Sub-mW Integrated Power Management Systems for Hearing aids - - PowerPoint PPT Presentation

Sub-mW Integrated Power Management Systems for Hearing aids Applications

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Yanhai Cao1, Frédéric Hasbani1, Dennis Øland Larsen1,2, Kim Rasmussen1

1GN Hearing A/S, Denmark, 2Technical University of Denmark

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

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A typical hearing aid platform

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4.6mm 4.25mm 1.6mm

Microphones Hybrid module Battery Loudspeaker Telecoil Wireless Push buttons Push button

A typical hearing aid platform

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ADC DSP 2.4GHz Power Management

DAC Battery

NVM

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)

Time/h 126 120 114 108 102 96 90 84 78 72 66 60 54 48 42 36 30 24 18 12 6 Voltage/V 1.50 1.45 1.40 1.35 1.30 1.25 1.20 1.15 1.10 1.05 1.00 0.95 Capacity/mAh 300 250 200 150 100 50 Capacity till 1.10 V: 200 mAh, 232 mAh incl. "reserve" capacity

5 Battery loaded In sleep V of RESOUND_Z13A C of RESOUND_Z13A

Energy densities of available and future hearing aid batteries

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Power supply disturbances in the audio input stage

q Supply ripple in the audio frequency range could couple into the signal path q Supply ripple at multiples of the A/D oversampling rate (typically 500k-1MHz) could couple into the signal path. LDOs with GOOD PSR!! Anti-aliasing filter to filter such noise

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Pre-amplifier Filter A/D Converter Digital Control Microphone signal Digital signal

Vsupply t Supply ripple

Power supply disturbances in the audio input stage

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.

Telecoil Amplifier Pre-amplifier filter A/D converter

Microphone Telecoil

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Power supply disturbances in the audio output stage

q One side of the output stage is depicted above q The supply noise from Vsupply is mixed with VDAC. This can result in noise folding into the audio band

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The resulting output spectrum & Sweeping the disturbance frequency

Spectrum with a 10mV@100KHz disturbance We observe that 250 kHz, 500 kHz, 750 kHz and 1 MHz are “immune” to noise, and the regions in between are sensitive

audio band

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Power supply disturbances in the radio

The radio chip is supplied by 1.8V q Large capacitors requried to lower the ripple from the DC-DC converter q The disturbance can be minimized by using: Ø A fixed-frequency voltage doubler followed by a LDO Ø 1.2V supply for the radio chip Ø Completely stop the DC-DC converter when the radio is receiving

t TX TX TX RX RX

Voltage

1.8V RF & Flash supply

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mute mute ramp up ramp up DC-DC Up converter Radio chip 1.2V 1.8V

Power distribution in hearing aids chipset

Power management system DSP RF & Flash Audio misc

v Ultra-Low Voltage § Load current range 100uA-4mA § Low ripple § High accuracy § High efficiency v 1.8V § Load current range 4uA-10mA § High efficiency with light load v Several voltage domains § Load current range 20uA-400uA § High PSRR § Extremely high PSRR for microphone supply

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Li-ion Zn-air

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 mm3 q Total volume of all external components 3 mm3

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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

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The current status of the rechargeability solution

GN hearing introduced its first wireless rechargeable hearing aids in August, 2018

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Integrated DC-DC converter for Li-Ion rechargeable solution Power management challenges:

q Small battery à efficiency q Small volume à power density q Wireless connectivity à EMI Li-Ion Inductive charging DSP 2.4 GHz Audio amp

• Mic. ADCs

Flash mem. Power converters

• 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

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

!"#!$% − V()* = IloadRout

Controllable variables Unknown variables

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The implemented regulator

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185nF 4.7uF Load current: 1mA-40mA

Output impedance regulation

q Output conductance used to design controller q Target: equidistant operating points in the Gout dimension

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Measurement

Output spectrum of pulse skipper q Load sweep of 1-10 mA

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Notice Fsw moves with load current

Measurement

Output spectrum of proposed controller q Has predictable spectrum

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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

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Thank you!

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