Overcome the design challenge of low quiescent current in small battery-powered devices

Thanks to advances in miniaturization, Bluetooth communication and embedded processing, modern hearing aids have more functions than ever before, from streaming music to being able to adjust hearing amplification through apps on smartphones.

Thanks to advances in miniaturization, Bluetooth communication and embedded processing, modern hearing aids have more functions than ever before, from streaming music to being able to adjust hearing amplification through apps on smartphones.

However, there is a price to pay for these enhanced functions: modern functions require more power. The increase in power consumption is a challenge for engineers designing hearing aids, mainly because the old version uses disposable zinc-air batteries. As shown in Figure 1, the battery life of these batteries is usually about two weeks. But when adding more features to hearing aids, such as enabling them to play music, the battery life may be reduced to hours. Therefore, engineers use rechargeable lithium batteries in the design of next-generation hearing aids (Figure 2).

Overcome the design challenge of low quiescent current in small battery-powered devices

Figure 2: The next-generation hearing aid rechargeable lithium battery with a compact form factor and smart phone connection increases the complexity of the power system in many ways. The most important thing is how to charge the battery safely and accurately. There are additional design considerations when using two hearing aids. Because the left and right earphones are not physically connected, they cannot be charged via a single cable at the same time. Therefore, almost all new hearing aids are now equipped with boxes with charging and storage functions.

The box is designed with a specific jack for each earphone to ensure proper charging. The charging of earphones must be accurate, because rechargeable hearing aids are usually 25 mAh-75 mAh, and the range of charging boxes is 300 mAh -700 mAh. This means that before the box needs to be recharged, the headset will last about 24 hours and the charging cycle will be about 10 times.

With the charging case, hearing aid designers can now consider using three different lithium batteries: one for the charging case and the other two for the earphones. The choice of battery charger plays an important role in the design.

It should also be noted that charging the battery through the battery (ie, charging the headset battery through the charging box battery) is not as simple as charging through a wall socket, because the voltage difference between the two batteries will not be very large. There must be an internal circuit to increase the voltage difference between the charging box and the headset to achieve full charging. As the battery discharges, its voltage is slowly decreasing. Observing the discharge curve shown in Figure 3, when the battery capacity is about 50%, the charging box voltage is about 3.6 V. But this means that if there is no boost, the charging box can only charge headphones up to 3.6 V, even if the energy stored in the charging box is enough to fully charge them.

A sample of the battery discharge curve of a lithium-ion battery; the typical average point voltage is 3.6 V, and the end-of-discharge voltage is 3 V (“characteristics of rechargeable batteries”). In this case, most engineers will consider stepping up cautiously. Although cautious boost is indeed effective, it usually increases solution size and inefficiency by adding additional boost and Inductor components to the power architecture.

To overcome these challenges, consider portable charging supported by quiescent current. For example, TI’s BQ25619 battery charger and BQ25155 linear charger support charging without external boost. In hearing aid applications, you can put BQ25619 in the charging case and BQ25155 in each earphone.

Moreover, it is not always necessary to boost the output of the charging box to 5V. By using the boost function of the BQ25619, it can be increased to the minimum voltage required to leave sufficient headroom between the charging box and the headset battery. This reduces unnecessary boost power loss and also increases earphone charging efficiency because of the reduced voltage difference.

BQ25155 is very suitable for headphones, because its minimum 3.4V input voltage can achieve longer charging without boosting, and its 43μA quiescent current can increase battery runtime. At the same time, the 7μA quiescent current of BQ25619 in factory mode can maximize the service life of the charging box. The 20mA charge termination current of BQ25619 enables it to charge small batteries with a 7% increase in capacity.

The good news is that these advantages are not limited to hearing aids: all dual-battery device systems, including earplugs and wearable patches, can benefit from these innovations. TI will continue to use the dual charger configuration in future designs, and its features are as follows:

・ Provide more efficient charging for earphones and charging boxes, while providing battery monitoring and protection, and reduce the total bill of materials through integrated boost.

・ Only one communication line is needed to reduce the number of pins for earphones and charging boxes.

With BQ25619 and BQ25155, you can improve the number of charging cycles that can be extracted from the charging box without increasing cost or solution size.

The Links:   LM64P103 SKM300GAL063D

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