Sub-mW Integrated Power Management Systems for Hearing aids Applications Yanhai Cao 1 , Frédéric Hasbani 1 , Dennis Øland Larsen 1,2 , Kim Rasmussen 1 1 GN Hearing A/S, Denmark, 2 Technical University of Denmark 1
Modern hearing aids Overview of features Invisible Size q There must be room for a decent battery q A minimum distance between microphones Advanced sound processing q Algorithms running on custom DSP 2.4 GHz Bluetooth directly to the hearing aid q Made for iPhone q The hearing aids can be used as a wireless stereo headset. q Stream audio from TV, smart phone, mini microphone directly to the HA. Extremely Low Power q Hearing aids running for the whole week on one battery 2
A typical hearing aid platform Microphones 4.25mm Push button 1.6mm Hybrid module Telecoil Wireless 4.6mm Loudspeaker Push buttons Battery 3
A typical hearing aid platform 2.4GHz DAC ADC DSP NVM Power Management Battery 4
The battery used in the hearing-aids Traditionally, hearing aids have used ZnAir batteries which are non-rechargeable Typical discharge voltage profile for a zinc air hearing aid battery, size 13 ZA13 IEC WL test: 5 mA dc current load for 15 mins. then 3 mA dc for 45 mins.; cut-off voltage = 1.10 V (Post-discharge 3 mA till 1.05 V) 1.50 300 1.45 In sleep 1.40 250 1.35 1.30 200 Capacity/mAh 1.25 Voltage/V 1.20 150 1.15 Battery loaded 1.10 100 1.05 Capacity till 1.10 V: 200 mAh, 232 mAh incl. "reserve" capacity 1.00 50 V of RESOUND_Z13A 0.95 C of RESOUND_Z13A 0 0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 96 102 108 114 120 126 Time/h 5
Energy densities of available and future hearing aid batteries 6
Power supply disturbances in the audio input stage Microphone Digital signal signal A/D Pre-amplifier Filter Converter Vsupply Supply ripple Digital Control t q Supply ripple in the audio frequency range could couple into the signal path LDOs with GOOD PSR!! q Supply ripple at multiples of the A/D oversampling rate (typically 500k-1MHz) could couple into the signal path. Anti-aliasing filter to filter such noise 7
Power supply disturbances in the audio input stage Microphone Pre-amplifier filter A/D converter Telecoil Telecoil Amplifier q Any varying current draw at audio frequency going though long wiring or inductors (eg. in inductive switching power management units) will create H-field which couples into the very sensitive telecoil. 8
Power supply disturbances in the audio output stage q One side of the output stage is depicted above q The supply noise from V supply is mixed with V DAC . This can result in noise folding into the audio band 9
The resulting output spectrum & Sweeping the disturbance frequency audio band We observe that 250 kHz, Spectrum with a 500 kHz, 750 kHz and 1 MHz 10mV@100KHz disturbance are “immune” to noise, and the regions in between are sensitive 10
Power supply disturbances in the radio The radio chip is supplied by 1.8V 1.2V q Large capacitors requried to lower the ripple from the DC-DC converter DC-DC q The disturbance can be minimized by using: Up converter A fixed-frequency voltage doubler followed by a LDO Ø 1.8V Ø 1.2V supply for the radio chip Radio chip Ø Completely stop the DC-DC converter when the radio is receiving mute mute Voltage RF & Flash supply 1.8V ramp up ramp up TX RX TX RX TX t 11
Power distribution in hearing aids chipset Li-ion Power management Zn-air v Ultra-Low Voltage system v 1.8V misc § Load current range 100uA-4mA § Load current range v Several voltage domains § Low ripple 4uA-10mA § High accuracy § High efficiency with § Load current range 20uA-400uA § High efficiency § High PSRR light load § Extremely high PSRR for microphone supply DSP RF & Flash Audio 12
Power management design due to the hearing aid size q Trend had always been to make invisible devices q Due to the number of passive components and the number of chips required for the hybrid module, power management design is becoming more challenging with the limited space q Total volume of all chips: 14 mm 3 q Total volume of all external components 3 mm 3 13
Challenges in the next generation Switched-capacitor DC-DC converter q Reduce ripples Ø Lower the ripple amplitude Ø More predictable spectrum q Reduce the size of the external capacitors Ø Small PCB pitch gives the opportunity of splitting a big capacitor into several small capacitors q Multiple outputs and reuse fly capacitors in different configurations 14
The current status of the rechargeability solution GN hearing introduced its first wireless rechargeable hearing aids in August, 2018 15
Integrated DC-DC converter for Li-Ion rechargeable solution DSP 2.4 GHz Inductive Power Li-Ion Audio amp charging converters Mic. ADCs Flash mem. Power management challenges: q Small battery à efficiency q Small volume à power density q Wireless connectivity à EMI D. Ø. Larsen, M. Vinter and I. Jørgensen, "Switched capacitor DC-DC converter with switch conductance modulation and Pseudo-fixed frequency control, " ESSCIRC 2017 – 43rd IEEE European Solid State Circuits Conference, Leuven, 2017, pp. 283-286 16
Output stage Unknown variables ! "# !$% − V ()* = I load R out Controllable variables q The equivalent output resistance needs to be controlled Ø Can be done with varying switch conductance and switching frequency q Gears could be added to cover larger range 17
The implemented regulator 185nF 4.7uF Load current: 1mA-40mA 18
Output impedance regulation q Output conductance used to design controller q Target: equidistant operating points in the G out dimension 19
Measurement Notice F sw moves with load current Output spectrum of pulse skipper q Load sweep of 1-10 mA 20
Measurement Output spectrum of proposed controller q Has predictable spectrum 21
Summary q Power management design for chipset used in hearing aids becomes more challenging due to more features and functionalities are desired q Rechargeable hearing aids with Lithium-Ion batteries is the trend q Supply disturburances in audio channel need to be thoroughly taken care q The proposed DC-DC converter in the recent study shows how we can achieve more predictable spectrum 22
Thank you! 23
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