The hearing aid market demands low power and small size as the devices become smaller and less obtrusive. The batteries have traditionally been primary cells with replacement on a regular basis. Richard Mount, Sales and Marketing Director, Swindon Silicon Systems explains that using a silver-zinc (Ag-Zn) rechargeable battery technology with a high energy density allows a hearing aid to run longer on a tiny battery.

Here he describes that, by combining in an ASIC, a switching regulator for maximum efficiency at high supply with linear regulation to maximise the output power with low supply, the cell output was maximised for an ecological and cost-effective solution.

Desirable Battery Features

Hearing aids are particularly sensitive to size constraints and the drive is always to make them as small as possible. This can make the replacement of the battery a cumbersome process, especially if the person is less dextrous which makes a rechargeable approach the preferred option. A high energy power density chemistry is desirable and Ag-Zn can offer this. This means for an equivalent size, it is possible to get a performance improvement or a reduction in size for the same performance.

The chemistry of the battery is important when considering disposal at the end of its life. A lot of technologies contain mercury which is highly toxic to the environment. If a battery can be recycled it prevents it being disposed of in landfill. Also, by using re-chargeable batteries there is less packaging and environmental impact. The cost of a battery technology does not just equate to the piece part price, it has to take into account all the above factors when selecting the appropriate battery technology.

Ag-Zn Battery Solution

Hearing aid designs typically require a battery voltage in the range 1.2V-1.45V for normal operation. A voltage of 1.05V-1.15V is detected as end of life and the hearing aid electronics will shut down. Any battery used has to work with current designs and offer significant advantages to be a viable option. A rechargeable battery based on Ag-Zn chemistry that can be used in portable hearing devices offers substantial advantages over zinc-air technology but has a two-stage discharge cycle that requires some additional circuitry in order to provide a drop-in replacement for existing designs. It is also important that this additional circuitry will function correctly with a Zinc-Air battery, so it can be used as an alternative source. The discharge profiles are shown below.

The change between the two plateaus happens in a short space of time, but it is important the ASIC switches regulating regimes in an appropriate automatic manner. The ASIC is supplied from the battery with a voltage range of 1V-2V, depending on which technology is used. The ASIC remains connected to the battery when charging and as such it detects the charging state and disables the output until the battery is disconnected from the charger.

Three potential ASIC architectures

The ASIC has to minimise losses whilst maintaining a small size and minimum external components. The DC-DC converter architecture is critical. There are three potential candidate architectures that were considered: linear regulation, switched capacitor regulation or an inductive switcher. Whilst an inductive switcher offers good efficiency there is potential for resonance due to magnetic coupling which rules it out in this case.

For capacitive switching regulators, the Dickson topology is typically used. For the input voltage range 1.9V-1.6V a 4:3 converter topology is appropriate. This gives good efficiency in the region of 90% with only three external switching capacitors and one decoupling capacitor that is shared with the linear regulator, (used for the lower voltages). The Dickson DC-DC converter generates an output voltage as a fixed ratio of the input voltage. With an ideal 4:3 converter, a 1.8V-1.65V input is converted to 1.35V-1.25V. For lower input voltages a linear regulator converts to 1.25V.

The linear regulator efficiency is defined by the simple relationship Vout/Vbat (ignoring control circuitry power consumption). Therefore, for Vbat=1.5V, the efficiency cannot exceed 83% but for lower Vbat the efficiency will increase.

The output voltage ripple is related to the choice of decoupling capacitor and switching frequency. To minimise ripple, a high value decoupling capacitor and high switching frequency are desirable. The maximum value decoupling capacitor is limited in this application by real estate, in this case a 0201 footprint, 470nF is the maximum value. Higher switching frequencies lead to higher switching losses and hence reduce efficiency. A switching frequency of 250kHz is implemented.

All switches are integrated in the ASIC with the capacitors off chip. Switch impedances in the range 2-3Ω were required to ensure that the converter can operate efficiently in switching mode.

For Ag-Zn, as Vbat drops, the regulation target is adjusted to mimic zinc air. When a zinc-air battery is used, Vbat will always be close to the Vout target of 1.25V. To maximize efficiency, the ASIC simply hard switches the regulator pass device to allow Vout to track Vbat. In this mode the impedance of the pass device determines the voltage drop and should be optimised.

The graph below illustrates efficiency for the ASIC.

Conclusions

In order to gain market penetration and depose existing solutions it is necessary to enhance one or more of the discussed desirable battery features. By applying a traditional, reliable and safe battery technology in a new way it has been possible to significantly improve the user experience whilst also addressing eco concerns with battery disposal. The small nature of these batteries coupled with a custom ASIC has made them ideal for this demanding application.

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