crystal/Reference Oven Controller 215 0 to 10A Sensing Over Two Ranges 216 Dual Sense Amplifier Can Have Different Sense Resistors and Gain an105fa AN105-14

  11. 12 NEGATIVE RAIL 14 E 6943 • TA01b 0.01µF 1µF POSITIVE OR 10 E 11 15 1µF R SHUNT 1/2 LTC6943 R SHUNT E Application Note 105 LOW SIDE This chapter discusses solutions for low side current Precision Current Sensing in Supply Rails (Figure 27) sensing. With these circuits the current flowing in the This is the same sampling architecture as used in the ground return or negative power supply line is monitored. front end of the LTC2053 and LTC6800, but sans op amp gain stage. This particular switch can handle up to 18V , so “Classic” High Precision Low Side Current Sense the ultrahigh precision concept can be utilized at higher (Figure 26) voltages than the fully integrated ICs mentioned. This This configuration is basically a standard noninverting circuit simply commutates charge from the flying sense amplifier. The op amp used must support common mode capacitor to the ground-referenced output capacitor so operation at the lower rail and the use of a zero-drift type that under DC input conditions the single-ended output (as shown) provides excellent precision. The output of voltage is exactly the same as the differential across the this circuit is referenced to the lower Kelvin contact, which sense resistor. A high precision buffer amplifier would could be ground in a single-supply application. Small-signal typically follow this circuit (such as an LTC2054). The range is limited by V OL for single-supply designs. Scaling commutation rate is user-set by the capacitor connected accuracy is set by the quality of the user-selected resistors. to Pin 14. For negative supply monitoring, Pin 15 would be tied to the negative rail rather than ground. 5V 5 OUT 3 I + 3V/AMP 1 LOAD CURRENT LTC2050HV IN MEASURED 4 – CIRCUIT, REFERRED 2 TO –5V 10Ω 10k TO 3mΩ MEASURED CIRCUIT 0.1µF LOAD CURRENT –5V 2050 TA08 I = 9 Figure 26. “Classic” High Precision Low Side Current Sense 6 7 Figure 27. Precision Current Sensing in Supply Rails an105fa AN105-15

  12. Application Note 105 LOW SIDE –48V Hot Swap Controller (Figure 28) op amp. The N-MOSFET drain delivers a metered current into the virtual ground of the second stage, configured as This load protecting circuit employs low side current a transimpedance amplifier (TIA). The second op amp is sensing. The N-MOSFET is controlled to soft-start the powered from a positive supply and furnishes a positive load (current ramping) or to disconnect the load in the output voltage for increasing load current. A dual op amp event of supply or load faults. An internal shunt regulator cannot be used for this implementation due to the different establishes a local operating voltage. supply voltages for each stage. This circuit is exceptionally precise due to the use of zero-drift op amps. The scaling –48V Low Side Precision Current Sense (Figure 29) accuracy is established by the quality of the user-selected The first stage amplifier is basically a complementary form resistors. Small-signal range is limited by V OL in single- of the “classic” high side current sense, designed to operate supply operation of the second stage. with telecom negative supply voltage. The Zener forms an inexpensive “floating” shunt-regulated supply for the first GND + R IN C L 3 × 1.8k IN SERIES 100µF 1/4W EACH C IN R3 LOAD GND 1 1µF 5.1k (SHORT PIN) V IN EN R1 402k LTC4252-1 * 1% 8 2 V OUT PWRGD OV R D 1M 9 7 UV DRAIN R2 10 6 Q1 TIMER GATE 32.4k IRF530S 3 4 C T 1% SS SENSE V EE 0.33µF R C R S 5 C1 C SS 10Ω 0.02Ω 10nF 68nF C C 18nF 425212 TA01 –48V * M0C207 Figure 28.–48V Hot Swap Controller 10k Q1 0.1µF 1% ZETEX ZVN3320F 5V 100Ω – 1% – 0.01µF 39k V OUT = 100V SENSE LTC2054 LTC2054 + + 0.1µF 100Ω BZX84C5V1 V Z = 5.1 0.003Ω 1% 3W –48V SUPPLY –48V LOAD – + 20545 TA01 I SENSE , V SENSE Figure 29.–48V Low Side Precision Current Sense an105fa AN105-16

  13. Application Note 105 LOW SIDE Fast Compact –48V Current Sense (Figure 30) –48V Current Monitor (Figures 31a and 31b) This amplifier configuration is essentially the complemen- In this circuit an economical ADC is used to acquire the tary implementation to the classic high side configuration. sense resistor voltage drop directly. The converter is The op amp used must support common mode operation powered from a “floating” high accuracy shunt-regulated at its lower rail. A “floating” shunt-regulated local supply supply and is configured to perform continuous conver- is provided by the Zener diode, and the transistor provides sions. The ADC digital output drives an opto-isolator, metered current to an output load resistance (1kΩ in this level-shifting the serial data stream to ground. For wider circuit). In this circuit, the output voltage is referenced to a supply voltage applications, the 13k biasing resistor may positive potential and moves downward when represent- be replaced with an active 4mA current source such as ing increasing –48V loading. Scaling accuracy is set by shown in Figure 31b. For complete dielectric isolation and/ the quality of resistors used and the performance of the or higher efficiency operation, the ADC may be powered NPN transistor. from a small transformer circuit as shown in Figure 31b. V OUT = 3V – 0.1Ω • I SENSE I SENSE = 0A TO 30A ACCURACY ≈ 3% V OUT R1 1k Q1 4.7k 1% FMMT493 V S = 3V 30.1Ω 1% – R1 REDUCES Q1 DISSIPATION 3.3k 0805 LT1797 × 3 + 0.1µF SETTLES TO 1% IN 2µs, 1V OUTPUT STEP BZX84C6V8 0.003Ω V Z = 6.8V 1% 3W –48V LOAD –48V SUPPLY – I SENSE + 1797 TA01 (–42V TO –56V) Figure 30. Fast Compact –48V Current Sense BAT54S 1µF 2 × a LT1790-5 5V 1µF 4.7µF 100kHz SELECT R FOR 3mA AT MINIMUM SUPPLY 13k DRIVE VOLTAGE, 10mA MAX CURRENT AT MAXIMUM b SUPPLY VOLTAGE DN341 F01 a 0.1µF LT1029 MIDCOM 50480 LOAD 4.7µF b Figure 31b + 5V 48V –48V – 10k LTC2433 1.05k 1 10 V CC 45.3k V CC F O V CC 6N139 2 9 V REF REF + 1.54k SCK MPSA92 8 2 108mV 3 8 REF – 590Ω 7 SDO 1k a 4 7 GND 6 DATA IN + CS 4.7µF 0.010Ω (INVERTED) 5 6 b IN – GND 3 5 V – FULL-SCALE = 5.4A –7V TO –100V –48V –48V Figure 31a Figure 31. –48V Current Monitor an105fa AN105-17

  14. Application Note 105 LOW SIDE –48V Hot Swap Controller (Figure 32) Simple Telecom Power Supply Fuse Monitor (Figure 33) This load protecting circuit employs low side current sensing. The N-MOSFET is controlled to soft-start the The LTC1921 provides an all-in-one telecom fuse and load (current ramping) or to disconnect the load in the supply voltage monitoring function. Three opto-isolated event of supply or load faults. An internal shunt regulator status flags are generated that indicate the condition of establishes a local operating voltage. the supplies and the fuses. GND + R IN C L 3 × 1.8k IN SERIES 100µF 1/4W EACH C IN R3 LOAD GND 1 1µF 5.1k (SHORT PIN) V IN EN R1 402k LTC4252-1 * 1% 8 2 V OUT PWRGD OV R D 1M 9 7 UV DRAIN R2 10 6 Q1 TIMER GATE 32.4k IRF530S 3 4 C T 1% SS SENSE V EE 0.33µF R C R S 5 C1 C SS 10Ω 0.02Ω 10nF 68nF C C 18nF 425212 TA01 –48V * M0C207 Figure 32.–48V Hot Swap Controller 47k 5V –48V RETURN FUSE STATUS R1 R2 100k 100k 3 MOC207 SUPPLY A SUPPLY B RTN 47k V A V B STATUS STATUS 1 4 5V OK OK 0 0 V A OUT F SUPPLY A OK UV OR OV 0 1 STATUS UV OR OV OK 1 0 8 V B UV OR OV UV OR OV 1 1 LTC1921 OK: WITHIN SPECIFICATION MOC207 2 OV: OVERVOLTAGE FUSE A UV: UNDERVOLTAGE 47k 5V 7 5 V FUSE A V FUSE B FUSE STATUS FUSE B OUT A SUPPLY B = V A = V B 0 STATUS = V A ≠ V B 1 ≠ V A = V B 1 MOC207 ≠ V A ≠ V B 1* 6 OUT B 0: LED/PHOTODIODE ON R3 1: LED/PHOTODIODE OFF 47k F1 D1 *IF BOTH FUSES (F1 AND F2) ARE OPEN, 1/4W SUPPLY A –48V OUT ALL STATUS OUTPUTS WILL BE HIGH –48V SINCE R3 WILL NOT BE POWERED F2 D2 SUPPLY B = LOGIC COMMON –48V Figure 33. Simple Telecom Power Supply Fuse Monitor an105fa AN105-18

  15. Application Note 105 LOW SIDE More Low Side Circuits Are Shown in Other Chapters: FIGURE TITLE 22 Wide Voltage Range Current Sensing 23 Smooth Current Monitor Output Signal by Simple Filtering 40 Monitor Current in Positive or Negative Supply Lines 122 Monitor Solenoid Current on the Low Side 127 Monitor both the ON Current and the Freewheeling Current In a Low Side Driven Solenoid 168 Monitoring a Fuse Protected Circuit an105fa AN105-19

  16. Application Note 105 NEGATIVE VOLTAGE This chapter discusses solutions for negative voltage –48V Hot Swap Controller (Figure 35) current sensing. This load protecting circuit employs low side current sensing. The N-MOSFET is controlled to soft-start the Telecom Supply Current Monitor (Figure 34) load (current ramping) or to disconnect the load in the The LT1990 is a wide common mode range difference event of supply or load faults. An internal shunt regulator amplifier used here to amplify the sense resistor drop by establishes a local operating voltage. ten. To provide the desired input range when using a single 5V supply, the reference potential is set to approximately 4V by the LT6650. The output signal moves downward from the reference potential in this connection so that a large output swing can be accommodated. + 5V LOAD I L 48V – 7 3 + G2 5 6 R S V OUT LT1990 8 2 – G1 4 1 –77V ≤ V CM ≤ 8V REF V OUT = V REF – (10 • I L • R S ) V REF = 4V 4 5 IN OUT 174k 1nF LT6650 1 GND FB 2 20k 1990 AI01 1µF Figure 34. Telecom Supply Current Monitor GND + R IN C L 3 × 1.8k IN SERIES 100µF 1/4W EACH C IN R3 LOAD GND 1 1µF 5.1k (SHORT PIN) V IN R1 EN 402k LTC4252-1 * 1% 8 2 V OUT PWRGD OV R D 1M 9 7 UV DRAIN R2 10 6 Q1 TIMER GATE 32.4k IRF530S 3 4 C T 1% SS SENSE V EE 0.33µF R C R S 5 C1 C SS 10Ω 0.02Ω 10nF 68nF C C 18nF 425212 TA01 –48V * M0C207 Figure 35.–48V Hot Swap Controller an105fa AN105-20

  17. Application Note 105 NEGATIVE VOLTAGE Fast Compact –48V Current Sense (Figure 37) –48V Low Side Precision Current Sense (Figure 36) This amplifier configuration is essentially the complemen- The first stage amplifier is basically a complementary form tary implementation to the classic high side configuration. of the “classic” high side current sense, designed to operate The op amp used must support common mode operation with telecom negative supply voltage. The Zener forms an at its lower rail. A “floating” shunt-regulated local supply inexpensive “floating” shunt-regulated supply for the first is provided by the Zener diode, and the transistor provides op amp. The N-MOSFET drain delivers a metered current metered current to an output load resistance (1kΩ in this into the virtual ground of the second stage, configured as circuit). In this circuit, the output voltage is referenced to a a transimpedance amplifier (TIA). The second op amp is positive potential and moves downward when represent- powered from a positive supply and furnishes a positive ing increasing –48V loading. Scaling accuracy is set by output voltage for increasing load current. A dual op amp the quality of resistors used and the performance of the cannot be used for this implementation due to the different NPN transistor. supply voltages for each stage. This circuit is exceptionally precise due to the use of zero-drift op amps. The scaling accuracy is established by the quality of the user-selected resistors. Small-signal range is limited by V OL in single- supply operation of the second stage. 10k Q1 0.1µF 1% ZETEX ZVN3320F 5V 100Ω – 1% 0.01µF – 39k LTC2054 V OUT = 100V SENSE LTC2054 + + 0.1µF 100Ω BZX84C5V1 V Z = 5.1 0.003Ω 1% 3W –48V SUPPLY –48V LOAD – + 20545 TA01 I SENSE , V SENSE Figure 36.–48V Low Side Precision Current Sense V OUT = 3V – 0.1Ω • I SENSE I SENSE = 0A TO 30A ACCURACY ≈ 3% V OUT R1 1k Q1 4.7k 1% FMMT493 V S = 3V 30.1Ω 1% – R1 REDUCES Q1 DISSIPATION 3.3k 0805 LT1797 × 3 + 0.1µF SETTLES TO 1% IN 2µs, 1V OUTPUT STEP BZX84C6V8 0.003Ω V Z = 6.8V 1% 3W –48V LOAD –48V SUPPLY – I SENSE + 1797 TA01 (–42V TO –56V) Figure 37. Fast Compact –48V Current Sense an105fa AN105-21

  18. Application Note 105 NEGATIVE VOLTAGE –48V Current Monitor (Figures 38a and 38b) Simple Telecom Power Supply Fuse Monitor (Figure 39) In this circuit an economical ADC is used to acquire the sense resistor voltage drop directly. The converter is The LTC1921 provides an all-in-one telecom fuse and powered from a “floating” high accuracy shunt-regulated supply voltage monitoring function. Three opto-isolated supply and is configured to perform continuous conver- status flags are generated that indicate the condition of sions. The ADC digital output drives an opto-isolator, the supplies and the fuses. level-shifting the serial data stream to ground. For wider supply voltage applications, the 13k biasing resistor may be replaced with an active 4mA current source such as shown to the right. For complete dielectric isolation BAT54S and/or higher efficiency operation, the ADC may be powered 1µF 2 × a LT1790-5 from a small transformer circuit as shown in Figure 38b. 5V 1µF 4.7µF 100kHz DRIVE SELECT R FOR 3mA AT MINIMUM SUPPLY b 13k VOLTAGE, 10mA MAX CURRENT AT MAXIMUM DN341 F01 SUPPLY VOLTAGE a MIDCOM 50480 LT1029 0.1µF LOAD 4.7µF Figure 38b b + 5V 48V –48V – LTC2433 10k V CC 1.05k 1 10 45.3k V CC F O V CC 6N139 V REF 2 9 REF + 1.54k SCK MPSA92 8 2 108mV 3 8 REF – 590Ω 7 SDO 1k a 4 7 GND 6 DATA IN + CS 4.7µF 0.010Ω (INVERTED) 5 6 b IN – GND 3 5 V – FULL-SCALE = 5.4A –7V TO –100V –48V –48V Figure 38a Figure 38. –48V Current Monitor 47k 5V –48V RETURN FUSE STATUS R1 R2 100k 100k 3 MOC207 SUPPLY A SUPPLY B RTN 47k V A V B STATUS STATUS 4 1 5V V A OUT F OK OK 0 0 SUPPLY A OK UV OR OV 0 1 STATUS UV OR OV OK 1 0 8 V B UV OR OV UV OR OV 1 1 LTC1921 OK: WITHIN SPECIFICATION MOC207 2 OV: OVERVOLTAGE FUSE A UV: UNDERVOLTAGE 47k 5V 7 5 V FUSE A V FUSE B FUSE STATUS FUSE B OUT A SUPPLY B = V A = V B 0 STATUS = V A ≠ V B 1 ≠ V A = V B 1 MOC207 ≠ V A ≠ V B 1* 6 OUT B 0: LED/PHOTODIODE ON R3 1: LED/PHOTODIODE OFF 47k F1 D1 *IF BOTH FUSES (F1 AND F2) ARE OPEN, 1/4W SUPPLY A –48V OUT ALL STATUS OUTPUTS WILL BE HIGH –48V SINCE R3 WILL NOT BE POWERED F2 D2 SUPPLY B = LOGIC COMMON –48V Figure 39. Simple Telecom Power Supply Fuse Monitor an105fa AN105-22

  19. Application Note 105 NEGATIVE VOLTAGE Monitor Current in Positive or Negative Supply Lines input connections. In both configurations the output is a (Figure 40) ground referred positive voltage. The negative supply to the LT6105 must be at least as negative as the supply line Using a negative supply voltage to power the LT6105 creates it is monitoring. a circuit that can be used to monitor the supply current in a positive or negative supply line by only changing the V OUT = 1V/A V OUT LT6105 4.99k V – –15V 1% V + 5V DC +IN –IN 100Ω 100Ω 20mΩ 1% 1% 1% + – +15V TO +15V POSITIVE LOAD SUPPLY CURRENT FLOW CURRENT FLOW TO –15V –15V – + LOAD NEGATIVE 20mΩ 100Ω 100Ω SUPPLY 1% 1% 1% LT6105 –IN +IN V + 5V DC V – –15V V OUT = 1V/A V OUT 4.99k 6105 F07 1% Figure 40. Monitor Current in Positive or Negative Supply Lines an105fa AN105-23

  20. Application Note 105 UNIDIRECTIONAL Unidirectional current sensing monitors the current flowing Unidirectional Current Sensing Mode only in one direction through a sense resistor. (Figures 42a and 42b) This is just about the simplest connection in which the Unidirectional Output into A/D with LT1787 may be used. The V BIAS pin is connected to ground, Fixed Supply at V S+ (Figure 41) and the V OUT pin swings positive with increasing sense Here the LT1787 is operating with the LTC1286 A/D con- current. The output can swing as low as 30mV . Accuracy is verter. The –IN pin of the A/D converter is biased at 1V by sacrificed at small output levels, but this is not a limitation the resistor divider R1 and R2. This voltage increases as in protection circuit applications or where sensed currents sense current increases, with the amplified sense voltage do not vary greatly. Increased low level accuracy can be appearing between the A/D converters –IN and +IN termi- obtained by level shifting V BIAS above ground. The level nals. The LTC1286 converter uses sequential sampling of shifting may be done with resistor dividers, voltage refer- its –IN and +IN inputs. Accuracy is degraded if the inputs ences or a simple diode. Accuracy is ensured if the output move between sampling intervals. A filter capacitor from signal is sensed differentially between V BIAS and V OUT . FIL + to FIL – as well as a filter capacitor from V BIAS to V OUT may be necessary if the sensed current changes more than R SENSE TO 1LSB within a conversion cycle. LOAD 2.5V TO C 60V 0.1µF R SENSE 5V 1 8 C1 FIL – FIL + 5V 1µF 1 8 LT1787HV FIL – FIL + V S– V S+ 2 7 LT1787 V S– V S+ R1 2 7 V BIAS 3 6 20k DNC V BIAS I OUT 5% 3 6 DNC R OUT 4 5 R OUT V EE V OUT V CC 4 5 V OUT CS V EE +IN V OUT LTC1286 CLK TO µP 1787 F08 –IN D OUT R2 V REF GND Figure 42a 5k 1787 F06 5% 0.30 Figure 41. Unidirectional Output into A/D 0.25 with Fixed Supply at V S+ OUTPUT VOLTAGE (V) 0.20 0.15 0.10 0.05 IDEAL 0 0 0.005 0.010 0.015 0.020 0.025 0.030 V S+ – V S– (V) 1787 F09 Figure 42b Figure 42. Unidirectional Current Sensing Mode an105fa AN105-24

  21. Application Note 105 UNIDIRECTIONAL 16-Bit Resolution Unidirectional Output Intelligent High Side Switch (Figure 44) into LTC2433 ADC (Figure 43) The LT1910 is a dedicated high side MOSFET driver with The LTC2433-1 can accurately digitize signal with source built in protection features. It provides the gate drive for a impedances up to 5kΩ. This LTC6101 current sense circuit power switch from standard logic voltage levels. It provides uses a 4.99kΩ output resistance to meet this requirement, shorted load protection by monitoring the current flow thus no additional buffering is necessary. to through the switch. Adding an LTC6101 to the same circuit, sharing the same current sense resistor, provides a linear voltage signal proportional to the load current for additional intelligent control. I LOAD V SENSE – + R IN 4V TO 60V 100Ω 4 3 L + – 2 5 O A 5V 1µF D 2 1 V OUT REF + V CC 1 4 9 IN + LTC6101 SCK 8 LTC2433-1 TO µP SDD R OUT 7 IN – C C 4.99k 5 REF – F O GND 3 6 10 R OUT V OUT = • V SENSE = 49.9V SENSE ADC FULL-SCALE = 2.5V 6101 TA06 R IN Figure 43. 16-Bit Resolution Unidirectional Output into LTC2433 ADC 10µF V LOGIC 14V 63V 47k 5 100Ω 3 1% 3 FAULT 8 1 4 R S LT1910 LTC6101 V O OFF ON 6 4 2 100Ω 4.99k 1µF 1 5 2 SUB85N06-5 V O = 49.9 • R S • I L L O I L FOR R S = 5mΩ, A V O = 2.5V AT I L = 10A (FULL-SCALE) D 6101 TA07 Figure 44. Intelligent High Side Switch an105fa AN105-25

  22. Application Note 105 UNIDIRECTIONAL 48V Supply Current Monitor with Isolated Output 12-Bit Resolution Unidirectional Output and 105V Survivability (Figure 45) into LTC1286 ADC (Figure 46) The HV version of the LTC6101 can operate with a total While the LT1787 is able to provide a bidirectional output, supply voltage of 105V . Current flow in high supply voltage in this application the economical LTC1286 is used to rails can be monitored directly or in an isolated fashion digitize a unidirectional measurement. The LT1787 has a as shown in this circuit. The gain of the circuit and the nominal gain of eight, providing a 1.25V full-scale output level of output current from the LTC6101 depends on the at approximately 100A of load current. particular opto-isolator used. I SENSE V SENSE + – V S LOAD R SENSE R IN 3 4 – + 5 2 V – LTC6101HV V LOGIC R OUT V OUT ANY OPTO-ISOLATOR V – N = OPTO-ISOLATOR CURRENT GAIN R SENSE V OUT = V LOGIC – I SENSE • • N • R OUT R IN 6101 TA08 Figure 45. 48V Supply Current Monitor with Isolated Output and 105V Survivability R SENSE 0.0016Ω I = 100A TO LOAD 2.5V TO 60V 1 8 FIL – FIL + LT1787HV V S– V S+ 2 7 R1 C1 5V 15k 1µF V BIAS 3 6 DNC R OUT V REF V CC 20k 4 5 CS V EE +IN LTC1286 CLK TO µP V OUT –IN D OUT GND C2 1787 TA01 LT1634-1.25 0.1µF V OUT = V BIAS + (8 • I LOAD • R SENSE ) Figure 46. 12-Bit Resolution Unidirectional Output into LTC1286 ADC an105fa AN105-26

  23. Application Note 105 UNIDIRECTIONAL More Unidirectional Circuits Are Shown in Other Chapters: FIGURE TITLE 20 Precision, Wide Dynamic Range High-side Current Sensing 21 Sensed Current Includes Monitor Circuit Supply Current 22 Wide Voltage Range Current Sensing 23 Smooth Current Monitor Output Signal by Simple Filtering 24 Power on Reset Pulse Using a TimerBlox Device 25 Accurate Delayed Power on Reset Pulse Using TimerBlox Devices 40 Monitor Current in Positive or Negative Supply Lines 93 High Voltage Current and Temperature Monitoring 104 Using Printed Circuit Sense Resistance 105 High Voltage, 5A High Side Current Sensing in Small Package 121 Single Output Provides 10A H-Bridge Current and Direction 122 Monitor Solenoid Current on the Low Side 123 Monitor Solenoid Current on the High Side 125 Large Input Voltage Range for Fused Solenoid Current Monitoring 126 Monitor both the ON Current and the Freewheeling Current Through a High Side Driven Solenoid 127 Monitor both the ON Current and the Freewheeling Current In a Low Side Driven Solenoid 129 Simple DC Motor Torque Control 130 Small Motor Protection and Control 131 Large Motor Protection and Control 143 Battery Stack Monitoring 148 Complete Single Cell Battery Protection 167 Monitor Current in an Isolated Supply Line 168 Monitoring a Fuse Protected Circuit 169 Circuit Fault Protection with Early Warning and Latching Load Disconnect 170 Use Comparator Output to Initialize Interrupt Routines 171 Current Sense with Over-current Latch and Power-On Reset with Loss of Supply 176 Directly Digitize Current with 16-Bit Resolution 177 Directly Digitizing Two Independent Currents 180 Complete Digital Current Monitoring 182 Power Sensing with Built In A to D Converter 183 Isolated Power Measurement 184 Fast Data Rate Isolated Power Measurement 185 Adding Temperature Measurement to Supply Power Measurement 186 Current, Voltage and Fuse Monitoring 187 Automotive Socket Power Monitoring 188 Power over Ethernet, PoE, Monitoring 189 Monitor Current, Voltage and Temperature 208 Remote Current Sensing with Minimal Wiring an105fa AN105-27

  24. Application Note 105 UNIDIRECTIONAL More Unidirectional Circuits Are Shown in Other Chapters: FIGURE TITLE 210 Crystal/Reference Oven Controller 211 Power Intensive Circuit Board Monitoring 212 Crystal/Reference Oven Controller 215 0A to 10A Sensing Over Two Ranges an105fa AN105-28

  25. Application Note 105 BIDIRECTIONAL Bidirectional current sensing monitors current flow in both Practical H-Bridge Current Monitor Offers Fault directions through a sense resistor. Detection and Bidirectional Load Information (Figure 48) Bidirectional Current Sensing with This circuit implements a differential load measurement Single-Ended Output (Figure 47) for an ADC using twin unidirectional sense measurements. Two LTC6101’s are used to monitor the current in a load Each LTC6101 performs high side sensing that rapidly in either direction. Using a separate rail-to-rail op amp to responds to fault conditions, including load shorts and combine the two outputs provides a single ended output. MOSFET failures. Hardware local to the switch module With zero current flowing the output sits at the reference (not shown in the diagram) can provide the protection potential, one-half the supply voltage for maximum out- logic and furnish a status flag to the control system. put swing or 2.5V as shown. With power supplied to the The two LTC6101 outputs taken differentially produce load through connection A the output will move positive a bidirectional load measurement for the control servo. between 2.5V and V CC . With connection B the output The ground-referenced signals are compatible with most ∆Σ ADCs. The ∆Σ ADC circuit also provides a “free” in- moves down between 2.5V and 0V . tegration function that removes PWM content from the V S measurement. This scheme also eliminates the need for analog-to-digital conversions at the rate needed to sup- port switch protection, thus reducing cost and complexity. B A B A LOAD – BATTERY BUS DIFF R S OUTPUT 100 Ω 100 Ω 0.1 TO ADC + 100 Ω R IN R IN I LTC6101 LTC6101 100 Ω R OUT R OUT R S R S 4 3 5 5 3 4 + FOR I M RANGE = ±100A, DIFF OUT =±2.5V – – R S = 1mΩ LTC6101 LTC6101 R IN = 200Ω R OUT = 4.99k + + I M 2 1 1 2 2.5V 5V REF 2.5k + DN374 F04 LT1490 V OUT Figure 48. Practical H-Bridge Current Monitor Offers Fault – 2.5V TO 5V (CONNECTION A) Detection and Bidirectional Load Information 2.5V TO 0V (CONNECTION B) 0A TO 1A IN EITHER DIRECTION 2.5k Figure 47. Bidirectional Current Sensing with Single-Ended Output an105fa AN105-29

  26. Application Note 105 BIDIRECTIONAL Conventional H-Bridge Current Monitor (Figure 49) Single-Supply 2.5V Bidirectional Operation with External Voltage Reference and I/V Converter Many of the newer electric drive functions, such as steer- (Figure 50) ing assist, are bidirectional in nature. These functions are generally driven by H-bridge MOSFET arrays using pulse- The LT1787’s output is buffered by an LT1495 rail-to-rail width modulation (PWM) methods to vary the commanded op amp configured as an I/V converter. This configuration torque. In these systems, there are two main purposes for is ideal for monitoring very low voltage supplies. The current monitoring. One is to monitor the current in the LT1787’s V OUT pin is held equal to the reference voltage load, to track its performance against the desired com- appearing at the op amp’s noninverting input. This al- mand (i.e., closed-loop servo law), and another is for fault lows one to monitor supply voltages as low as 2.5V . The detection and protection features. op amp’s output may swing from ground to its positive supply voltage. The low impedance output of the op amp A common monitoring approach in these systems is to may drive following circuitry more effectively than the amplify the voltage on a “flying” sense resistor, as shown. high output impedance of the LT1787. The I/V converter Unfortunately, several potentially hazardous fault scenarios configuration also works well with split supply voltages. go undetected, such as a simple short to ground at a motor terminal. Another complication is the noise introduced by I SENSE the PWM activity. While the PWM noise may be filtered for R SENSE TO CHARGER/ purposes of the servo law, information useful for protection LOAD C1 2.5V + V SENSE(MAX) becomes obscured. The best solution is to simply provide 1µF 1 8 FIL – FIL + two circuits that individually protect each half-bridge and LT1787 V S– V S+ 2 7 report the bidirectional load current. In some cases, a 2.5V V BIAS 3 6 smart MOSFET bridge driver may already include sense DNC C3 resistors and offer the protection features needed. In these R OUT 1000pF 4 5 V EE situations, the best solution is the one that derives the load V OUT – information with the least additional circuitry. A1 V OUT A + 2.5V LT1495 1M BATTERY BUS 5% LT1389-1.25 + 1787 F07 Figure 50. Single-Supply 2.5V Bidirectional Operation with External Voltage Reference and I/V Converter + R S DIFF AMP – I M DN374 F03 Figure 49. Conventional H-Bridge Current Monitor an105fa AN105-30

  27. Application Note 105 BIDIRECTIONAL Battery Current Monitor (Figure 51) Fast Current Sense with Alarm (Figure 52) One LT1495 dual op amp package can be used to establish The LT1995 is shown as a simple unity gain difference separate charge and discharge current monitoring outputs. amplifier. When biased with split supplies the input current The LT1495 features Over-the-Top operation allowing can flow in either direction providing an output voltage of the battery potential to be as high as 36V with only a 5V 100mV per Amp from the voltage across the 100mΩ sense amplifier supply voltage. resistor. With 32MHz of bandwidth and 1000V/µs slew rate the response of this sense amplifier is fast. Adding a simple comparator with a built in reference voltage circuit I L R SENSE such as the LT6700-3 can be used to generate an overcur- CHARGE 0.1Ω rent flag. With the 400mV reference the flag occurs at 4A. 12V DISCHARGE 5V 15V TO –15V 15V R A R A – – I A2 A1 LT6700-3 1/2 LT1495 1/2 LT1495 R A R A P1 10k + + 10k LT1995 0.1Ω + G = 1 REF M1 2N3904 2N3904 SENSE ( ) FLAG – OUTPUT R B DISCHARGE CHARGE OUTPUT V O = I L R SENSE 100mV/A R A OUT OUT 4A LIMIT –15V FOR R A = 1k, R B = 10k R B R B V O 400mV = 1V/A I L 1495 TA05 1995 TA05 Figure 51. Battery Current Monitor Figure 52. Fast Current Sense with Alarm an105fa AN105-31

  28. Application Note 105 BIDIRECTIONAL Bidirectional Current Sense with Separate Bidirectional Absolute Value Current Sense Charge/Discharge Output (Figure 53) (Figure 54) In this circuit the outputs are enabled by the direction of The high impedance current source outputs of two current flow. The battery current when either charging LTC6101’s can be directly tied together. In this circuit or discharging enables only one of the outputs. For ex- the voltage at V OUT continuously represents the absolute ample when charging, the V OUT D signal goes low since value of the magnitude of the current into or out of the the output MOSFET of that LTC6101 turns completely off battery. The direction or polarity of the current flow is not while the other LT6101, V OUT C, ramps from low to high discriminated. in proportion to the charging current. The active output reverses when the charger is removed and the battery discharges into the load. I DISCHARGE I CHARGE R SENSE CHARGER R IN C R IN D 100 100 R IN D R IN C 100 100 4 3 3 4 V BATT + + – – 2 5 5 2 L O A D 1 1 LTC6101 LTC6101 + + R OUT D R OUT C V OUT D V OUT C 4.99k 4.99k – – 6101 TA02 V OUT D = I DISCHARGE • R SENSE ( ) WHEN I DISCHARGE ≥ 0 R OUT D DISCHARGING: R IN D V OUT C = I CHARGE • R SENSE ( ) WHEN I CHARGE ≥ 0 R OUT C CHARGING: R IN C Figure 53. Bidirectional Current Sense with Separate Charge/Discharge Output I DISCHARGE I CHARGE R SENSE CHARGER R IN C R IN D R IN D R IN C 4 3 3 4 V BATT + + – – 2 5 5 2 L O A D 1 1 LTC6101 LTC6101 + V OUT R OUT – 6101 TA05 V OUT = I DISCHARGE • R SENSE ( ) WHEN I DISCHARGE ≥ 0 R OUT DISCHARGING: R IN D V OUT = I CHARGE • R SENSE ( ) WHEN I CHARGE ≥ 0 R OUT CHARGING: R IN C Figure 54. Bidirectional Absolute Value Current Sense an105fa AN105-32

  29. Application Note 105 BIDIRECTIONAL Full-Bridge Load Current Monitor (Figure 55) Low Power, Bidirectional 60V Precision High Side Current Sense (Figure 56) The LT1990 is a difference amplifier that features a very wide common mode input voltage range that can far Using a very precise zero-drift amplifier as a pre-amp exceed its own supply voltage. This is an advantage to allows for the use of a very small sense resistor in a high reject transient voltages when used to monitor the current voltage supply line. A floating power supply regulates the in a full-bridge driven inductive load such as a motor. The voltage across the pre-amplifier on any voltage rail up to LT6650 provides a voltage reference of 1.5V to bias up the the 60V limit of the LT1787HV circuit. Overall gain of this output away from ground. The output will move above or circuit is 1000. A 1mA change in current in either direction below 1.5V as a function of which direction the current through the 10mΩ sense resistor will produce a 10mV in the load is flowing. As shown, the amplifier provides change in the output voltage. a gain of 10 to the voltage developed across resistor R S . +V SOURCE 5V LT1990 900k 10k 8 7 100k 1M 2 – R S 6 V OUT – + 1M 3 + V REF = 1.5V I L 4 10k 5 IN OUT 1nF 54.9k LT6650 40k 900k GND FB 100k 40k 20k –12V ≤ V CM ≤ 73V V OUT = V REF ± (10 • I L • R S ) 1 1990 TA01 1µF Figure 55. Full-Bridge Load Current Monitor POSITIVE SENSE 10mΩ – + 5 3 BAT54 V SENSE 1 PRECISION LTC1754-5 BIDIRECTIONAL 100Ω 1N4686 3 GAIN OF 125 5 + 0.1µF 3.9V Z 2 4 6 1 10µF 10µF LTC2054 1µF 100Ω 4 0.1µF – 2 12.4k 33Ω 2 7 V S– V S+ 2N5401 1 8 V OUT = 2.5V ON 5V MPSA42 PRECISION POWER SUPPLY +1000* V SENSE OFF 0V 5 (NOTE: POSITIVE BIDIRECTIONAL LT1787HV CURRENT SENSE HIGH VOLTAGE 4.7µF 35.7k INCLUDES CIRCUIT LEVEL SHIFT 6 SUPPLY CURRENT) AND GAIN OF 8 2.5V REF 4 20545 TA06 Figure 56. Low Power, Bidirectional 60V Precision High Side Current Sense an105fa AN105-33

  30. Application Note 105 BIDIRECTIONAL Split or Single Supply Operation, Bidirectional Output Bidirectional Precision Current Sensing (Figure 58) into A/D (Figure 57) This circuit uses two LTC6102 devices, one for each di- In this circuit, split supply operation is used on both the rection of current flow through a single sense resistance. LT1787 and LT1404 to provide a symmetric bidirectional While each output only provides a result in one particular measurement. In the single-supply case, where the LT1787 direction of current, taking the two output signals differ- Pin 6 is driven by V REF , the bidirectional measurement entially provides a bipolar signal to other circuitry such range is slightly asymmetric due to V REF being somewhat as an ADC. Since each circuit has its own gain resistors, greater than midspan of the ADC input range. bilinear scaling is possible (different scaling depending on direction). 1Ω 1% I S = ±125mA V CC 5V 1 8 V SRCE FIL – FIL + ≈4.75V LT1787 V S– V S+ 10µF 2 7 16V V BIAS 3 6 1 DNC 7 20k CONV V OUT (±1V) 4 5 V EE 2 6 CLOCKING V EE A IN LTC1404 CLK –5V CIRCUITRY V OUT 3 V REF OPTIONAL SINGLE 5 DOUT SUPPLY OPERATION: GND 10µF DISCONNECT V BIAS 16V 4 8 FROM GROUND AND CONNECT IT TO V REF . 10µF D OUT REPLACE –5V SUPPLY 16V WITH GROUND. V EE 1787 TA02 OUTPUT CODE FOR ZERO –5V CURRENT WILL BE ~2430 Figure 57. Split or Single Supply Operation, Bidirectional Output into A/D I CHARGE I DISCHARGE R SENSE CHARGER R IN C R IN D 100Ω 100Ω R IN C R IN D 100Ω 100Ω +IN –INS –INS +IN V BATT –INF –INF + – – + L V – V + V + V – O A 0.1µF 0.1µF V REG V REG D OUT OUT LTC6102 LTC6102 + + R OUT C R OUT D V OUT C V OUT D 4.99k 4.99k – – 6102 TA02 V OUT D = I DISCHARGE • R SENSE ( ) WHEN I DISCHARGE ≥ 0 R OUT D DISCHARGING: R IN D V OUT C = I CHARGE • R SENSE ( ) WHEN I CHARGE ≥ 0 R OUT C CHARGING: R IN C Figure 58. Bidirectional Precision Current Sensing an105fa AN105-34

  31. Application Note 105 BIDIRECTIONAL Differential Output Bidirectional 10A Current Sense Absolute Value Output Bidirectional Current Sensing (Figure 59) (Figure 60) The LTC6103 has dual sense amplifiers and each measures Connecting an LTC6103 so that the outputs each represent current in one direction through a single sense resistance. opposite current flow through a shared sense resistance, The outputs can be taken together as a differential output but with the outputs driving a common load, results in a to subsequent circuitry such as an ADC. Values shown positive only output function while sensing bidirectionally. are for 10A maximum measurement. 10mΩ + V BATT 200Ω 200Ω LOAD CHARGER 4V < V BATT < 60V 8 7 6 5 +INA –INA –INB +INB + – – + V SA V SB V – OUTA OUTB LTC6103 1 4 2 + DIFFERENTIAL OUTPUT* – ±2.5V FS (+ IS CHARGE CURRENT) 4.99k 4.99k +OUTPUT MAY BE TAKEN SINGLE ENDED AS CHARGE CURRENT MONITOR * 6103 TA02 –OUTPUT MAY BE TAKEN SINGLE ENDED AS DISCHARGE CURRENT MONITOR OUTPUT SWING MAY BE LIMITED FOR V BATT BELOW 6V Figure 59. Differential Output Bidirectional 10A Current Sense 20mΩ + V BATT 200Ω 200Ω LOAD CHARGER 8 7 6 5 +INA –INA –INB +INB + – – + V SA V SB V – OUTA OUTB LTC6103 1 4 2 V OUT 2.5V FS 4.99k 6103 TA03 Figure 60. Absolute Value Output Bidirectional Current Sensing an105fa AN105-35

  32. Application Note 105 BIDIRECTIONAL More Bidirectional Circuits Are Shown in Other Chapters: FIGURE TITLE 104 Using Printed Circuit Sense Resistance 120 Bidirectional Current Sensing in H-Bridge Drivers 124 Monitor H-Bridge Motor Current Directly 128 Fixed Gain DC Motor Current Monitor 136 Coulomb Counting Battery Gas Gauge 142 Monitor Charge and Discharge Currents at One Output 145 High Voltage Battery Coulomb Counting 146 Low Voltage Battery Coulomb Counting 147 Single Cell Lithium-Ion Battery Coulomb Counter 178 Digitize a Bidirectional Current Using a Single Sense Amplifier and ADC 179 Digitizing Charging and Loading Current in a Battery Monitor 181 Ampere-Hour Gauge 209 Use Kelvin Connections to Maintain High Current Accuracy 216 Dual Sense Amplifier Can Have Different Sense Resistors and Gain an105fa AN105-36

  33. Application Note 105 AC Sensing current in AC power lines is quite tricky in the former can be connected directly to the converter. Up to sense that both the current and voltage are continuously 75A of AC current is measurable without breaking the signal changing polarity. Transformer coupling of signals to drive path from a power source to a load. The accurate operating ground referenced circuitry is often a good approach. range of the circuit is determined by the selection of the transformer termination resistor. All of the math is built Single-Supply RMS Current Measurement (Figure 61) in to the LTC1966 to provide a DC output voltage that is proportional to the true RMS value of the current. This is The LT1966 is a true RMS-to-DC converter that takes a valuable in determining the power/energy consumption single-ended or differential input signal with rail-to-rail of AC-powered appliances. range. The output of a PCB mounted current sense trans- V + LTC1966 IN1 AC CURRENT V OUT V OUT = 4mV DC /A RMS 75A MAX T1 10Ω C AVE 50Hz TO 400Hz IN2 OUT RTN 1µF GND EN 100k V SS 1966 TA08 100k 0.1µF T1: CR MAGNETICS CR8348-2500-N www.crmagnetics.com Figure 61. Single-Supply RMS Current Measurement More AC Circuits Are Shown in Other Chapters: FIGURE TITLE 120 Bidirectional Current Sensing in H-Bridge Drivers 124 Monitor H-Bridge Motor Current Directly 128 Fixed Gain DC Motor Current Monitor an105fa AN105-37

  34. Application Note 105 DC DC current sensing is for measuring current flow that is LT1991 measuring the voltage. The LT6100 senses the changing at a very slow rate. current by measuring the voltage across the 10Ω resistor, applies a gain of 50, and provides a ground referenced Micro-Hotplate Voltage and Current Monitor output. The I to V gain is therefore 500mV/mA, which (Figure 62) makes sense given the 10mA full-scale heater current and the 5V output swing of the LT6100. The LT1991’s task is Materials science research examines the properties and the opposite, applying precision attenuation instead of interactions of materials at various temperatures. Some gain. The full-scale voltage of the heater is a total of 40V of the more interesting properties can be excited with (±20), beyond which the life of the heater may be reduced localized nano-technology heaters and detected using the in some atmospheres. The LT1991 is set up for an attenua- presence of interactive thin films. tion factor of 10, so that the 40V full-scale differential drive While the exact methods of detection are highly complex becomes 4V ground referenced at the LT1991 output. In and relatively proprietary, the method of creating localized both cases, the voltages are easily read by 0V–5V PC I/O heat is as old as the light bulb. Shown is the schematic cards and the system readily software controlled. of the heater elements of a Micro-hotplate from Boston Microsystems (www.bostonmicrosystems.com). The Battery Current Monitor (Figure 63) physical dimensions of the elements are tens of microns. One LT1495 dual op amp package can be used to estab- They are micromachined out of SiC and heated with simple lish separate charge and discharge current monitoring DC electrical power, being able to reach 1000°C without outputs. The LT1495 features Over-the-Top operation damage. allowing the battery potential to be as high as 36V with The power introduced to the elements, and thereby their only a 5V amplifier supply voltage. temperature, is ascertained from the voltage-current product with the LT6100 measuring the current and the I L R SENSE CHARGE 0.1Ω V DR+ 12V DISCHARGE 5V 10Ω R A R A – 1% – V S– V S+ A2 A1 1/2 LT1495 1/2 LT1495 R A R A I HOTPLATE – + + + 5V V CC CURRENT MONITOR 2N3904 2N3904 ( ) LT6100 V OUT = 500mV/mA R B DISCHARGE CHARGE V EE V O = I L R SENSE A2 A4 R A OUT OUT MICRO-HOTPLATE FOR R A = 1k, R B = 10k R B R B BOSTON MICROSYSTEMS V O = 1V/A MHP100S-005 I L 5V 1495 TA05 5V M9 Figure 63. Battery Current Monitor M3 VOLTAGE M1 LT1991 MONITOR P1 V DR+ – V DR– V OUT = P3 10 P9 V DR– 6100 TA06 www.bostonmicrosystems.com Figure 62. Micro-Hotplate Voltage and Current Monitor an105fa AN105-38

  35. Application Note 105 DC Bidirectional Battery-Current Monitor (Figure 64) V OS performance of op amps at the supply is generally not factory trimmed, thus less accurate than other solutions. This circuit provides the capability of monitoring current The finite current gain of the bipolar transistor is a small in either direction through the sense resistor. To allow source of gain error. negative outputs to represent charging current, V EE is connected to a small negative supply. In single-supply High Side Current Sense and Fuse Monitor (Figure 66) operation (V EE at ground), the output range may be offset The LT6100 can be used as a combination current sen- upwards by applying a positive reference level to V BIAS sor and fuse monitor. This part includes on-chip output (1.25V for example). C3 may be used to form a filter in buffering and was designed to operate with the low supply conjunction with the output resistance (R OUT ) of the part. voltage (≥2.7V), typical of vehicle data acquisition systems, This solution offers excellent precision (very low V OS ) and while the sense inputs monitor signals at the higher bat- a fixed nominal gain of 8. tery bus potential. The LT6100 inputs are tolerant of large R SENSE TO input differentials, thus allowing the blown-fuse operating CHARGER/ LOAD C1 condition (this would be detected by an output full-scale 15V 1µF 1 8 FIL – FIL + indication). The LT6100 can also be powered down while LT1787 V S– V S+ maintaining high impedance sense inputs, drawing less 2 7 than 1µA max from the battery bus. V BIAS 3 6 DNC R OUT 4 5 V EE OUTPUT R SENSE V OUT TO LOAD FUSE 2mΩ C2 C3* BATTERY –5V 1µF 1000pF BUS + 1 8 1787 F02 V S– V S+ *OPTIONAL ADC 2 7 POWER V CC A4 Figure 64. Bidirectional Battery-Current Monitor ≥2.7V C2 – + 0.1µF “Classic” Positive Supply Rail Current Sense 3 6 FIL A2 (Figure 65) This circuit uses generic devices to assemble a function OUT 4 5 OUTPUT V EE 2.5V = 25A similar to an LTC6101. A rail-to-rail input type op amp is LT6100 required since input voltages are right at the upper rail. DN374 F02 The circuit shown here is capable of monitoring up to 44V Figure 66. High Side Current Sense and Fuse Monitor applications. Besides the complication of extra parts, the 5V 200Ω 0.2Ω + Q1 LT1637 2N3904 200Ω – 0V TO 4.3V I LOAD 2k LOAD 1637 TA02 V OUT = (2Ω)(I LOAD ) Figure 65. “Classic” Positive Supply Rail Current Sense an105fa AN105-39

  36. LOAD LT6100 FIL Application Note 105 DC Gain of 50 Current Sense (Figure 67) Dual LTC6101’s Allow High-Low Current Ranging (Figure 68) The LT6100 is configured for a gain of 50 by grounding both A2 and A4. This is one of the simplest current sensing Using two current sense amplifiers with two values of amplifier circuits where only a sense resistor is required. sense resistors is an easy method of sensing current over a wide range. In this circuit the sensitivity and resolution of measurement is 10 times greater with low currents, less I SENSE R SENSE V SUPPLY than 1.2A, than with higher currents. A comparator detects 6.4V TO 48V higher current flow, up to 10A, and switches sensing over V S+ V S– to the high current circuitry. – + 5V V CC V OUT 50 • R SENSE • I SENSE V EE A2 A4 6100 TA04 Figure 67. Gain of 50 Current Sense V LOGIC CMPZ4697 (3.3V TO 5V) 7 10k M1 3 + Si4465 V IN 4 – R SENSE HI I LOAD 8 Q1 10m 5 V OUT CMPT5551 R SENSE LO 40.2k 6 100m 301 301 301 301 4.7k 1.74M LTC1540 4 3 4 3 2 1 HIGH + – + – 2 5 2 5 RANGE V IN 619k INDICATOR (I LOAD > 1.2A) 1 1 HIGH CURRENT RANGE OUT LTC6101 LTC6101 250mV/A 7.5k V LOGIC BAT54C LOW CURRENT RANGE OUT 2.5V/A R5 ( V LOGIC +5V ) ≤ V IN ≤ 60V 7.5k 6101 F03b 0 ≤ I LOAD ≤ 10A Figure 68. Dual LTC6101’s Allow High-Low Current Ranging an105fa AN105-40

  37. Application Note 105 DC Two Terminal Current Regulator (Figure 69) power to the circuit with batteries, any voltage potential at the inputs are handled. The LT1495 is a micropower op The LT1635 combines an op amp with a 200mV reference. amp so the quiescent current drain from the batteries is Scaling this reference voltage to a potential across resistor very low and thus no on/off switch is required. R3 forces a controlled amount of current to flow from the +terminal to the –terminal. Power is taken from the loop. 100pF + (R2 + R3)V REF I OUT = R1 (R1)(R3) 10M 2 7 – R4 – 10k 6 1/2 – 1.5V LT1635 LT1495 3 INPUT 1/2 4 + + CURRENT LT1495 1 R2 + 1.5V R1 8 9k R3 I S = 3µA WHEN I IN = 0 2k NO ON/OFF SWITCH R2 R3 FULL-SCALE – REQUIRED ADJUST 1635 TA05 0µA TO µA 200µA Figure 69. Two Terminal Current Regulator 1495 TA06 Figure 71. 0nA to 200nA Current Meter High Side Power Supply Current Sense (Figure 70) Over-The-Top Current Sense (Figure 72) The low offset error of the LTC6800 allows for unusually low sense resistance while retaining accuracy. This circuit is a variation on the “classic” high side cir- cuit, but takes advantage of Over-the-Top input capability to separately supply the IC from a low voltage rail. This 1.5mΩ V REGULATOR provides a measure of fault protection to downstream circuitry by virtue of the limited output swing set by the low 2 8 – OUT voltage supply. The disadvantage is V OS in the Over-the- 7 100mV/A LTC6800 Top mode is generally inferior to other modes, thus less OF LOAD 3 6 + CURRENT 10k accurate. The finite current gain of the bipolar transistor 5 4 0.1µF is a source of small gain error. I LOAD LOAD 150Ω 3V TO 44V R1 200Ω 6800 TA01 3V Figure 70. High Side Power Supply Current Sense R S + 0.2Ω Q1 LT1637 2N3904 0nA to 200nA Current Meter (Figure 71) – V OUT (0V TO 2.7V) I LOAD A floating amplifier circuit converts a full-scale 200nA R2 2k flowing in the direction indicated at the inputs to 2V at V OUT LOAD I LOAD = (R S )(R2/R1) 1637 TA06 the output of the LT1495. This voltage is converted to a current to drive a 200µA meter movement. By floating the Figure 72. Over-The-Top Current Sense an105fa AN105-41

  38. Application Note 105 DC Conventional H-Bridge Current Monitor (Figure 73) Single-Supply 2.5V Bidirectional Operation with External Voltage Reference and I/V Converter Many of the newer electric drive functions, such as steer- (Figure 74) ing assist, are bidirectional in nature. These functions are generally driven by H-bridge MOSFET arrays using pulse- The LT1787’s output is buffered by an LT1495 rail-to-rail width modulation (PWM) methods to vary the commanded op amp configured as an I/V converter. This configuration torque. In these systems, there are two main purposes for is ideal for monitoring very low voltage supplies. The current monitoring. One is to monitor the current in the LT1787’s V OUT pin is held equal to the reference voltage load, to track its performance against the desired com- appearing at the op amp’s non-inverting input. This al- mand (i.e., closed-loop servo law), and another is for fault lows one to monitor supply voltages as low as 2.5V . The detection and protection features. op amp’s output may swing from ground to its positive supply voltage. The low impedance output of the op amp A common monitoring approach in these systems is to may drive following circuitry more effectively than the amplify the voltage on a “flying” sense resistor, as shown. high output impedance of the LT1787. The I/V converter Unfortunately, several potentially hazardous fault scenarios configuration also works well with split supply voltages. go undetected, such as a simple short to ground at a motor terminal. Another complication is the noise introduced by I SENSE R SENSE TO the PWM activity. While the PWM noise may be filtered for CHARGER/ purposes of the servo law, information useful for protection LOAD C1 2.5V + V SENSE(MAX) 1µF becomes obscured. The best solution is to simply provide 1 8 FIL – FIL + LT1787 two circuits that individually protect each half-bridge and V S– V S+ 2 7 report the bidirectional load current. In some cases, a 2.5V V BIAS 3 6 DNC smart MOSFET bridge driver may already include sense C3 R OUT resistors and offer the protection features needed. In these 1000pF 4 5 V EE situations, the best solution is the one that derives the load V OUT – information with the least additional circuitry. A1 V OUT A + 2.5V LT1495 1M 5% LT1389-1.25 BATTERY BUS 1787 F07 + Figure 74. Single-Supply 2.5V Bidirectional Operation with External Voltage Reference and I/V Converter + R S DIFF AMP – I M DN374 F03 Figure 73. Conventional H-Bridge Current Monitor an105fa AN105-42

  39. Application Note 105 DC Battery Current Monitor (Figure 75) I L R SENSE CHARGE 0.1Ω One LT1495 dual op amp package can be used to establish separate charge and discharge current monitoring outputs. DISCHARGE 12V 5V The LT1495 features Over-the-Top operation allowing R A R A – the battery potential to be as high as 36V with only a 5V – A2 A1 amplifier supply voltage. 1/2 LT1495 R A R A 1/2 LT1495 + + Fast Current Sense with Alarm (Figure 76) 2N3904 2N3904 ( ) The LT1995 is shown as a simple unity-gain difference R B DISCHARGE CHARGE V O = I L R SENSE amplifier. When biased with split supplies the input R A OUT OUT FOR R A = 1k, R B = 10k R B R B current can flow in either direction providing an output V O = 1V/A voltage of 100mV per Amp from the voltage across the I L 1495 TA05 100mΩ sense resistor. With 32MHz of bandwidth and Figure 75. Battery Current Monitor 1000V/µs slew rate the response of this sense amplifier is fast. Adding a simple comparator with a built in refer- 15V TO –15V 15V ence voltage circuit such as the LT6700-3 can be used to I generate an over current flag. With the 400mV reference LT6700-3 P1 10k the flag occurs at 4A. 10k LT1995 0.1Ω + G = 1 REF M1 Positive Supply Rail Current Sense (Figure 77) SENSE FLAG OUTPUT – OUTPUT 100mV/A This is a configuration similar to an LT6100 implemented 4A LIMIT –15V with generic components. A rail-to-rail or Over-the-Top 400mV input op amp type is required (for the first section). The 1995 TA05 first section is a variation on the classic high side where the P-MOSFET provides an accurate output current into Figure 76. Fast Current Sense with Alarm R2 (compared to a BJT). The second section is a buffer to allow driving ADC ports, etc., and could be configured with gain if needed. As shown, this circuit can handle up V CC R1 to 36V operation. Small-signal range is limited by V OL in 200Ω single-supply operation. Rs – – 0.2Ω Q1 1/2 LT1366 1/2 LT1366 TP0610L + + ( ) I LOAD R2 V O = I LOAD • R S R1 R2 = I LOAD • 20Ω 20k LOAD 1366 TA01 Figure 77. Positive Supply Rail Current Sense an105fa AN105-43

  40. Application Note 105 DC LT6100 Load Current Monitor (Figure 78) TO LOAD R SENSE + This is the basic LT6100 circuit configuration. The internal C1 5V 1 8 0.1µF circuitry, including an output buffer, typically operates from V S– V S+ a low voltage supply, such as the 3V shown. The moni- 2 7 V CC A4 + tored supply can range anywhere from V CC + 1.4V up to C2 3V – + 0.1µF 48V . The A2 and A4 pins can be strapped various ways to 3 6 provide a wide range of internally fixed gains. The input FIL A2 leads become very Hi-Z when V CC is powered down, so as not to drain batteries for example. Access to an internal OUT 4 5 OUTPUT V EE signal node (Pin 3) provides an option to include a filtering LT6100 function with one added capacitor. Small-signal range is 6100 F04 limited by V OL in single-supply operation. Figure 78. LT6100 Load Current Monitor 1A Voltage-Controlled Current Sink (Figure 79) V + This is a simple controlled current sink, where the op amp drives the N-MOSFET gate to develop a match between V + R L the 1Ω sense resistor drop and the V IN current command. I OUT + V IN Since the common mode voltage seen by the op amp is 100Ω 1/2 Si9410DY LT1492 near ground potential, a “single-supply” or rail-to-rail type N-CHANNEL – 100pF is required in this application. 1k LTC6101 Supply Current Included as Load in I OUT = V IN 1Ω Measurement (Figure 80) 1Ω t r < 1µs 1492/93 TA06 This is the basic LTC6101 high side sensing supply-monitor Figure 79. 1A Voltage-Controlled Current Sink configuration, where the supply current drawn by the IC is included in the readout signal. This configuration is use- V + ful when the IC current may not be negligible in terms of overall current draw, such as in low power battery-powered R IN applications. R SENSE should be selected to limit voltage R SENSE 4 3 drop to <500mV for best linearity. If it is desirable not to include the IC current in the readout, as in load monitor- + – ing, Pin 5 may be connected directly to V + instead of the LOAD 2 5 load. Gain accuracy of this circuit is limited only by the precision of the resistors selected by the user. 1 V OUT LTC6101 R OUT 6101 F06 Figure 80. LTC6101 Supply Current Included as Load in Measurement an105fa AN105-44

  41. Application Note 105 DC V + Powered Separately from Load Supply (Figure 81) 4.4V TO 48V 3V SUPPLY 2 7 6 The inputs of the LTC6101 can function from 1.4V above V CC A4 A2 LT6100 the device positive supply to 48V DC. In this circuit the V S+ current flow in the high voltage rail is directly translated 8 to a 0V to 3V range. V OUT 5 V OUT = 2.5V R SENSE 3mΩ I SENSE = 33A V S– Simple High Side Current Sense Using the LTC6101 1 (Figure 82) V EE FIL LOAD 6100 TA01a This is a basic high side current monitor using the LTC6101. 4 3 220pF CONFIGURED FOR GAIN = 25V/V The selection of R IN and R OUT establishes the desired gain of this circuit, powered directly from the battery bus. The current output of the LTC6101 allows it to be located re- Figure 81. V+ Powered Separately from Load Supply motely to R OUT . Thus, the amplifier can be placed directly BATTERY BUS at the shunt, while R OUT is placed near the monitoring electronics without ground drop errors. This circuit has a fast 1µs response time that makes it ideal for providing R IN R SENSE 100Ω MOSFET load switch protection. The switch element may 0.01Ω 3 4 be the high side type connected between the sense resistor LOAD – + and the load, a low side type between the load and ground 2 5 or an H-bridge. The circuit is programmable to produce up to 1mA of full-scale output current into R OUT , yet draws a mere 250µA supply current when the load is off. 1 V OUT LT6101 4.99V = 10A R OUT “Classic” High Precision Low Side Current Sense 4.99k (Figure 83) V OUT = I LOAD (R SENSE • R OUT /R IN ) DN374 F01 This configuration is basically a standard noninverting Figure 82. Simple High Side Current Sense Using the LTC6101 amplifier. The op amp used must support common mode operation at the lower rail and the use of a zero-drift type (as shown) provides excellent precision. The output of 5V this circuit is referenced to the lower Kelvin contact, which 3 5 OUT + could be ground in a single-supply application. Small-signal 3V/AMP 1 LOAD CURRENT range is limited by V OL for single-supply designs. Scaling LTC2050HV IN MEASURED 4 – CIRCUIT, REFERRED accuracy is set by the quality of the user-selected resistors. 2 TO –5V 10Ω 10k TO 3mΩ MEASURED CIRCUIT LOAD CURRENT 0.1µF –5V 2050 TA08 Figure 83. “Classic” High Precision Low Side Current Sense an105fa AN105-45

  42. Application Note 105 DC More DC Circuits Are Shown in Other Chapters: FIGURE TITLE 20 Precision, Wide Dynamic Range High-side Current Sensing 22 Wide Voltage Range Current Sensing 58 Bidirectional Precision Current Sensing 59 Differential Output Bidirectional 10A Current Sense 60 Absolute Value Output Bidirectional Current Sensing 142 Monitor Charge and Discharge Currents at One Output 178 Digitize a Bi-Directional Current Using a Single Sense Amplifier and ADC 208 Remote Current Sensing with Minimal Wiring 209 Use Kelvin Connections to Maintain High Current Accuracy 216 Dual Sense Amplifier Can Have Different Sense Resistors and Gain an105fa AN105-46

  43. Application Note 105 LEVEL SHIFTING V + Powered Separately from Load Supply (Figure 85) Quite often it is required to sense current flow in a sup- ply rail that is a much higher voltage potential than the The inputs of the LTC6101 can function from 1.4V above supply voltage for the system electronics. Current sense the device positive supply to 48V DC. In this circuit the circuits with high voltage capability are useful to translate current flow in the high voltage rail is directly translated information to lower voltage signals for processing. to a 0V to 3V range. Over-The-Top Current Sense (Figure 84) Voltage Translator (Figure 86) This circuit is a variation on the “classic” high side cir- This is a convenient usage of the LTC6101 current sense cuit, but takes advantage of Over-the-Top input capability amplifier as a high voltage level translator. Differential to separately supply the IC from a low voltage rail. This voltage signals riding on top of a high common mode provides a measure of fault protection to downstream voltage (up to 105V with the LTC6101HV) get converted to circuitry by virtue of the limited output swing set by the low a current, through R IN , and then scaled down to a ground voltage supply. The disadvantage is V OS in the Over-the- referenced voltage across R OUT . Top mode is generally inferior to other modes, thus less accurate. The finite current gain of the bipolar transistor is a source of small gain error. + R IN V IN 3V TO 44V 4 3 – R1 200Ω – + 3V 2 5 R S + 0.2Ω Q1 + V TRANSLATE LT1637 – 2N3904 – V OUT 1 (0V TO 2.7V) I LOAD V OUT LTC6101 R2 R OUT 2k V OUT LOAD I LOAD = (R S )(R2/R1) 1637 TA06 Figure 84. Over-The-Top Current Sense Figure 86. Voltage Translator 4.4V TO 48V 3V SUPPLY 2 7 6 V CC A4 A2 LT6100 V S+ 8 V OUT 5 R SENSE V OUT = 2.5V 3mΩ I SENSE = 33A V S– 1 V EE FIL LOAD 6100 TA01a 4 3 220pF CONFIGURED FOR GAIN = 25V/V Figure 85. V + Powered Separately from Load Supply an105fa AN105-47

  44. Application Note 105 LEVEL SHIFTING Low Power, Bidirectional 60V Precision High Side voltage across the pre-amplifier on any voltage rail up to Current Sense (Figure 87) the 60V limit of the LT1787HV circuit. Overall gain of this circuit is 1000. A 1mA change in current in either direction Using a very precise zero-drift amplifier as a pre-amp through the 10mΩ sense resistor will produce a 10mV allows for the use of a very small sense resistor in a high change in the output voltage. voltage supply line. A floating power supply regulates the POSITIVE SENSE 10mΩ – + 5 3 BAT54 V SENSE 1 PRECISION LTC1754-5 BIDIRECTIONAL 100Ω 1N4686 3 GAIN OF 125 5 + 0.1µF 3.9V Z 2 4 6 1 10µF 10µF LTC2054 1µF 100Ω 4 0.1µF – 2 12.4k 33Ω 2 7 V S+ V S– 2N5401 1 8 V OUT = 2.5V ON 5V MPSA42 POWER SUPPLY PRECISION +1000* V SENSE OFF 0V 5 (NOTE: POSITIVE BIDIRECTIONAL LT1787HV CURRENT SENSE HIGH VOLTAGE 4.7µF 35.7k INCLUDES CIRCUIT LEVEL SHIFT 6 SUPPLY CURRENT) AND GAIN OF 8 2.5V REF 4 20545 TA06 Figure 87. Low Power, Bidirectional 60V Precision High Side Current Sense More Level Shifting Circuits Are Shown in Other Chapters: FIGURE TITLE 40 Monitor Current in Positive or Negative Supply Lines an105fa AN105-48

  45. Application Note 105 HIGH VOLTAGE Monitoring current flow in a high voltage line often re- Measuring Bias Current Into an Avalanche Photo quires floating the supply of the measuring circuits up Diode (APD) Using an Instrumentation Amplifier near the high voltage potentials. Level shifting and isola- (Figures 89a and 89b) tion components are then often used to develop a lower The upper circuit (a) uses an instrumentation amplifier output voltage indication. (IA) powered by a separate rail (>1V above V IN ) to mea- sure across the 1kΩ current shunt. The lower figure (b) Over-The-Top Current Sense (Figure 88) is similar but derives its power supply from the APD bias This circuit is a variation on the “classic” high side cir- line. The limitation of these circuits is the 35V maximum cuit, but takes advantage of Over-the-Top input capability APD voltage, whereas some APDs may require 90V or to separately supply the IC from a low voltage rail. This more. In the single-supply configuration shown, there is provides a measure of fault protection to downstream also a dynamic range limitation due to V OL to consider. circuitry by virtue of the limited output swing set by the low The advantage of this approach is the high accuracy that voltage supply. The disadvantage is V OS in the Over-the- is available in an IA. Top mode is generally inferior to other modes, thus less accurate. The finite current gain of the bipolar transistor 1k is a source of small gain error. 1% V IN BIAS OUTPUT 10V TO 33V TO APD 35V 3V TO 44V – R1 CURRENT 200Ω MONITOR OUTPUT LT1789 0mA TO 1mA = 0V TO 1V + 3V R S + 0.2Ω AN92 F02b Q1 LT1637 2N3904 Figure 89a – V OUT (0V TO 2.7V) I LOAD R2 2k 1N4684 V OUT 1k LOAD I LOAD = 3.3V (R S )(R2/R1) 1% 1637 TA06 V IN BIAS OUTPUT 10V TO 35V TO APD 10M – Figure 88. Over-The-Top Current Sense CURRENT MONITOR OUTPUT LT1789 0mA TO 1mA = 0V TO 1V + A = 1 AN92 F02b Figure 89b Figure 89. Measuring Bias Current Into an Avalanche Photo Diode (APD) Using an Instrumentation Amplifier an105fa AN105-49

  46. Application Note 105 HIGH VOLTAGE Simple 500V Current Monitor (Figure 90) 48V Supply Current Monitor with Isolated Output and 105V Survivability (Figure 91) Adding two external MOSFETs to hold off the voltage allows the LTC6101 to connect to very high potentials and monitor The HV version of the LTC6101 can operate with a total the current flow. The output current from the LTC6101, supply voltage of 105V . Current flow in high supply voltage which is proportional to the sensed input voltage, flows rails can be monitored directly or in an isolated fashion through M1 to create a ground referenced output voltage. as shown in this circuit. The gain of the circuit and the level of output current from the LTC6101 depends on the particular opto-isolator used. DANGER! Lethal Potentials Present — Use Caution V SENSE I SENSE + – V S LOAD I SENSE V SENSE 500V – + R SENSE R IN 3 4 R SENSE R IN 100Ω 4 3 – + 5 2 V – L + DANGER!! – 2 5 O HIGH VOLTAGE!! A D LTC6101HV 1 62V LTC6101 V LOGIC CMZ59448 R OUT M1 V OUT V OUT M2 ANY OPTO-ISOLATOR R OUT M1 AND M2 ARE FQD3P50 TM 2M 4.99k R OUT V – V OUT = • V SENSE = 49.9 V SENSE R IN N = OPTO-ISOLATOR CURRENT GAIN 6101 TA09 R SENSE V OUT = V LOGIC – I SENSE • • N • R OUT 6101 TA08 Figure 90. Simple 500V Current Monitor R IN Figure 91. 48V Supply Current Monitor with Isolated Output and 105V Survivability an105fa AN105-50

  47. Application Note 105 HIGH VOLTAGE Low Power, Bidirectional 60V Precision High Side voltage across the pre-amplifier on any voltage rail up to Current Sense (Figure 92) the 60V limit of the LT1787HV circuit. Overall gain of this circuit is 1000. A 1mA change in current in either direction Using a very precise zero-drift amplifier as a pre-amp al- through the 10mΩ sense resistor will produce a 10mV lows for the use of a very small sense resistor in a high change in the output voltage. voltage supply line. A floating power supply regulates the POSITIVE SENSE 10mΩ + – 5 3 BAT54 V SENSE 1 PRECISION LTC1754-5 BIDIRECTIONAL 100Ω 1N4686 3 5 GAIN OF 125 + 0.1µF 3.9V Z 2 4 6 1 10µF LTC2054 10µF 1µF 100Ω 4 0.1µF – 2 12.4k 33Ω 2 7 V S– V S+ 2N5401 1 8 V OUT = 2.5V ON 5V MPSA42 POWER SUPPLY PRECISION +1000* V SENSE OFF 0V 5 (NOTE: POSITIVE BIDIRECTIONAL LT1787HV CURRENT SENSE HIGH VOLTAGE 4.7µF 35.7k INCLUDES CIRCUIT LEVEL SHIFT 6 SUPPLY CURRENT) AND GAIN OF 8 2.5V REF 4 20545 TA06 Figure 92. Low Power, Bidirectional 60V Precision High Side Current Sense an105fa AN105-51

  48. Application Note 105 HIGH VOLTAGE High Voltage Current and Temperature Monitoring referenced voltage proportional to the load current and is (Figure 93) measured as a single ended input by the ADC. A divided down representation of the supply voltage is a second Combining an LTC2990 ADC converter with a high voltage input. An external NPN transistor serves as a remote LTC6102HV current sense amplifier allows the measure- temperature sensor. ment of very high voltage rails, up to 104V , and very high current loads. The current sense amplifier outputs a ground R SENSE 1mΩ 1% V IN 5V TO 105V I LOAD R IN 0A TO 10A 20Ω 1% 0.1µF +IN –INS – + –INF V – V + V REG OUT LTC6102HV R OUT 200k 0.1µF 4.99k 1% 1% 4.75k 0.1µF 1% 5V 0.1µF MMBT3904 V CC V1 V2 2-WIRE SDA V3 I 2 C LTC2990 SCL INTERFACE 470pF ADR0 ADR1 V4 GND 2990 TA02 ALL CAPACITORS ±20% VOLTAGE, CURRENT AND TEMPERATURE CONFIGURATION: CONTROL REGISTER: 0x58 T AMB REG 4, 5 0.0625°C/LSB V LOAD REG 6, 7 13.2mVLSB V2(I LOAD ) REG 8, 9 1.223mA/LSB T REMOTE REG A, B 0.0625°C/LSB V CC REG E, F 2.5V + 305.18µV/LSB Figure 93. High Voltage Current and Temperature Monitoring an105fa AN105-52

  49. Application Note 105 HIGH VOLTAGE More High Voltage Circuits Are Shown In Other Chapters: FIGURE TITLE 22 Wide Voltage Range Current Sensing 23 Smooth Current Monitor Output Signal by Simple Filtering 105 High Voltage, 5A High Side Current Sensing in Small Package 124 Monitor H-Bridge Motor Current Directly 128 Fixed Gain DC Motor Current Monitor 167 Monitor Current in an Isolated Supply Line 168 Monitoring a Fuse Protected Circuit 179 Digitizing Charging and Loading Current in a Battery Monitor 182 Power Sensing with Built In A to D Converter 183 Isolated Power Measurement 184 Fast Data Rate Isolated Power Measurement 185 Adding Temperature Measurement to Supply Power Measurement 186 Current, Voltage and Fuse Monitoring 187 Automotive Socket Power Monitoring 188 Power over Ethernet, PoE, Monitoring an105fa AN105-53

  50. Application Note 105 LOW VOLTAGE Single-Supply 2.5V Bidirectional Operation with 1.25V Electronic Circuit Breaker (Figure 95) External Voltage Reference and I/V Converter The LTC4213 provides protection and automatic circuit (Figure 94) breaker action by sensing drain-to-source voltage drop The LT1787’s output is buffered by an LT1495 rail-to-rail across the N-MOSFET . The sense inputs have a rail-to-rail op amp configured as an I/V converter. This configuration common mode range, so the circuit breaker can protect is ideal for monitoring very low voltage supplies. The bus voltages from 0V up to 6V . Logic signals flag a trip LT1787’s V OUT pin is held equal to the reference voltage condition (with the READY output signal) and reinitialize appearing at the op amp’s noninverting input. This al- the breaker (using the ON input). The ON input may also lows one to monitor supply voltages as low as 2.5V . The be used as a command in a “smart switch” application. op amp’s output may swing from ground to its positive supply voltage. The low impedance output of the op amp SI4864DY V IN V OUT may drive following circuitry more effectively than the 1.25V 1.25V 3.5A high output impedance of the LT1787. The I/V converter SENSEP GATE SENSEN configuration also works well with split supply voltages. V BIAS V CC V BIAS 2.3V TO 6V LTC4213 10k I SENSE R SENSE TO OFF ON ON READY GND I SEL CHARGER/ LOAD C1 2.5V + V SENSE(MAX) 1µF 1 8 FIL – FIL + 4213 TA01 LT1787 V S– V S+ 2 7 Figure 95. 1.25V Electronic Circuit Breaker 2.5V V BIAS 3 6 DNC C3 R OUT 1000pF 4 5 V EE V OUT – A1 V OUT A + 2.5V LT1495 1M 5% LT1389-1.25 1787 F07 Figure 94. Single-Supply 2.5V Bidirectional Operation with External Voltage Reference and I/V Converter an105fa AN105-54

  51. Application Note 105 HIGH CURRENT (100mA to Amps) Sensing high currents accurately requires excellent control than the max current spec allowed unless the max current of the sensing resistance, which is typically a very small is limited in another way, such as with a Schottky diode value to minimize losses, and the dynamic range of the across R SENSE . This will reduce the high current measure- measurement circuitry ment accuracy by limiting the result, while increasing the low current measurement resolution. This approach can Kelvin Input Connection Preserves Accuracy Despite be helpful in cases where an occasional large burst of Large Load Currents (Figure 96) current may be ignored. Kelvin connection of the –IN and +IN inputs to the sense resistor should be used in all but the lowest power ap- V + plications. Solder connections and PC board interconnec- tions that carry high current can cause significant error R SENSE D SENSE in measurement due to their relatively large resistances. By isolating the sense traces from the high current paths, 6101 F03a this error can be reduced by orders of magnitude. A sense LOAD resistor with integrated Kelvin sense terminals will give Figure 97. Shunt Diode Limits Maximum Input Voltage to Allow the best results. Better Low Input Resolution Without Over-Ranging the LTC6101 V + Kelvin Sensing (Figure 98) R IN In any high current, >1A, application, Kelvin contacts to R SENSE the sense resistor are important to maintain accuracy. 4 3 This simple illustration from a battery charger application + – LOAD shows two voltage-sensing traces added to the pads of the 2 5 current sense resistor. If the voltage is sensed with high impedance amplifier inputs, no IxR voltage drop errors are developed. 1 V OUT LTC6101 R OUT DIRECTION OF CHARGING CURRENT 6101 F02 Figure 96. Kelvin Input Connection Preserves Accuracy Despite R SENSE Large Load Currents 4008 F12 Shunt Diode Limits Maximum Input Voltage to Allow Better Low Input Resolution Without Over-Ranging CSP BAT the LTC6101 (Figure 97) Figure 98. Kelvin Sensing If low sense currents must be resolved accurately in a system that has very wide dynamic range, more gain can be taken in the sense amplifier by using a smaller value for resistor R IN . This can result in an operating current greater an105fa AN105-55

  52. Application Note 105 HIGH CURRENT (100mA to Amps) 0A to 33A High Side Current Monitor with Filtering Single Supply RMS Current Measurement (Figure 99) (Figure 100) High current sensing on a high voltage supply rail is eas- The LT1966 is a true RMS-to-DC converter that takes a ily accomplished with the LT6100. The sense amplifier is single-ended or differential input signal with rail-to-rail biased from a low 3V supply and pin strapped to a gain range. The output of a PCB mounted current sense trans- of 25V/V to output a 2.5V full-scale reading of the current former can be connected directly to the converter. Up to flow. A capacitor at the FIL pin to ground will filter out 75A of AC current is measurable without breaking the signal noise of the system (220pF produces a 12kHz lowpass path from a power source to a load. The accurate operating corner frequency). range of the circuit is determined by the selection of the transformer termination resistor. All of the math is built in to the LTC1966 to provide a DC output voltage that is proportional to the true RMS value of the current. This is valuable in determining the power/energy consumption of AC-powered appliances. 4.4V TO 48V 3V SUPPLY 2 7 6 V CC A4 A2 LT6100 V S+ 8 V OUT 5 R SENSE V OUT = 2.5V 3mΩ I SENSE = 33A V S– 1 V EE FIL LOAD 6100 TA01a 4 3 220pF CONFIGURED FOR GAIN = 25V/V Figure 99. 0A to 33A High Side Current Monitor with Filtering V + LTC1966 IN1 AC CURRENT V OUT V OUT = 4mV DC /A RMS 75A MAX T1 10Ω C AVE 50Hz TO 400Hz IN2 OUT RTN 1µF GND EN 100k V SS 1966 TA08 100k 0.1µF T1: CR MAGNETICS CR8348-2500-N www.crmagnetics.com Figure 100. Single Supply RMS Current Measurement an105fa AN105-56

  53. Application Note 105 HIGH CURRENT (100mA to Amps) Dual LTC6101’s Allow High-Low Current Ranging measurement is 10 times greater with low currents, less (Figure 101) than 1.2A, than with higher currents. A comparator detects higher current flow, up to 10A, and switches sensing over Using two current sense amplifiers with two values of to the high current circuitry. sense resistors is an easy method of sensing current over a wide range. In this circuit the sensitivity and resolution of V LOGIC CMPZ4697 (3.3V TO 5V) 7 10k 3 M1 + Si4465 V IN 4 – R SENSE HI I LOAD 8 Q1 10m 5 CMPT5551 V OUT 40.2k R SENSE LO 6 100m 301 301 301 301 4.7k 1.74M LTC1540 4 3 4 3 2 1 HIGH + + – – 2 5 2 5 RANGE V IN 619k INDICATOR (I LOAD > 1.2A) 1 1 HIGH CURRENT RANGE OUT LTC6101 LTC6101 250mV/A 7.5k V LOGIC BAT54C LOW CURRENT RANGE OUT 2.5V/A ( V LOGIC +5V ) ≤ V IN ≤ 60V R5 7.5k 0 ≤ I LOAD ≤ 10A 6101 F03b Figure 101. Dual LTC6101’s Allow High-Low Current Ranging an105fa AN105-57

  54. Application Note 105 HIGH CURRENT (100mA to Amps) LDO Load Balancing (Figure 102) and servo’ed to match the master regulator output volt- age. The precise low offset voltage of the LTC6078 dual op As system design enhancements are made there is often amp (10µV) balances the load current provided by each the need to supply more current to a load than originally regulator to within 1mA. This is achieved using a very expected. A simple way to modify power amplifiers or small 10mΩ current sense resistor in series with each voltage regulators, as shown here, is to parallel devices. output. This sense resistor can be implemented with PCB When paralleling devices it is desired that each device copper traces or thin gauge wire. shares the total load current equally. In this circuit two adjustable “slave” regulator output voltages are sensed V IN BALLAST RESISTANCE: IN OUT + 1.8V TO 20V IDENTICAL LENGTH LT1763 0.01µF 10µF THERMALLY MATED 10µF WIRE OR PCB TRACE SHDN BYP FB R1 R2 2k 2k ( ) R1 IN OUT V OUT = 1.22V 1 + R2 LT1763 0.01µF 10µF SHDN BYP 100Ω I LOAD LOAD FB 2k 2k 1k 0.1µF – A 10k + IN OUT LT1763 0.01µF 10µF SHDN BYP 100Ω FB 2k 2k 0 ≤ I LOAD ≤ 1.5A 1k 0.1µF 1.22V ≤ V OUT ≤ V DD V DD LDO LOADS MATCH TO WITHIN 1mA WITH 10mΩ OF BALLAST – RESISTANCE (2 INCHES OF AWG 28 GAUGE STRANDED WIRE) B A, B: LTC6078 10k + 60789 TA09 Figure 102. LDO Load Balancing an105fa AN105-58

  55. Application Note 105 HIGH CURRENT (100mA to Amps) Sensing Output Current (Figure 103) in a microprocessor controlled system. For closed loop control of the current to a load an LT1787 can monitor the The LT1970 is a 500mA power amplifier with voltage output current. The LT1880 op amp provides scaling and programmable output current limit. Separate DC voltage level shifting of the voltage applied to an A-to-D converter inputs and an output current sensing resistor control the for a 5mV/mA feedback signal. maximum sourcing and sinking current values. These control voltages could be provided by a D-to-A converter V CC 0V TO 1V 12V VC SRC VC SNK EN +IN V CC V + ISRC R S ISNK 0.2Ω TSD OUT LT1970 SENSE + SENSE – R LOAD FILTER V – –IN V EE COMMON R4 LT1787 255k V S– V S+ –12V BIAS R G R F –12V 12V R1 20k 60.4k – V EE V OUT R2 LT1880 2.5V 10k ±5mV/mA + R3 1kHz FULL CURRENT 20k –12V BANDWIDTH –12V 0V TO 5V A/D 1970 F10 OPTIONAL DIGITAL FEEDBACK Figure 103. Sensing Output Current an105fa AN105-59

  56. Application Note 105 HIGH CURRENT (100mA to Amps) Using Printed Circuit Sense Resistance (Figure 104) High Voltage, 5A High Side Current Sensing in Small Package (Figure 105) The outstanding LTC6102 precision allows the use of sense resistances fabricated with conventional printed The LT6106 is packaged in a small SOT-23 package but circuit techniques. For “one ounce” copperclad, the trace still operates over a wide supply range of 3V to 44V . Just resistance is approximately (L/W)·0.0005Ω and can carry two resistors set the gain (10 in circuit shown) and the about 4A per mm of trace width. The example below shows output is a voltage referred to ground. a practical 5A monitoring solution with both L and W set to 2.5mm. The resistance is subject to about +0.4%/ºC 3V TO 36V temperature change and the geometric tolerances of the fabrication process, so this will not generally be for high 100Ω accuracy work, but can be useful in various low cost 0.02Ω protection and status monitoring functions. +IN –IN – + LOAD CURRENT CARRYING TRACE V – V + L FROM TO LOAD R SENSE * W SUPPLY 10A MAX OUT R IN– R IN+ V OUT LT6106 200mV/A 1k 6106 TA01a C REG V – OUTPUT Figure 105. High Voltage, 5A High Side Current Sensing LTC6102 R OUT in Small Package * 2.5mm × 2.5mm V – 1oz COPPER 500µΩ DN423 F02 Figure 104. Using Printed Circuit Sense Resistance More High Current Circuits Are Shown in Other Chapters: FIGURE TITLE 59 Differential Output Bidirectional 10A Current Sense 93 High Voltage Current and Temperature Monitoring 121 Single Output Provides 10A H-Bridge Current and Direction 179 Digitizing Charging and Loading Current in a Battery Monitor 209 Use Kelvin Connections to Maintain High Current Accuracy 215 0 to 10A Sensing Over Two Ranges an105fa AN105-60

  57. LOAD LT6100 LOAD FIL FIL LT6100 Application Note 105 LOW CURRENT (Picoamps to Milliamps) For low current applications the easiest way to sense cur- I SENSE R SENSE V SUPPLY rent is to use a large sense resistor. This however causes 6.4V TO 48V larger voltage drops in the line being sensed which may V S+ V S– not be acceptable. Using a smaller sense resistor and taking gain in the sense amplifier stage is often a better – + approach. Low current implies high source impedance 5V V CC measurements which are subject approach. Low current implies high source impedance measurements which are subject to noise pickup and often require filtering of V OUT 50 • R SENSE • I SENSE some sort. V EE A2 A4 6100 TA04 Filtered Gain of 20 Current Sense (Figure 106) The LT6100 has pin strap connections to establish a variety Figure 107. Gain of 50 Current Sense of accurate gain settings without using external compo- nents. For this circuit grounding A2 and leaving A4 open 0nA to 200nA Current Meter (Figure 108) set a gain of 20. Adding one external capacitor to the FIL A floating amplifier circuit converts a full-scale 200nA pin creates a lowpass filter in the signal path. A capacitor of flowing in the direction indicated at the inputs to 2V at 1000pF as shown sets a filter corner frequency of 2.6KHz. the output of the LT1495. This voltage is converted to a current to drive a 200µA meter movement. By floating the I SENSE R SENSE V SUPPLY power to the circuit with batteries, any voltage potential 4.4V TO 48V at the inputs are handled. The LT1495 is a micropower op V S+ V S– amp so the quiescent current drain from the batteries is very low and thus no on/off switch is required. – + 3V V CC 100pF V OUT 1000pF R1 20 • R SENSE • I SENSE 10M V EE A2 A4 6100 TA03 R4 – –3dB AT 2.6kHz 10k 1/2 – 1.5V LT1495 INPUT 1/2 Figure 106. Filtered Gain of 20 Current Sense + CURRENT LT1495 R2 + 1.5V 9k Gain of 50 Current Sense (Figure 107) R3 I S = 3µA WHEN I IN = 0 2k NO ON/OFF SWITCH FULL-SCALE The LT6100 is configured for a gain of 50 by grounding REQUIRED ADJUST both A2 and A4. This is one of the simplest current sensing 0µA TO µA 200µA amplifier circuits where only a sense resistor is required. 1495 TA06 Figure 108. 0nA to 200nA Current Meter an105fa AN105-61

  58. Application Note 105 LOW CURRENT (Picoamps to Milliamps) Lock-In Amplifier Technique Permits 1% Accurate shunt, modulating it into a differential square wave signal APD Current Measurement Over 100nA to 1mA Range which feeds A1 through 0.2µF AC coupling capacitors. (Figure 109) A1’s single-ended output biases demodulator S2, which presents a DC output to buffer amplifier A2. A2’s output Avalanche Photodiodes, APDs, require a small amount of is the circuit output. current from a high voltage supply. The current into the diode is an indication of optical signal strength and must Switch S3 clocks a negative output charge pump which be monitored very accurately. It is desirable to power all supplies the amplifier’s V– pins, permitting output swing to of the support circuitry from a single 5V supply. (and below) zero volts. The 100k resistors at Q1 minimize its on-resistance error contribution and prevent destruc- This circuit utilizes AC carrier modulation techniques to tive potentials from reaching A1 (and the 5V rail) if either meet APD current monitor requirements. It features 0.4% 0.2µF capacitor fails. A2’s gain of 1.1 corrects for the slight accuracy over the sensed current range, runs from a 5V attenuation introduced by A1’s input resistors. In practice, supply and has the high noise rejection characteristics of it may be desirable to derive the APD bias voltage regula- carrier based “lock in” measurements. tor’s feedback signal from the indicated point, eliminating The LTC1043 switch array is clocked by its internal the 1kΩ shunt resistor’s voltage drop. Verifying accuracy oscillator. Oscillator frequency, set by the capacitor at involves loading the APD bias line with 100nA to 1mA and Pin 16, is about 150Hz. S1 clocking biases Q1 via level noting output agreement. shifter Q2. Q1 chops the DC voltage across the 1k current FOR OPTIONAL “ZERO CURRENT” FEEDBACK TO APD BIAS REGULATOR, SEE APPENDIX A 1k* APD 1% V OUT = 20V TO 90V HIGH VOLTAGE TO APD 1µF 1µF BIAS INPUT 100k* 100k* 100V 100V Q1 1N4690 1M* 5V 5.6V 5V 0.2µF 6 – 1µF + 20k 2 A1 OUTPUT S2 A2 10k LT1789 0V TO 1V = 30k 1µF LT1006 + 0mA TO 1mA 5 – Q2 0.2µF 20k* –3.5V MPSA42 1M* –3.5V 20k 200k* 12 13 14 S1 –3.5V TO AMPLIFIERS 5V 18 22µF 5V 3 S3 + * = 0.1% METAL FILM RESISTOR 22µF 15 1µF 100V = TECATE CMC100105MX1825 + # CIRCLED NUMBERS = LTC1043 PIN NUMBER = 1N4148 16 17 4 = TP0610L 0.056µF 5V AN92 F04 Figure 109. Lock-In Amplifier Technique Permits 1% Accurate APD Current Measurement Over 100nA to 1mA Range an105fa AN105-62

  59. Application Note 105 LOW CURRENT (Picoamps to Milliamps) DC-Coupled APD Current Monitor (Figure 110) drop across the ground referred 1kΩ resistor identical to the drop across the 1kΩ current shunt and, hence, APD Avalanche Photodiodes, APDs, require a small amount of current. This relationship holds across the 20V to 90V APD current from a high voltage supply. The current into the bias voltage range. The 5.6V zener assures A1’s inputs diode is an indication of optical signal strength and must are always within their common mode operating range be monitored very accurately. It is desirable to power all and the 10MΩ resistor maintains adequate Zener current of the support circuitry from a single 5V supply. when APD current is at very low levels. This circuit’s DC-coupled current monitor eliminates the Two output options are shown. A2, a chopper stabilized previous circuit’s trim but pulls more current from the amplifier, provides an analog output. Its output is able to APD bias supply. A1 floats, powered by the APD bias rail. swing to (and below) zero because its V – pin is supplied The 15V Zener diode and current source Q2 ensure A1 with a negative voltage. This potential is generated by us- never is exposed to destructive voltages. The 1kΩ current ing A2’s internal clock to activate a charge pump which, shunt’s voltage drop sets A1’s positive input potential. A1 in turn, biases A2’s V – Pin 3. A second output option balances its inputs by feedback controlling its negative input substitutes an A-to-D converter, providing a serial format via Q1. As such, Q1’s source voltage equals A1’s positive digital output. No V – supply is required, as the LTC2400 input voltage and its drain current sets the voltage across A-to-D will convert inputs to (and slightly below) zero volts. its source resistor. Q1’s drain current produces a voltage FOR OPTIONAL “ZERO CURRENT” FEEDBACK TO 1N4690 APD BIAS REGULATOR, SEE APPENDIX A 1k* 5.6V APD CURRENT SHUNT V OUT = 20V TO 90V HIGH VOLTAGE TO APD BIAS INPUT 10M 1k* 51K + + 1N4702 1µF A1 15V LT1077 51k – Q1 100k Q2 ZVP0545A 5V MPSA42 10k LT1460 5V 1k* 2.5V Hi-Z OUTPUT 0V TO 1V = 0mA TO 1mA 1k* V IN V REF F O LTC2400 BUFFERED OUTPUT SCK 5V 5V A-TO-D 0mA TO 1mA = 0V TO 1V DIGITAL SDO INTERFACE * = 0.1% METAL FILM RESISTOR + CS 1k 10µF A2 = BAT85 LTC1150 OPTIONAL + – 39k DIGITAL OUTPUT CLK OUT Q2 10µF V – 2N3904 + 100k ≈ –3.5V HERE OPTIONAL BUFFERED OUTPUT AN92 F05 Figure 110. DC-Coupled APD Current Monitor an105fa AN105-63

  60. Application Note 105 LOW CURRENT (Picoamps to Milliamps) Six Decade (10nA to 10mA) Current Log Amplifier current sensing. In this circuit a six decade range of current (Figure 111) pulled from the circuit input terminal is converted to an output voltage in logarithmic fashion increasing 150mV Using precision quad amplifiers like the LTC6079, (10µV for every decade of current change. offset and <1pA bias current) allow for very wide range – C + 100Ω – B 100Ω + 33µF Q1 Q2 100k 133k V DD – 1000pF – A 1.58k + D PRECISION + RESISTOR PT146 I IN V OUT 1k LT6650 V CC +3500ppm/°C IN OUT 60789 TA07 GND 10nA ≤ I IN ≤ 10mA 1µF 1µF Q1, Q2: DIODES INC. DMMT3906W A TO D: LTC6079 V OUT ≈ 150mV • log (I IN ) + 1.23V, I IN IN AMPS Figure 111. Six Decade (10nA to 10mA) Current Log Amplifier an105fa AN105-64

  61. LTC1153 • TA01 Application Note 105 MOTORS AND INDUCTIVE LOADS A common monitoring approach in these systems is to The largest challenge in measuring current through induc- amplify the voltage on a “flying” sense resistor, as shown. tive circuits is the transients of voltage that often occur. Unfortunately, several potentially hazardous fault scenarios Current flow can remain continuous in one direction while go undetected, such as a simple short to ground at a motor the voltage across the sense terminals reverses in polarity. terminal. Another complication is the noise introduced by Electronic Circuit Breaker (Figure 112) the PWM activity. While the PWM noise may be filtered for purposes of the servo law, information useful for protection The LTC1153 is an electronic circuit breaker. Sensed cur- becomes obscured. The best solution is to simply provide rent to a load opens the breaker when 100mV is developed two circuits that individually protect each half-bridge and between the supply input, V S , and the drain sense pin, DS. report the bidirectional load current. In some cases, a To avoid transient, or nuisance trips of the break compo- smart MOSFET bridge driver may already include sense nents RD and CD delay the action for 1ms. A thermistor resistors and offer the protection features needed. In these can also be used to bias the shutdown input to monitor situations, the best solution is the one that derives the load heat generated in the load and remove power should the information with the least additional circuitry. temperature exceed 70°C in this example. A feature of the LTC1153 is timed automatic reset which will try to reconnect the load after 200ms using the 0.22μF timer BATTERY BUS + capacitor shown. ON/OFF IN V S C D *R SEN R D C T 0.01µF 0.1Ω 100k 0.22µF C T DS + R S Z5U LTC1153 DIFF AMP TO µP STATUS G IRLR024 – I M 51k 51k GND SHUTDOWN 5V SENSITIVE **70°C 5V LOAD PTC DN374 F03 ALL COMPONENTS SHOWN ARE SURFACE MOUNT. * IMS026 INTERNATIONAL MANUFACTURING SERVICE, INC. (401) 683-9700 Figure 113. Conventional H-Bridge Current Monitor ** RL2006-100-70-30-PT1 KEYSTONE CARBON COMPANY (814) 781-1591 Motor Speed Control (Figure 114) Figure 112. Electronic Circuit Breaker This uses an LT1970 power amplifier as a linear driver of a DC motor with speed control. The ability to source Conventional H-Bridge Current Monitor (Figure 113) and sink the same amount of output current provides for bidirectional rotation of the motor. Speed control is Many of the newer electric drive functions, such as steer- managed by sensing the output of a tachometer built on ing assist, are bidirectional in nature. These functions are to the motor. A typical feedback signal of 3V/1000rpm is generally driven by H-bridge MOSFET arrays using pulse- compared with the desired speed-set input voltage. Be- width modulation (PWM) methods to vary the commanded cause the LT1970 is unity-gain stable, it can be configured torque. In these systems, there are two main purposes for as an integrator to force whatever voltage across the mo- current monitoring. One is to monitor the current in the tor as necessary to match the feedback speed signal with load, to track its performance against the desired com- the set input signal. Additionally, the current limit of the mand (i.e., closed-loop servo law), and another is for fault amplifier can be adjusted to control the torque and stall detection and protection features. current of the motor. an105fa AN105-65

  62. Application Note 105 MOTORS AND INDUCTIVE LOADS OV TO 5V TORQUE/STALL CURRENT CONTROL 15V VC SRC VC SNK EN +IN V CC V + ISRC R S ISNK 1Ω TSD LT1970 OUT SENSE + SENSE – 12V DC FILTER MOTOR V – –IN V EE COMMON GND 15V C1 R1 –15V 1µF TACH 1.2k FEEDBACK REVERSE 3V/1000rpm R4 R5 49.9k 49.9k R2 1970 F13 10k FORWARD R3 1.2k –15V Figure 114. Motor Speed Control Practical H-Bridge Current Monitor Offers Fault – BATTERY BUS DIFF Detection and Bidirectional Load Information OUTPUT (Figure 115) TO ADC + R IN R IN LTC6101 LTC6101 This circuit implements a differential load measurement R OUT R OUT R S R S for an ADC using twin unidirectional sense measurements. + Each LTC6101 performs high side sensing that rapidly responds to fault conditions, including load shorts and FOR I M RANGE = ±100A, DIFF OUT = ±2.5V MOSFET failures. Hardware local to the switch module R S = 1mΩ R IN = 200Ω (not shown in the diagram) can provide the protection R OUT = 4.99k logic and furnish a status flag to the control system. I M The two LTC6101 outputs taken differentially produce a bidirectional load measurement for the control servo. The ground-referenced signals are compatible with most Δ Σ ADCs. The Δ Σ ADC circuit also provides a “free” in- DN374 F04 tegration function that removes PWM content from the Figure 115. Practical H-Bridge Current Monitor Offers Fault measurement. This scheme also eliminates the need for Detection and Bidirectional Load Information analog-to-digital conversions at the rate needed to sup- port switch protection, thus reducing cost and complexity. an105fa AN105-66

  63. LTC1153 • TA07 Application Note 105 MOTORS AND INDUCTIVE LOADS Lamp Driver (Figure 116) Intelligent High Side Switch (Figure 117) The inrush current created by a lamp during turn-on can The LT1910 is a dedicated high side MOSFET driver with be 10 to 20 times greater than the rated operating cur- built in protection features. It provides the gate drive for a rent. This circuit shifts the trip threshold of an LTC1153 power switch from standard logic voltage levels. It provides electronic circuit breaker up by a factor of 11:1 (to 30A) for shorted load protection by monitoring the current flow 100ms while the bulb is turned on. The trip threshold then to through the switch. Adding an LTC6101 to the same drops down to 2.7A after the inrush current has subsided. circuit, sharing the same current sense resistor, provides a linear voltage signal proportional to the load current for additional intelligent control. 12V + 470µF 10k 0.02Ω IN V S 100k 5V C T DS 0.33µF VN2222LL LTC1153 STATUS G 0.1µF 1M GND SD IRFZ34 12V 12V/2A BULB Figure 116. Lamp Driver 10µF V LOGIC 63V 14V 47k 5 100Ω 3 1% FAULT 8 3 1 4 R S OFF ON LT1910 LTC6101 V O 6 4 2 100Ω 4.99k 1µF 2 1 5 SUB85N06-5 V O = 49.9 • R S • I L L O I L FOR R S = 5mΩ, A V O = 2.5V AT I L = 10A (FULL-SCALE) D 6101 TA07 Figure 117. Intelligent High Side Switch an105fa AN105-67

  64. 1N4148 LTC1153 • TA08 Application Note 105 MOTORS AND INDUCTIVE LOADS Full-Bridge Load Current Monitor (Figure 119) Relay Driver (Figure 118) The LT1990 is a difference amplifier that features a very This circuit provides reliable control of a relay by using an wide common mode input voltage range that can far electronic circuit breaker circuit with two-level over-current exceed its own supply voltage. This is an advantage to protection. Current flow is sensed through two separate reject transient voltages when used to monitor the current resistors, one for the current into the relay coil and the in a full-bridge driven inductive load such as a motor. The other for the current through the relay contacts. When LT6650 provides a voltage reference of 1.5V to bias up the 100mV is developed between the V S supply pin and the output away from ground. The output will move above or drain sense pin, DS, the N-channel MOSFET is turned off below 1.5V as a function of which direction the current opening the contacts. As shown, the relay coil current is in the load is flowing. As shown, the amplifier provides limited to 350mA and the contact current to 5A. a gain of 10 to the voltage developed across resistor R S . 12V + 100µF 2Ω 0.02Ω 10k IN V S 0.01µF 5V C T DS 1µF MTD3055E LTC1153 STATUS G TO 12V LOAD 15V GND SD 1N4001 COIL CURRENT LIMITED TO 350mA CONTACT CURRENT LIMITED TO 5A Figure 118. Relay Driver +V SOURCE 5V LT1990 900k 10k 8 7 100k 1M 2 – R S 6 V OUT + – 1M 3 + I L V REF = 1.5V 4 10k 5 IN OUT 1nF 54.9k LT6650 40k 900k GND FB 100k 40k 20k –12V ≤ V CM ≤ 73V 1 V OUT = V REF ± (10 • I L • R S ) 1990 TA01 1µF Figure 119. Full-Bridge Load Current Monitor an105fa AN105-68

  65. Application Note 105 MOTORS AND INDUCTIVE LOADS Bidirectional Current Sensing in H-Bridge Drivers two outputs form a bidirectional measurement for subse- (Figure 120) quent circuitry, such as an ADC. In this configuration, any load fault to ground will also be detected so that bridge Each channel of an LTC6103 provides measurement of the protection can be implemented. This arrangement avoids supply current into a half-bridge driver section. Since only the high frequency common mode rejection problem that one of the half-bridge sections will be conducting current can cause problems in “flying” sense resistor circuits. in the measurable direction at any given time, only one output at a time will have a signal. Taken differentially, the V + 4V TO 60V 10mΩ 10mΩ 200Ω 200Ω 8 7 6 5 +INA –INA –INB +INB + + – – V SA V SB V – OUTA OUTB LTC6103 1 4 2 + DIFFERENTIAL 4.99k OUTPUT ±2.5V FS (MAY BE LIMITED IF V + < 6V) ±10A FS – 4.99k – + PWM* PWM* 6103 TA04 *USE “SIGN-MAGNITUDE” PWM FOR ACCURATE LOAD CURRENT CONTROL AND MEASUREMENT Figure 120. Bidirectional Current Sensing in H-Bridge Drivers an105fa AN105-69

  66. Application Note 105 MOTORS AND INDUCTIVE LOADS Single Output Provides 10A H-Bridge Current and 24V DC Direction (Figure 121) 24V, 3W The output voltage of the LTC6104 will be above or below 1N5818 SOLENOID 1Ω the external 2.5V reference potential depending on which 1% – + side of the H-bridge is conducting current. Monitoring the 200Ω 200Ω current in the bridge supply lines eliminates fast voltage 1% 1% changes at the inputs to the sense amplifiers. LT6105 –IN +IN 5V/ON 2N7000 0V/OFF 3V TO 18V 0.1µF V + 5V DC V BATTERY 4 1µF (8V TO 60V) 6 V – LT1790-2.5 V OUT = 25mV/mA V OUT 249Ω 249Ω 1 2 7 6 4.99k 10m 10m 4.99k 6105 F04 8 5 1% LTC6104 V O 2 V O = 2.5V ±2V (±10A FS) 4 Figure 122. Monitor Solenoid Current on the Low Side 24V DC PWM* PWM* I M 24V/OFF 1Ω 19V/ON TP0610L 1% 6104 TA02 – + 200Ω 200Ω *USE “SIGN-MAGNITUDE” PWM FOR ACCURATE 1% 1% 1N914 LOAD CURRENT CONTROL AND MEASUREMENT 24V, 3W 2k 2k 1N5818 SOLENOID 1% 1% Figure 121. Single Output Provides 10A H-Bridge Current and Direction LT6105 –IN +IN Monitor Solenoid Current on the Low Side (Figure 122) V + 5V DC Driving an inductive load such as a solenoid creates large transients of common mode voltage at the inputs to a current sense amplifier. When de-energized the voltage V – V OUT = 25mV/mA V OUT across the solenoid reverses (also called the freewheel state) and tries to go above its power supply voltage but 4.99k 6105 F06 1% is clamped by the freewheel diode. The LT6105 senses the solenoid current continuously over an input voltage range of 0V to one diode drop above the 24V supply. Figure 123. Monitor Solenoid Current on the High Side Monitor Solenoid Current on the High Side across the solenoid reverses (also called the freewheel (Figure 123) state) and tries to go below ground but is clamped by the Driving an inductive load such as a solenoid creates large freewheel diode. The LT6105 senses the solenoid current transients of common mode voltage at the inputs to a continuously with pull-up resistors keeping the inputs current sense amplifier. When de-energized the voltage within the most accurate input voltage range. an105fa AN105-70

  67. Application Note 105 MOTORS AND INDUCTIVE LOADS Monitor H-Bridge Motor Current Directly Large Input Voltage Range for Fused Solenoid Current (Figures 124a and 124b) Monitoring (Figure 125) The LT1999 is a differential input amplifier with a very wide, The LT1999 has series resistors at each input. This allows –5V to 80V , input common mode voltage range. With an the input to be overdriven in voltage without damaging the AC CMRR greater than 80dB at 100kHz allows the direct amplifier. The amplifier will monitor the current through the measurement of the bidirectional current in an H-bridge positive and negative voltage swings of a solenoid driver. driven load. The large and fast common mode input volt- The large differential input with a blown protective fuse age swings are rejected at the output. The amplifier gain is will force the output high and not damage the LT1999. fixed at 10, 20 or 50 requiring only a current sense resistor and supply bypass capacitors external to the amplifier. V + LT1999 V S 5V V + 2µA 1 8 SHDN V SHDN + R G – V OUT 4k V +IN 2 – 7 R S 2.5V V + V OUT 0.8k + V –IN V +IN (20V/DIV) V + V OUT (2V/DIV) 0.8k 160k 4k 3 V REF 6 V +IN V + 5V 160k 0.1µF 4 5 0.1µF 1999 TA01a 1999 TA01b TIME (10µs/DIV) Figure 124a Figure 124b Figure 124. Monitor H-Bridge Motor Current Directly V + V S LT1999 V + 5V 2µA 1 8 SHDN ON OFF V SHDN V SHDN + R G I LOAD – 4k V +IN 2 – 7 V OUT V + V OUT 0.8k + FUSE V + R SENSE 0.8k 160k V REF 4k V –IN 3 V REF 6 V + 160k 5V 0.1µF STEERING LOAD 4 5 DIODE 0.1µF 1999 F05 Figure 125. Large Input Voltage Range for Fused Solenoid Current Monitoring an105fa AN105-71

  68. Application Note 105 MOTORS AND INDUCTIVE LOADS Monitor Both the ON Current and the Freewheeling Monitor Both the ON Current and the Freewheeling Current Through a High Side Driven Solenoid Current In a Low Side Driven Solenoid (Figure 127) (Figure 126) Placing the current sense resistor inside the loop created Placing the current sense resistor inside the loop created by a grounded solenoid and the freewheeling clamp diode by a grounded solenoid and the freewheeling clamp diode allows for continuous monitoring of the solenoid current allows for continuous monitoring of the solenoid current while being energized or switched OFF . The LT1999 oper- while being energized or switched OFF . The LT1999 oper- ates accurately with an input common mode voltage up ates accurately with an input common mode voltage down to 80V . In this circuit the input is clamped at one diode to –5V below ground. above the solenoid supply voltage. V + V S LT1999 V + OFF 5V 2µA ON 1 8 SHDN V SHDN + R G – 4k V +IN 2 – 7 V + V OUT 0.8k + V + R SENSE 0.8k 160k 4k V –IN 3 V REF 6 5V V + 0.1µF 160k 4 SOLENOID 5 0.1µF 1999 F07a Figure 126. Monitor Both the ON Current and the Freewheeling Current Through a High Side Driven Solenoid V + V S LT1999 5V V + 2µA 1 8 SHDN V SHDN SOLENOID + R G – 4k V +IN 2 – 7 V + V OUT 0.8k + V + R SENSE 0.8k 160k 4k V –IN 3 V REF 6 ON 5V V + 160k 0.1µF OFF 4 5 0.1µF 1999 F08a Figure 127. Monitor Both the ON Current and the Freewheeling Current In a Low Side Driven Solenoid an105fa AN105-72

  69. Application Note 105 MOTORS AND INDUCTIVE LOADS Fixed Gain DC Motor Current Monitor (Figure 128) referenced to one-half supply so the direction of motor rotation is indicated by the output being above or below With no critical external components the LT1999 can be the DC output voltage when stopped. connected directly across a sense resistor in series with an H-bridge driven motor. The amplifier output voltage is 5V V + V + LT1999-20 10µF 1 2µA 8 SHDN V SHDN + 80k 0.1µF 24V – 4k 2 – 7 V +IN V + V OUT C4 0.8k + 1000µF V + V –IN 0.8k 160k 4k 3 V BRIDGE 6 H-BRIDGE V REF 160k 0.1µF 5V V + 5V 5 4 PWM INPUT R SENSE 1999 F09 0.025Ω OUTA PWM IN DIRECTION 24V MOTOR OUTB BRAKE INPUT GND Figure 128. Fixed Gain DC Motor Current Monitor an105fa AN105-73

  70. Application Note 105 MOTORS AND INDUCTIVE LOADS Simple DC Motor Torque Control (Figure 129) match the set point current value through an amplifier and a PWM motor drive circuit. The LTC6992-1 produces The torque of a spinning motor is directly proportional to a PWM signal from 0% to 100% duty cycle for a 0V to 1V the current through it. In this circuit the motor current is change at the MOD input pin. monitored and compared to a DC set point voltage. The motor current is sensed by an LT6108-1 and forced to V MOTOR 100µF 1k 0.1Ω 8 1 SENSEHI SENSELO CURRENT SET POINT (0V TO 5V) 6 7 BRUSHED V + OUTA V OUT DC MOTOR 1µF 0.47µF (0A TO 5A) 1N5818 100k LT6108-1 9k 5V MABUCHI 2 5 RS-540SH RESET EN/ RST INC 5 2 1k 4 V + – 1 6 3 6 MOD OUT IRF640 OUTC 3 + V – 7 LTC6246 LTC6992-1 4 100k 78.7k 3 4 SET DIV 5V GND 280k 1M 2 610812 TA04 Figure 129. Simple DC Motor Torque Control an105fa AN105-74

  71. Application Note 105 MOTORS AND INDUCTIVE LOADS Small Motor Protection and Control (Figure 130) Large Motor Protection and Control (Figure 131) DC motor operating current and temperature can be digi- For high voltage/current motors, simple resistor divid- tized and sent to a controller which can then adjust the ers can scale the signals applied to an LTC2990 14-bit applied control voltage. Stalled rotor or excessive loading converter. Proportional DC motor operating current and on the motor can be sensed. temperature can be digitized and sent to a controller which can then adjust the applied control voltage. Stalled rotor or excessive loading on the motor can be sensed. LOAD PWR = I • V 0.1Ω MOTOR CONTROL VOLTAGE 1% 0V DC TO 5V DC 0A TO ±2.2A 5V 0.1µF V CC V1 V2 MMBT3904 2-WIRE SDA V3 I 2 C LTC2990 SCL INTERFACE 470pF ADR0 MOTOR ADR1 V4 GND T MOTOR 2990 TA04 T INTERNAL CURRENT AND TEMPERATURE CONFIGURATION: VOLTAGE AND TEMPERATURE CONFIGURATION: CONTROL REGISTER: 0x59 CONTROL REGISTER: 0x58 T AMB REG 4, 5 0.0625°C/LSB T AMB REG 4, 5 0.0625°C/LSB I MOTOR REG 6, 7 194µA/LSB V MOTOR REG 8, 9 305.18µVLSB T MOTOR REG A, B 0.0625°C/LSB T MOTOR REG A, B 0.0625°C/LSB V CC REG E, F 2.5V + 305.18µV/LSB V CC REG E, F 2.5V + 305.18µV/LSB Figure 130. Small Motor Protection and Control LOAD PWR = I • V 0.01Ω MOTOR CONTROL VOLTAGE 1W, 1% 0V TO 40V 0A TO 10A 71.5k 71.5k 1% 1% 10.2k 10.2k 1% 1% 5V 0.1µF V CC V1 V2 MMBT3904 2-WIRE SDA V3 I 2 C LTC2990 SCL INTERFACE 470pF ADR0 MOTOR ADR1 V4 GND T MOTOR 2990 TA05 T INTERNAL VOLTAGE AND TEMPERATURE CONFIGURATION: CURRENT AND TEMPERATURE CONFIGURATION: CONTROL REGISTER: 0x58 CONTROL REGISTER: 0x59 T AMB REG 4, 5 0.0625°C/LSB T AMB REG 4, 5 0.0625°C/LSB V MOTOR REG 8, 9 2.44mVLSB I MOTOR REG 6, 7 15.54mA/LSB T MOTOR REG A, B 0.0625°C/LSB T MOTOR REG A, B 0.0625°C/LSB V CC REG E, F 2.5V + 305.18µV/LSB V CC REG E, F 2.5V + 305.18µV/LSB Figure 131. Large Motor Protection and Control an105fa AN105-75

  72. Application Note 105 BATTERIES The science of battery chemistries and the charging and R SENSE TO CHARGER/ 3.3V discharging characteristics is a book of its own. This chap- C1 LOAD TO 1µF ter is intended to provide a few examples of monitoring 60V 1 8 FIL – FIL + 3.3V current flow into and out of batteries of any chemistry. LT1787HV V S– V S+ 2 7 20k 5% Input Remains Hi-Z when LT6100 is Powered Down V BIAS 3 6 DNC (Figure 132) C2 R OUT LT1634-1.25 4 5 1µF V EE This is the typical configuration for an LT6100, monitoring V OUT C3* the load current of a battery. The circuit is powered from 1000pF a low voltage supply rail rather than the battery being *OPTIONAL OUTPUT 1787 F04 monitored. A unique benefit of this configuration is that Figure 133. Charge/Discharge Current Monitor on Single Supply when the LT6100 is powered down, its battery sense inputs with Shifted V BIAS remain high impedance, drawing less than 1µA of current. This is due to an implementation of Linear Technology’s Battery Current Monitor (Figure 134) Over-The-Top input technique at its front end. One LT1495 dual op amp package can be used to establish separate charge and discharge current monitoring outputs. I SENSE R SENSE TO LOAD The LT1495 features Over-the-Top operation allowing + BATTERY V S– V S+ the battery potential to be as high as 36V with only a 5V LT6100 4.1V TO 48V amplifier supply voltage. POWER DOWN OK – + V CC I L 3V R SENSE V CC CHARGE 0V 0.1Ω INPUTS REMAIN FIL Hi-Z 12V DISCHARGE 5V V OUT R A R A V EE A2 A4 – – 6100 F08 A2 A1 1/2 LT1495 1/2 LT1495 R A R A + + Figure 132. Input Remains Hi-Z when LT6100 is Powered Down 2N3904 2N3904 ( ) Charge/Discharge Current Monitor on Single Supply R B DISCHARGE CHARGE V O = I L R SENSE with Shifted V BIAS (Figure 133) R A OUT OUT FOR R A = 1k, R B = 10k R B R B Here the LT1787 is used in a single-supply mode with the V O = 1V/A I L V BIAS pin shifted positive using an external LT1634 voltage 1495 TA05 reference. The V OUT output signal can swing above and Figure 134. Battery Current Monitor below V BIAS to allow monitoring of positive or negative currents through the sense resistor. The choice of refer- ence voltage is not critical except for the precaution that adequate headroom must be provided for V OUT to swing without saturating the internal circuitry. The component values shown allow operation with V S supplies as low as 3.1V . an105fa AN105-76

  73. 1620/21 • F04 Application Note 105 BATTERIES Input Current Sensing Application (Figure 135) Coulomb Counter (Figure 136) The LT1620 is coupled with an LT1513 SEPIC battery char- The LTC4150 is a micropower high side sense circuit that ger IC to create an input over current protected charger includes a V/F function. Voltage across the sense resistor circuit. The programming voltage (V CC – V PROG ) is set to is cyclically integrated and reset to provide digital transi- 1.0V through a resistor divider (R P1 and R P2 ) from the tions that represent charge flow to or from the battery. A 5V input supply to ground. In this configuration, if the polarity bit indicates the direction of the current. Supply input current drawn by the battery charger combined potential for the LTC4150 is 2.7V to 8.5V . In the free-running with the system load requirements exceeds a current mode (as shown, with CLR and INT connected together) limit threshold of 3A, the battery charger current will be the pulses are approximately 1μs wide and around 1Hz reduced by the LT1620 such that the total input supply full-scale. current is limited to 3A. CHARGER R SENSE 5V LOAD + + C1 22µF 1µF R P1 C2 4.7µF 3k 1 8 1µF SENSE AVG 1% R L R L 2 7 I OUT PROG SENSE – SENSE + V DD R P2 LT1620MS8 C F+ 3 6 INT 12k V CC GND 1% 4.7µF CLR LTC4150 µP CHG C F– DISCHG 4 5 IN + IN – POL R1 SHDN GND 0.033Ω TO + 4150 TA01a SYSTEM LOAD 22µF L1B Figure 136. Coulomb Counter 10µH MBRS340 V BATT = 12.3V 7 V IN 5 V SW Li-Ion Gas Gauge (Figure 137) 4.7µF L1A 57k LT1513 + 10µH This is the same as the Coulomb Counter circuit, except that 22µF 6 S/S 2 Li-ION × 2 V FB RUN the microprocessor clears the integration cycle complete 24Ω 4 GND 3 I FB 6.4k condition with software, so that a relatively slow polling GND routine may be used. TAB V C 0.22µF 8 1 R SENSE 0.1Ω 0.1µF NiMH Charger (Figure 138) X7R The LTC4008 is a complete NiMH battery pack controller. It provides automatic switchover to battery power when Figure 135. Input Current Sensing Application the external DC power source is removed. When power is connected the battery pack is always kept charged and ready for duty. an105fa AN105-77

  74. Application Note 105 BATTERIES POWER-DOWN SWITCH LOAD C L 2.5V 47µF R L R L 3k 3k 1 10 SENSE + INT 9 R SENSE LTC4150 CLR 0.1Ω 8 2 V DD SENSE – C2 2-CELL 3 + C F+ 7 4.7µF Li-Ion GND µP C F 6V ~ 8.4V 4.7µF 4 C F– 5 6 SHDN POL SHUTDOWN Figure 137. Li-Ion Gas Gauge Q3 INPUT SWITCH DCIN 0V TO 20V C1 R8 0.1µF 147k 0.25% BATMON DCIN V LOGIC R CL C4 R11 R12 0.02Ω V FB INFET 0.1µF 100k 100k 1% SYSTEM R1 5.1k 1% I CL I CL LTC4008 CLP LOAD C2 ACP ACP/SHDN CLN R SENSE 20µF L1 0.025Ω FAULT FAULT TGATE Q1 10µH 1% FLAG FLAG BGATE NiMH R10 32.4k 1% BATTERY Q2 D1 NTC PGND C3 PACK 20µF R T CSP R4 3.01k 1% I TH BAT R5 3.01k 1% R9 R7 GND PROG C7 CHARGING 13.3k 6.04k 0.47µF CURRENT 0.25% 1% C5 MONITOR 0.0047µF THERMISTOR D1: MBRS130T3 RT C6 10k R6 Q1: Si4431ADY 150k 0.12µF NTC 26.7k Q2: FDC645N 1% 4008 TA02 Figure 138. NiMH Charger an105fa AN105-78

  75. Application Note 105 BATTERIES Single Cell Li-Ion Charger (Figure 139) V IN 5V TO 22V Controlling the current flow in lithium-ion battery chargers is essential for safety and extending useful battery life. Intelligent battery charger ICs can be used in fairly simple 0.1µF 10µF V CC circuits to monitor and control current, voltage and even BAT GATE battery pack temperature for fast and safe charging. 2k LTC4002ES8-4.2 CHARGE 6.8µH Li-Ion Charger (Figure 140) STATUS CHRG SENSE Just a few external components are required for this single 68mΩ Li-Ion cell charger. Power for the charger can come from COMP BAT a wall adapter or a computer’s USB port. NTC GND + 0.47µF Li-Ion 22µF BATTERY 2.2k 10k Battery Monitor (Figure 141) T 4002 TA01 NTC NTC: DALE NTHS-1206N02 Op amp sections A and B form classical high side sense circuits in conjunction with Q1 and Q2 respectively. Each Figure 139. Single Cell Li-Ion Charger section handles a different polarity of battery current flow and delivers metered current to load resistor R G . Sec- 800mA (WALL) tion C operates as a comparator to provide a logic signal LTC4076 500mA (USB) WALL ADAPTER DCIN BAT indicating whether the current is a charge or discharge USB USBIN HPWR + 4.2V flow. S1 sets the section D buffer op-amp gain to +1 or PORT 1µF SINGLE CELL IUSB +10. Rail-to-rail op amps are required in this circuit, such Li-Ion BATTERY 2k IDC ITERM 1µF as the LT1491 quad in the example. 1% GND 1.24k 1k 1% 1% 4076 TA01 Figure 140. Li-Ion Charger R S RA 0.2Ω 2k Q1 CHARGER + 2N3904 VOLTAGE A R A ' I BATT – 1/4 LT1491 2k C – LOGIC 1/4 LT1491 + R B 2k Q2 + LOGIC HIGH (5V) = CHARGING 2N3904 LOGIC LOW (0V) = DISCHARGING B R B ' 1/4 LT1491 2k – LOAD + D + R G V OUT 1/4 LT1491 10k V BATT = 12V – S1 10k 90.9k V OUT V OUT S1 = OPEN, GAIN = 1 R A = R B I BATT = = AMPS (R S )(R G /R A )(GAIN) GAIN S1 = CLOSED, GAIN = 10 V S = 5V, 0V 1490/91 TA01 Figure 141. Battery Monitor an105fa AN105-79

  76. Application Note 105 BATTERIES Monitor Charge and Discharge Currents at One Output Battery Stack Monitoring (Figure 143) (Figure 142) The comparators used in the LT6109 can be used sepa- Current from a battery to a load or from a charger to the rately. In this battery stack monitoring circuit a low on battery can be monitored using a single sense resistor either comparator output will disconnect the load from and the LTC6104. Discharging load current will source the battery. One comparator watches for an overcurrent a current at the output pin in proportion to the voltage condition (800mA) and the other for a low voltage condi- across the sense resistor. Charging current into the battery tion (30V). These threshold values are fully programmable will sink a current at the output pin. The output voltage using resistor divider networks. above or below the voltage V REF will indicate charging or discharging of the battery. I CHARGE V SENSE + – CHARGER R SENSE I DISCHARGE R IN R IN 8 7 6 5 +INA –INA –INB +INB I LOAD + + – – A B + V S V S LOAD CURRENT V – MIRROR OUT LTC6104 1 4 + R OUT V OUT + V REF – – 6104 TA03 Figure 142. Monitor Charge and Discharge Currents at One Output 12 LITHIUM 40V CELL STACK SENSE IRF9640 0.1Ω LOW TO LOAD + 10µF 1M 100k 6.2V R10 + 100Ω INC2 10 1 + SENSEHI SENSELO 0.1µF 13.3k 8 9 V + OUTA V OUT 0.8A 5V LT6109-1 + OVERCURRENT 9.53k DETECTION 2 6 10k RESET EN/ RST INC1 100k 4 475Ω OUTC1 3 7 OUTC2 INC2 2N7000 30V V – UNDERVOLTAGE DETECTION 5 6109 TA02 Figure 143. Battery Stack Monitoring an105fa AN105-80

  77. Application Note 105 BATTERIES Coulomb Counting Battery Gas Gauge (Figure 144) High Voltage Battery Coulomb Counting (Figure 145) The LTC4150 converts the voltage across a sense resis- When coulomb counting, after each interrupt interval tor to a microprocessor interrupt pulse train. The time the internal counter needs to be cleared for the next between each interrupt pulse is directly proportional to the time interval. This can be accomplished by the µP or the current flowing through the sense resistor and therefore LTC4150 can clear itself. In this circuit the IC is powered the number of coulombs travelling to or from the battery from a battery supply which is at a higher voltage than power source. A polarity output indicates the direction of the interrupt counting µP supply. current flow. By counting interrupt pulses with the polarity adding or subtracting from the running total, an indication of the total change in charge on a battery is determined. This acts as a battery gas gauge to indicate where the battery charge is between full or empty. CHARGER R SENSE LOAD + 4.7µF R L R L SENSE – SENSE + V DD C F+ INT 4.7µF CLR LTC4150 µP CHG C F– DISCHG POL SHDN GND 4150 TA01a Figure 144. Coulomb Counting Battery Gas Gauge POWER-DOWN SWITCH LOAD C L PROCESSOR 47µF V CC R L R L 1 10 SENSE + INT 9 LTC4150 CLR R SENSE 8 2 SENSE – V DD C2 + 3 2.7V TO 8.5V C F+ 4.7µF 7 GND BATTERY µP C F 4.7µF 4 C F– 5 6 SHDN POL 4150 F05 Figure 145. High Voltage Battery Coulomb Counting an105fa AN105-81

  78. Application Note 105 BATTERIES Low Voltage Battery Coulomb Counting (Figure 146) Single Cell Lithium-Ion Battery Coulomb Counter (Figure 147) When coulomb counting, after each interrupt interval the internal counter needs to be cleared for the next time in- This is a circuit which will keep track of the total change terval. This can be accomplished by the µP or the LTC4150 in charge of a single cell Li-Ion battery power source. The can clear itself. In this circuit the IC is powered from a maximum battery current is assumed to be 500mA due battery supply which is at a lower voltage than the interrupt to the 50mV full-scale sense voltage requirement of the counting µP supply. The CLR signal must be attenuated LTC4150. The µP supply is greater than the battery supply. because the INT pin is pulled to a higher voltage. POWER-DOWN SWITCH LOAD C L PROCESSOR 47µF V CC R L R L 1 10 SENSE + INT 9 LTC4150 R SENSE CLR R2 8 2 V DD SENSE – C2 + 3 C F+ 7 4.7µF BATTERY GND R1 µP V BATTERY < V CC C F 4.7µF 4 C F– 5 6 SHDN POL SHUTDOWN 4150 F06 R4 R3 Figure 146. Low Voltage Battery Coulomb Counting POWER-DOWN SWITCH LOAD C L 5.0V 47µF R L R L 3k 3k 1 10 SENSE + INT 9 R SENSE R2 LTC4150 CLR 0.1Ω 76.8k 8 2 SENSE – V DD C2 SINGLE-CELL 3 + C F+ 7 4.7µF R1 Li-Ion GND µP 75k C F 3.0V ~ 4.2V 4.7µF 4 C F– 5 6 SHDN POL SHUTDOWN R4 76.8k R3 75k 4150 F08 Figure 147. Single Cell Lithium-Ion Battery Coulomb Counter an105fa AN105-82

  79. • • • Application Note 105 BATTERIES Complete Single Cell Battery Protection (Figure 148) and signal the termination or initiation of cell charging. The ADC can be continually reconfigured for single-ended Voltage, current and battery temperature can all be moni- or differential measurements to produce the required tored by a single LTC2990 ADC to 14-bit resolution. Each information. of these parameters can detect an excessive condition BATTERY I AND V MONITOR 15mΩ* CHARGING CURRENT 5V 0.1µF V CC V1 V2 MMBT3904 2-WIRE SDA V3 + I 2 C V(t) T(t) I(t) LTC2990 SCL NiMH INTERFACE 470pF ADR0 BATTERY 100% 100% 100% ADR1 V4 GND T BATT 2990 TA07 T INTERNAL *IRC LRF3W01R015F VOLTAGE AND TEMPERATURE CONFIGURATION: CURRENT AND TEMPERATURE CONFIGURATION: CONTROL REGISTER: 0x58 CONTROL REGISTER: 0x59 T AMB REG 4, 5 0.0625°C/LSB T AMB REG 4, 5 0.0625°C/LSB V BAT REG 8, 9 305.18µVLSB I BAT REG 6, 7 1.295mA/LSB T BAT REG A, B 0.0625°C/LSB T BAT REG A, B 0.0625°C/LSB V CC REG E, F 2.5V + 305.18µV/LSB V CC REG E, F 2.5V + 305.18µV/LSB Figure 148. Complete Single Cell Battery Protection More Battery Circuits Are Shown in Other Chapters: FIGURE TITLE 21 Sensed Current Includes Monitor Circuit Supply Current 58 Bidirectional Precision Current Sensing 179 Digitizing Charging and Loading Current in a Battery Monitor 181 Ampere-Hour Gauge 209 Use Kelvin Connections to Maintain High Current Accuracy 216 Dual Sense Amplifier Can Have Different Sense Resistors and Gain an105fa AN105-83

  80. Application Note 105 HIGH SPEED Current monitoring is not normally a particularly high speed A common monitoring approach in these systems is to requirement unless excessive current flow is caused by a amplify the voltage on a “flying” sense resistor, as shown. fault of some sort. The use of fast amplifiers in conventional Unfortunately, several potentially hazardous fault scenarios current sense circuits is usually sufficient to obtain the go undetected, such as a simple short to ground at a motor response time desired. terminal. Another complication is the noise introduced by the PWM activity. While the PWM noise may be filtered for Fast Compact –48V Current Sense (Figure 149) purposes of the servo law, information useful for protection becomes obscured. The best solution is to simply provide This amplifier configuration is essentially the complemen- two circuits that individually protect each half-bridge and tary implementation to the classic high side configuration. report the bidirectional load current. In some cases, a The op amp used must support common mode operation smart MOSFET bridge driver may already include sense at its lower rail. A “floating” shunt-regulated local supply resistors and offer the protection features needed. In these is provided by the Zener diode, and the transistor provides situations, the best solution is the one that derives the load metered current to an output load resistance (1kΩ in this information with the least additional circuitry. circuit). In this circuit, the output voltage is referenced to a positive potential and moves downward when represent- BATTERY BUS ing increasing –48V loading. Scaling accuracy is set by + the quality of resistors used and the performance of the NPN transistor. Conventional H-Bridge Current Monitor (Figure 150) + R S Many of the newer electric drive functions, such as steer- DIFF AMP ing assist, are bidirectional in nature. These functions are – I M generally driven by H-bridge MOSFET arrays using pulse- width modulation (PWM) methods to vary the commanded torque. In these systems, there are two main purposes for current monitoring. One is to monitor the current in the load, to track its performance against the desired com- DN374 F03 mand (i.e., closed-loop servo law), and another is for fault Figure 150. Conventional H-Bridge Current Monitor detection and protection features. V OUT = 3V – 0.1Ω • I SENSE I SENSE = 0A TO 30A ACCURACY ≈ 3% V OUT R1 1k Q1 4.7k 1% FMMT493 V S = 3V 30.1Ω 1% – R1 REDUCES Q1 DISSIPATION 3.3k 0805 LT1797 × 3 + 0.1µF SETTLES TO 1% IN 2s, 1V OUTPUT STEP BZX84C6V8 0.003Ω V Z = 6.8V 1% 3W –48V SUPPLY –48V LOAD – I SENSE + 1797 TA01 (–42V TO –56V) Figure 149. Fast Compact –48V Current Sense an105fa AN105-84

  81. Application Note 105 HIGH SPEED Single-Supply 2.5V Bidirectional Operation with may drive following circuitry more effectively than the External Voltage Reference and I/V Converter high output impedance of the LT1787. The I/V converter (Figure 151) configuration also works well with split supply voltages. The LT1787’s output is buffered by an LT1495 rail-to-rail Battery Current Monitor (Figure 152) op amp configured as an I/V converter. This configuration One LT1495 dual op amp package can be used to establish is ideal for monitoring very low voltage supplies. The separate charge and discharge current monitoring outputs. LT1787’s V OUT pin is held equal to the reference voltage The LT1495 features Over-the-Top operation allowing appearing at the op amp’s non-inverting input. This al- the battery potential to be as high as 36V with only a 5V lows one to monitor supply voltages as low as 2.5V . The amplifier supply voltage. op amp’s output may swing from ground to its positive supply voltage. The low impedance output of the op amp I SENSE TO R SENSE CHARGER/ LOAD C1 2.5V + V SENSE(MAX) 1µF 1 8 FIL – FIL + LT1787 V S– V S+ 2 7 2.5V V BIAS 3 6 DNC C3 R OUT 1000pF 4 5 V EE V OUT – A1 V OUT A + 2.5V LT1495 1M 5% LT1389-1.25 1787 F07 Figure 151. Single-Supply 2.5V Bidirectional Operation with External Voltage Reference and I/V Converter I L R SENSE CHARGE 0.1Ω 12V DISCHARGE 5V R A R A – – A2 A1 1/2 LT1495 1/2 LT1495 R A R A + + 2N3904 2N3904 ( ) R B DISCHARGE CHARGE V O = I L R SENSE R A OUT OUT FOR R A = 1k, R B = 10k R B R B V O = 1V/A I L 1495 TA05 Figure 152. Battery Current Monitor an105fa AN105-85

  82. LT1022 • TA07 7 Application Note 105 HIGH SPEED Fast Current Sense with Alarm (Figure 153) Fast Differential Current Source (Figure 154) The LT1995 is shown as a simple unity gain difference This is a variation on the Howland configuration, where amplifier. When biased with split supplies the input current load current actually passes through a feedback resistor can flow in either direction providing an output voltage of as an implicit sense resistance. Since the effective sense 100mV/A from the voltage across the 100mΩ sense resis- resistance is relatively large, this topology is appropriate tor. With 32MHz of bandwidth and 1000V/µs slew rate the for producing small controlled currents. response of this sense amplifier is fast. Adding a simple comparator with a built in reference voltage circuit such as the LT6700-3 can be used to generate an overcurrent flag. With the 400mV reference the flag occurs at 4A. 15V R* R* 10pF 2 I OUT = V IN2 – V IN1 15V TO –15V 15V V IN1 – R 6 LT1022 I R* 3 LT6700-3 V IN2 + P1 10k 4 10k R* LT1995 0.1Ω + –15V I OUT R L G = 1 REF M1 SENSE FLAG OUTPUT – OUTPUT * MATCH TO 0.01% 100mV/A 4A LIMIT –15V FULL-SCALE POWER BANDWIDTH = 1MHz FOR I OUT R = 8V P-P = 400kHz FOR I OUT R = 20V P-P 400mV MAXIMUM I OUT = 10mA P-P I OUTP-P • R L COMMON MODE VOLTAGE AT LT1022 INPUT = 1995 TA05 2 Figure 153. Fast Current Sense with Alarm Figure 154. Fast Differential Current Source More High Speed Circuits Are Shown in Other Chapters: FIGURE TITLE 22 Wide Voltage Range Current Sensing 124 Monitor H-Bridge Motor Current Directly 128 Fixed Gain DC Motor Current Monitor 143 Battery Stack Monitoring 168 Monitoring a Fuse Protected Circuit 169 Circuit Fault Protection with Early Warning and Latching Load Disconnect 170 Use Comparator Output to Initialize Interrupt Routines an105fa AN105-86

  83. Application Note 105 FAULT SENSING The lack of current flow or the dramatic increase of current R SENSE flow very often indicates a system fault. In these circuits R1 it is important to not only detect the condition, but also 100 ensure the safe operation of the detection circuitry itself. 4 3 + – L 2 5 System faults can be destructive in many unpredictable O A ways. V BATT D 1 High Side Current Sense and Fuse Monitor LTC6101 (Figure 155) D1 R2 4.99k The LT6100 can be used as a combination current sen- 6101 F07 sor and fuse monitor. This part includes on-chip output buffering and was designed to operate with the low supply Figure 156. Schottky Prevents Damage During Supply Reversal voltage (≥2.7V), typical of vehicle data acquisition systems, while the sense inputs monitor signals at the higher bat- Additional Resistor R3 Protects Output During Supply tery bus potential. The LT6100 inputs are tolerant of large Reversal (Figure 157) input differentials, thus allowing the blown-fuse operating condition (this would be detected by an output full-scale If the output of the LTC6101 is wired to an independently indication). The LT6100 can also be powered down while powered device that will effectively short the output to maintaining high impedance sense inputs, drawing less another rail or ground (such as through an ESD protection than 1µA max from the battery bus. clamp) during a reverse supply condition, the LTC6101’s output should be connected through a resistor or Schottky R SENSE TO LOAD diode to prevent excessive fault current. FUSE 2mΩ BATTERY BUS + 1 8 R SENSE V S– V S+ ADC 2 7 R1 POWER V CC A4 V BATT 100 ≥2.7V C2 – + 0.1µF 4 3 + – 3 6 L 2 5 FIL A2 O A D R3 4 OUT 5 OUTPUT 1k 1 V EE LTC6101 2.5V = 25A ADC LT6100 D1 R2 DN374 F02 4.99k 6101 F08 Figure 155. High Side Current Sense and Fuse Monitor Schottky Prevents Damage During Supply Reversal Figure 157. Additional Resistor R3 Protects Output During (Figure 156) Supply Reversal The LTC6101 is not protected internally from external reversal of supply polarity. To prevent damage that may Electronic Circuit Breaker (Figure 158) occur during this condition, a Schottky diode should be The LT1620l current sense amplifier is used to detect an added in series with V – . This will limit the reverse current overcurrent condition and shut off a P-MOSFET load switch. through the LTC6101. Note that this diode will limit the A fault flag is produced in the overcurrent condition and low voltage performance of the LTC6101 by effectively a self-reset sequence is initiated. reducing the supply voltage to the part by V D . an105fa AN105-87

  84. LTC1153 • TA01 LT1620/21 • TA03 Application Note 105 FAULT SENSING Si9434DY 0.033Ω 5V AT 1A 5V PROTECTED 0.1µF 1k FAULT C DELAY 100Ω 1N4148 1 8 33k SENSE AVG 2 7 PROG 100k 2N3904 I OUT LT1620MS8 3 6 4.7k 33k GND V CC 4 5 +IN –IN 2N3904 TYPICAL DC TRIP AT 1.6A 3A FAULT TRIPS IN 2ms WITH C DELAY = 1.0µF Figure 158. Electronic Circuit Breaker Electronic Circuit Breaker (Figure 159) ON/OFF IN VS C D The LTC1153 is an electronic circuit breaker. Sensed cur- R D *R SEN C T 0.01µF 100k 0.1Ω 0.22µF rent to a load opens the breaker when 100mV is developed C T DS Z5U between the supply input, V S , and the drain sense pin, DS. LTC1153 TO µP STATUS G IRLR024 To avoid transient, or nuisance trips of the break compo- 51k nents RD and CD delay the action for 1ms. A thermistor 51k GND SHUTDOWN can also be used to bias the shutdown input to monitor 5V SENSITIVE **70°C heat generated in the load and remove power should the 5V LOAD PTC temperature exceed 70°C in this example. A feature of the LTC1153 is timed automatic reset which will try to ALL COMPONENTS SHOWN ARE SURFACE MOUNT. reconnect the load after 200ms using the 0.22μF timer * IMS026 INTERNATIONAL MANUFACTURING SERVICE, INC. (401) 683-9700 ** RL2006-100-70-30-PT1 KEYSTONE CARBON COMPANY (814) 781-1591 capacitor shown. Figure 159. Electronic Circuit Breaker 1.25V Electronic Circuit Breaker (Figure 160) The LTC4213 provides protection and automatic circuit SI4864DY V IN V OUT breaker action by sensing drain-to-source voltage drop 1.25V 1.25V across the N-MOSFET . The sense inputs have a rail-to-rail 3.5A common mode range, so the circuit breaker can protect SENSEP GATE SENSEN V BIAS V CC bus voltages from 0V up to 6V . Logic signals flag a trip V BIAS 2.3V TO 6V condition (with the READY output signal) and reinitialize LTC4213 10k the breaker (using the ON input). The ON input may also OFF ON ON READY GND I SEL be used as a command in a “smart switch” application. 4213 TA01 Figure 160. 1.25V Electronic Circuit Breaker an105fa AN105-88

  85. Application Note 105 FAULT SENSING Lamp Outage Detector (Figure 161) the op amp, so part substitutions are discouraged (how- ever, this same circuit also works properly with an LT1716 In this circuit, the lamp is monitored in both the on and off comparator, also an Over-the-Top part). condition for continuity. In the off condition, the filament pull-down action creates a small test current in the 5kΩ that Simple Telecom Power Supply Fuse Monitor is detected to indicate a good lamp. If the lamp is open, the (Figure 162) 100kΩ pull-up, or the relay contact, provides the op amp The LTC1921 provides an all-in-one telecom fuse and bias current through the 5kΩ, that is opposite in polarity. supply-voltage monitoring function. Three opto-isolated When the lamp is powered and filament current is flowing, status flags are generated that indicate the condition of the drop in the 0.05Ω sense resistor will exceed that of the the supplies and the fuses. 5kΩ and a lamp-good detection will still occur. This circuit requires particular Over-the-Top input characteristics for Conventional H-Bridge Current Monitor (Figure 163) 5V TO 44V 3V 1M Many of the newer electric drive functions, such as steer- ing assist, are bidirectional in nature. These functions are LAMP 100k ON/OFF generally driven by H-bridge MOSFET arrays using pulse- 5k – width modulation (PWM) methods to vary the commanded 0.5Ω LT1637 OUT torque. In these systems, there are two main purposes for + current monitoring. One is to monitor the current in the load, to track its performance against the desired com- mand (i.e., closed-loop servo law), and another is for fault OUT = 0V FOR GOOD BULB 3V FOR OPEN BULB detection and protection features. 1637 TA05 Figure 161. Lamp Outage Detector 47k 5V –48V RETURN FUSE STATUS R1 R2 100k 100k MOC207 3 SUPPLY A SUPPLY B RTN 47k V A V B STATUS STATUS 4 1 5V V A OUT F OK OK 0 0 SUPPLY A OK UV OR OV 0 1 STATUS UV OR OV OK 1 0 8 V B UV OR OV UV OR OV 1 1 LTC1921 OK: WITHIN SPECIFICATION MOC207 2 OV: OVERVOLTAGE FUSE A UV: UNDERVOLTAGE 47k 5V 7 5 V FUSE A V FUSE B FUSE STATUS FUSE B OUT A SUPPLY B = V A = V B 0 STATUS = V A ≠ V B 1 ≠ V A = V B 1 MOC207 ≠ V A ≠ V B 1* 6 OUT B 0: LED/PHOTODIODE ON R3 1: LED/PHOTODIODE OFF 47k F1 D1 *IF BOTH FUSES (F1 AND F2) ARE OPEN, 1/4W SUPPLY A ALL STATUS OUTPUTS WILL BE HIGH –48V OUT –48V SINCE R3 WILL NOT BE POWERED F2 D2 SUPPLY B = LOGIC COMMON –48V Figure 162. Simple Telecom Power Supply Fuse Monitor an105fa AN105-89

  86. Application Note 105 FAULT SENSING A common monitoring approach in these systems is to I SENSE TO R SENSE amplify the voltage on a “flying” sense resistor, as shown. CHARGER/ LOAD Unfortunately, several potentially hazardous fault scenarios C1 2.5V + V SENSE(MAX) 1µF 1 8 go undetected, such as a simple short to ground at a motor FIL – FIL + LT1787 terminal. Another complication is the noise introduced by V S– V S+ 2 7 the PWM activity. While the PWM noise may be filtered for 2.5V V BIAS 3 6 DNC purposes of the servo law, information useful for protection C3 R OUT 1000pF becomes obscured. The best solution is to simply provide 4 5 V EE V OUT two circuits that individually protect each half-bridge and – report the bidirectional load current. In some cases, a A1 V OUT A + 2.5V LT1495 smart MOSFET bridge driver may already include sense 1M 5% LT1389-1.25 resistors and offer the protection features needed. In these 1787 F07 situations, the best solution is the one that derives the load information with the least additional circuitry. Figure 164. Single-Supply 2.5V Bidirectional Operation with External Voltage Reference and I/V Converter BATTERY BUS + Battery Current Monitor (Figure 165) One LT1495 dual op amp package can be used to establish separate charge and discharge current monitoring outputs. + R S The LT1495 features Over-the-Top operation allowing DIFF the battery potential to be as high as 36V with only a 5V AMP – I M amplifier supply voltage. I L R SENSE CHARGE 0.1Ω DN374 F03 12V DISCHARGE 5V R A R A Figure 163. Conventional H-Bridge Current Monitor – – A2 A1 1/2 LT1495 1/2 LT1495 R A R A Single-Supply 2.5V Bidirectional Operation with + + External Voltage Reference and I/V Converter (Figure 164) 2N3904 2N3904 ( ) R B DISCHARGE CHARGE V O = I L R SENSE The LT1787’s output is buffered by an LT1495 rail-to-rail R A OUT OUT op amp configured as an I/V converter. This configuration FOR R A = 1k, R B = 10k R B R B V O is ideal for monitoring very low voltage supplies. The = 1V/A I L 1495 TA05 LT1787’s V OUT pin is held equal to the reference voltage appearing at the op amp’s non-inverting input. This al- Figure 165. Battery Current Monitor lows one to monitor supply voltages as low as 2.5V . The op amp’s output may swing from ground to its positive supply voltage. The low impedance output of the op amp may drive following circuitry more effectively than the high output impedance of the LT1787. The I/V converter configuration also works well with split supply voltages. an105fa AN105-90

  87. Application Note 105 FAULT SENSING Fast Current Sense with Alarm (Figure 166) I SENSE V SENSE + – V S LOAD The LT1995 is shown as a simple unity gain difference R SENSE R IN amplifier. When biased with split supplies the input current –IN +IN can flow in either direction providing an output voltage of – 100mV/A from the voltage across the 100mΩ sense resis- + V – V + tor. With 32MHz of bandwidth and 1000V/µs slew rate the response of this sense amplifier is fast. Adding a simple comparator with a built in reference voltage circuit such 1/2 LTC6103 OUT as the LT6700-3 can be used to generate an overcurrent V LOGIC flag. With the 400mV reference the flag occurs at 4A. R OUT 15V 15V TO –15V V OUT I LT6700-3 P1 10k ANY OPTO-ISOLATOR 10k LT1995 + 0.1Ω G = 1 REF V – 6103 TA07 M1 SENSE N = OPTO-ISOLATOR CURRENT GAIN FLAG – OUTPUT OUTPUT 100mV/A R SENSE 4A LIMIT V OUT = V LOGIC – I SENSE • • N • R OUT –15V R IN 400mV Figure 167. Monitor Current in an Isolated Supply Line 1995 TA05 Figure 166. Fast Current Sense with Alarm TO LOAD R SENSE FUSE V S– V S+ DC SOURCE (≤ 44V) R IN1 R IN2 C1 Monitor Current in an Isolated Supply Line 0.1µF (Figure 167) –IN +IN V + Using the current sense amplifier output current to directly + C2 5V modulate the current in a photo diode is a simple method – + 0.1µF to monitor an isolated 48V industrial/telecom power supply. Current faults can be signaled to nonisolated monitoring circuitry. V – OUT Monitoring a Fuse Protected Circuit (Figure 168) OUTPUT LT6105 R OUT Current sensing a supply line that has a fuse for overcurrent 6105 F03 protection requires a current sense amplifier with a wide differential input voltage rating. Should the fuse blow open Figure 168. Monitoring a Fuse Protected Circuit the full load supply voltage appears across the inputs to the sense amplifier. The LT6105 can work with input voltage differentials up to 44V . The LT6105 output slews at 2V/µs so can respond quickly to fast current changes. When the fuse opens the LT6105 output goes high and stays there. an105fa AN105-91

  88. Application Note 105 FAULT SENSING Circuit Fault Protection with Early Warning and Use Comparator Output to Initialize Interrupt Routines Latching Load Disconnect (Figure 169) (Figure 170) With a precision current sense amplifier driving two built The comparator outputs can connect directly to I/O or in comparators, LT6109-2 can provide current overload interrupt inputs to any microcontroller. A low level at protection to a load circuit. The internal comparators have OUTC2 can indicate an undercurrent condition while a low a fixed 400mV reference. The current sense output is level at OUTC1 indicates an overcurrent condition. These resistor divided down so that one comparator will trip at interrupts force service routines in the microcontroller. an early warning level and the second at a danger level of current to the load (100mA and 250mA in this example). The comparator outputs latch when tripped so they can be used as a circuit breaker to disconnect and protect the load until a reset pulse is provided. 0.1Ω IRF9640 12V TO LOAD 6.2V 10µF 1k 100Ω 3.3V SENSEHI SENSELO V + OUTA V OUT 10k 1.62k 100k LT6109-2 6.04k RESET EN/ RST INC2 1k 100mA WARNING 2.37k OUTC2 250mA DISCONNECT 2N2700 OUTC1 INC1 V – 1.6k 610912 TA01a Figure 169. Circuit Fault Protection with Early Warning and Latching Load Disconnect 0.1Ω EXAMPLE V + TO LOAD 5V OUTC2 GOES LOW 100Ω 10 1 SENSEHI SENSELO 9 8 V OUT V + OUTA MCU INTERUPT ADC IN AtMega1280 5V 5 LT6109-1 2k PB0 RESET 7 6 2 10k EN/ RST INC2 PB1 UNDERCURRENT ROUTINE 7 3 6.65k PCINT2 OUTC2 2 4 6 5V PCINT3 OUTC1 INC1 3 V – ADC2 V OUT /ADC IN 1.33k 10k 1 RESET COMPARATORS 5 PB5 6109 TA03 Figure 170. Use Comparator Output to Initialize Interrupt Routines an105fa AN105-92

  89. Application Note 105 FAULT SENSING Current Sense with Overcurrent Latch and Power-On arrangement can create a latching output when an over- Reset with Loss of Supply (Figure 171) current condition is sensed. The same logic gate can also generate an active low power-on reset signal. The LT6801-2 has a normal nonlatching comparator built in. An external logic gate configured in a positive feedback 5V 7 V + LT6108-2 V + R IN 100Ω 8 SENSEHI – R3 R SENSE 10k 1 SENSELO OUTA 6 + I LOAD V – R7 V + R1 9.53k 24.9k INC 5 VTH – 3 OUTC R8 499Ω 400mV + REFERENCE V – 4 R5* R9* V DD 100k 30k Q1* C1 R4* R2 2N2222 0.1µF 3.4k 200k 610812 TA06 *OPTIONAL COMPONENT R6 1M Figure 171. Current Sense with Overcurrent Latch and Power-On Reset with Loss of Supply an105fa AN105-93

  90. Application Note 105 FAULT SENSING More Fault Sensing Circuits Are Shown in Other Chapters: FIGURE TITLE 120 Bidirectional Current Sensing in H-Bridge Drivers 125 Large Input Voltage Range for Fused Solenoid Current Monitoring 136 Coulomb Counting Battery Gas Gauge 143 Battery Stack Monitoring 145 High Voltage Battery Coulomb Counting 146 Low Voltage Battery Coulomb Counting 147 Single Cell Lithium-Ion Battery Coulomb Counter 211 Power Intensive Circuit Board Monitoring an105fa AN105-94

  91. Application Note 105 DIGITIZING In many systems the analog voltage quantity indicating inputs and an output current sensing resistor control the current flow must be input to a system controller. In this maximum sourcing and sinking current values. These chapter several examples of the direct interface of a cur- control voltages could be provided by a D-to-A converter rent sense amplifier to an A to D converter are shown. in a microprocessor controlled system. For closed loop control of the current to a load an LT1787 can monitor the Sensing Output Current (Figure 172) output current. The LT1880 op amp provides scaling and level shifting of the voltage applied to an A-to-D converter The LT1970 is a 500mA power amplifier with voltage for a 5mV/mA feedback signal. programmable output current limit. Separate DC voltage V CC 0V TO 1V 12V VC SRC VC SNK EN +IN V CC V + ISRC R S ISNK 0.2Ω TSD LT1970 OUT SENSE + SENSE – R LOAD FILTER V – –IN V EE COMMON R4 LT1787 255k V S– V S+ –12V BIAS R G R F –12V 12V R1 20k 60.4k – V EE V OUT R2 LT1880 2.5V 10k ±5mV/mA + R3 1kHz FULL CURRENT –12V 20k BANDWIDTH –12V 0V TO 5V A/D 1970 F10 OPTIONAL DIGITAL FEEDBACK Figure 172. Sensing Output Current an105fa AN105-95

  92. Application Note 105 DIGITIZING Split or Single-Supply Operation, Bidirectional Output 16-Bit Resolution Unidirectional Output into LTC2433 into A/D (Figure 173) ADC (Figure 174) In this circuit, split supply operation is used on both the The LTC2433-1 can accurately digitize signal with source LT1787 and LT1404 to provide a symmetric bidirectional impedances up to 5kΩ. This LTC6101 current sense circuit measurement. In the single-supply case, where the LT1787 uses a 4.99kΩ output resistance to meet this requirement, Pin 6 is driven by V REF , the bidirectional measurement thus no additional buffering is necessary. range is slightly asymmetric due to V REF being somewhat greater than midspan of the ADC input range. 1Ω 1% I S = ±125mA V CC 5V 1 8 V SRCE FIL – FIL + ≈4.75V LT1787 V S– V S+ 10µF 2 7 16V V BIAS 3 6 1 DNC 7 CONV 20k V OUT (±1V) 4 5 2 V EE 6 CLOCKING V EE A IN LTC1404 CLK –5V CIRCUITRY 3 V OUT V REF 5 OPTIONAL SINGLE DOUT SUPPLY OPERATION: GND 10µF DISCONNECT V BIAS 16V 4 8 FROM GROUND 10µF AND CONNECT IT TO V REF . D OUT 16V REPLACE –5V SUPPLY WITH GROUND. V EE 1787 TA02 OUTPUT CODE FOR ZERO –5V CURRENT WILL BE ~2430 Figure 173. Split or Single-Supply Operation, Bidirectional Output into A/D I LOAD V SENSE + – R IN 4V TO 60V 100Ω 4 3 L + – 2 5 O A 5V 1µF D 2 1 REF + V OUT V CC 1 4 9 IN + LTC6101 SCK 8 LTC2433-1 SDD TO µP R OUT 7 IN – C C 4.99k 5 REF – F O GND 3 6 10 R OUT V OUT = • V SENSE = 49.9V SENSE ADC FULL-SCALE = 2.5V 6101 TA06 R IN Figure 174. 16-Bit Resolution Unidirectional Output into LTC2433 ADC an105fa AN105-96

  93. Application Note 105 DIGITIZING 12-Bit Resolution Unidirectional Output Directly Digitize Current with 16-Bit Resolution into LTC1286 ADC (Figure 175) (Figure 176) While the LT1787 is able to provide a bidirectional output, The low offset precision of the LTC6102 permits direct in this application the economical LTC1286 is used to digitization of a high side sensed current. The LTC2433 digitize a unidirectional measurement. The LT1787 has a is a 16-bit delta sigma converter with a 2.5V full-scale nominal gain of eight, providing a 1.25V full-scale output range. A resolution of 16 bits has an LSB value of only at approximately 100A of load current. 40µV . In this circuit the sense voltage is amplified by a factor of 50. This translates to a sensed voltage resolution of only 0.8µV per count. The LTC6102 DC offset typically contributes only four LSB’s of uncertainty. R SENSE 0.0016Ω I = 100A TO LOAD 2.5V TO 60V 1 8 FIL – FIL + LT1787HV V S– V S+ 2 7 R1 C1 5V 15k 1µF V BIAS 3 6 DNC R OUT V REF V CC 20k 4 5 CS V EE +IN LTC1286 CLK TO µP V OUT –IN D OUT GND C2 1787 TA01 LT1634-1.25 0.1µF V OUT = V BIAS + (8 • I LOAD • R SENSE ) Figure 175. 12-Bit Resolution Unidirectional Output into LTC1286 ADC + R IN V SENSE 4V TO 60V 100Ω – +IN –INS –INF L + – I LOAD O V – V + A D 0.1µF 5V 1µF V REG 2 1 EN POWER ENABLE REF + V OUT V CC OUT 4 9 IN + LTC6102-1 SCK 8 LTC2433-1 SDD TO µP R OUT 7 IN – C C 4.99k 5 REF – GND F O 3 6 10 R OUT V OUT = • V SENSE = 49.9V SENSE ADC FULL-SCALE = 2.5V 6102 TA05 R IN Figure 176. Directly Digitize Current with 16-Bit Resolution an105fa AN105-97

  94. Application Note 105 DIGITIZING Directly Digitizing Two Independent Currents Digitize a Bidirectional Current Using a Single-Sense (Figure 177) Amplifier and ADC (Figure 178) With two independent current sense amplifiers in the The dual LTC6104 can be connected in a fashion to source LTC6103, two currents from different sources can be or sink current at its output depending on the direction simultaneously digitized by a 2-channel 16-bit ADC such of current flow through the sense resistor. Biasing the as the LTC2436-1. While shown to have the same gain on amplifier output resistor and the V REF input of the ADC to each channel, it is not necessary to do so. Two different an external 2.5V LT1004 voltage reference allows a 2.5V current ranges can be gain scaled to match the same full- full-scale input voltage to the ADC for current flowing in scale range for each ADC channel. either direction. V A+ V B+ V SENSE V SENSE I LOAD I LOAD – + + – LOAD LOAD R IN R IN 100Ω 100Ω 8 7 6 5 5V 1µF +INA –INA –INB +INB + + – – 2 1 V SA V SB 6 CH1 13 7 V – 12 OUTA OUTB LTC2436-1 TO µP LTC6103 4 1 4 2 11 5 CH0 R OUT R OUT 3,8,9,10,14,15,16 4.99k 4.99k 6103 TA01a Figure 177. Directly Digitizing Two Independent Currents I LOAD V SENSE – + TO CHARGER/LOAD + R SENSE 12V R IN R IN 100Ω 100Ω 8 7 6 5 +INA –INA –INB +INB + – – + A B + R1 V S V S C1 2.3k 5V 1µF V REF V REF V CC CURRENT CS +IN V – OUT MIRROR LTC6104 LTC1286 CLK TO µP 1 4 –IN D OUT C2 R OUT GND LT1004-2.5 0.1µF 2.5k 6104 TA01a Figure 178. Digitize a Bidirectional Current Using a Single-Sense Amplifier and ADC an105fa AN105-98

  95. Application Note 105 DIGITIZING Digitizing Charging and Loading Current in a Battery Complete Digital Current Monitoring (Figure 180) Monitor (Figure 179) An LTC2470 16-bit delta sigma A-to-D converter can A 16-bit digital output battery current monitor can be directly digitize the output of the LT6109 representing implemented with just a single sense resistor, an LT1999 a circuit load current. At the same time the comparator and an LTC2344 delta sigma ADC. With a fixed gain of outputs connect to MCU interrupt inputs to immediately ten and DC biased output the digital code indicates the signal programmable threshold over and undercurrent instantaneous loading or charging current (up to 10A) of conditions. a system battery power source. 0.025Ω CHARGER BAT 42V 5V V + LT1999-10 LOAD V + 5V 2µA 0.1µF 1 8 V SHDN SHDN 0.1µF 10µF + 40k – 4k V +IN 2 – V OUT 7 V + V CC V REF CS + +IN 0.8k + V + V OUT LTC2433-1 SCK 0.8k 160k 4k V –IN 3 V REF – 6 –IN SDO 160k V + 5V 0.1µF 4 5 0.1µF 1999 TA02 Figure 179. Digitizing Charging and Loading Current in a Battery Monitor SENSE SENSE 0.1Ω 0.1µF HIGH LOW IN OUT V CC V REF 100Ω COMP 10 1 SENSEHI SENSELO 9 8 V + IN + OUTA LTC2470 TO V CC 0.1µF LT6109-1 MCU 2k V CC RESET 2 7 EN/ RST INC2 10k 3 10k OUTC2 6.65k 4 6 OUTC1 INC1 V – 1.33k 5 OVERCURRENT 6109 TA05 UNDERCURRENT Figure 180. Complete Digital Current Monitoring an105fa AN105-99

  96. Application Note 105 DIGITIZING Ampere-Hour Gauge (Figure 181) Power Sensing with Built-In A-to-D Converter (Figure 182) With specific scaling of the current sense resistor, the LTC4150 can be set to output exactly 10,000 interrupt The LTC4151 contains a dedicated current sense input pulses for one Amp-hr of charge drawn from a battery channel to a 3-channel 12-bit delta-sigma ADC. The ADC source. With such a base-10 round number of pulses a directly and sequentially measures the supply voltage series of decade counters can be used to create a visual (102V full-scale), supply current (82mV full-scale) and a 5-digit display. This schematic is just the concept. The separate analog input channel (2V full-scale). The 12-bit polarity output can be used to direct the interrupt pulses resolution data for each measurement is output through an I 2 C link. to either the count-up or count-down clock input to display total net charge. CHARGER LOAD SENSE + INT CD40110B LTC4150 CLR 1.2Ω 1.1Ω 100mΩ SENSE – CD40110B + CD40110B SENSE RESISTANCE = 0.0852Ω CD40110B I MAX = 588mA 10,000 PULSES = 1Ah CD40110B 4150 F09 Figure 181. Ampere-Hour Gauge 3.3V 0.02Ω V IN V OUT 7V to 80V V DD 2k 2k SENSE + SENSE – µCONTROLLER SHDN V IN LTC4151 SCL SCL ADR1 SDA SDA MEASURED 4151 TA01 ADR0 ADIN VOLTAGE GND Figure 182. Power Sensing with Built-In A-to-D Converter an105fa AN105-100

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