The holidays are upon us and one of the hottest gifts this season will be wearables. Style and convenience is usually what people look for. But what about the battery? Charging a battery that is smaller than your thumb is totally different than charging a battery for smartphones or tablets. Wearable electronics are usually small in size such as smart watches, sports and fitness trackers and even clothing. Due to their physical constraints, the battery size and capacity is limited, even though longer battery run time becomes more critical for good user experience.
So, are you ready to design your power management solution to achieve the longest battery run time for your wearable devices? Selecting the right battery charger is almost the first thing you need to think about.
There are four main features an ideal charger IC for wearables should have:
- Small size. When the total device size is around 20mm x 20mm and the battery itself occupies more than half of it, a 4 mm x 4 mm charger IC is no longer suitable. The latest linear charger bq25100 offers a package size of only 1.6 mm by 0.9 mm and the total solution size is as small as 2.1 mm by 2.2 mm.
- Low maximum charging current. Over 90% of the wearable devices use a battery with the capacity less than 300 mAH and the state of art power density is 150 mAH/cm3. The bq25100 supports up to 250 mA battery charge current which is sufficient to charge almost all the wearable devices on the market today within two hours.
- High accuracy in termination current control. With the limitation in battery capacity, every mAh is important for the device run time. An accurate control at a low termination level prevents the battery from terminating early and therefore extends the battery run time. For a 41 mAH battery, the charge cycle with 1 mA termination current can bring an extra 2 mAH capacity than that with 4 mA termination current, which increases the usable battery capacity by 5%. The smaller the battery, the more critical the termination control. Think about a 20 mAH battery, if you cannot control the termination current below 5 mA, then you would lose more than 10% of the battery capacity even before you start using it. The bq25100 can accurately program and control the termination current down to 1 mA, which is the best across the industry today.
- Low battery leakage current. While we are maximizing the effective battery capacity, we should minimize unnecessary power consumption at the same time. Again, bq25100 has superior performance in this area. It has maximum 75 nA spec for battery leakage current from 0°C to 125°C and it would take 152.2 years to discharge a 100 mAH battery. Therefore, in reality the current is negligible as the self-discharge rate of the battery cell is much faster than that.
Now that you have the ideal charger, let’s discuss the charging process with some exciting new technologies like wireless charging.
The TIDA-00318 is a reference design circuit dedicated to wireless charging for wearables. It has the low power wireless receiver bq51003 and linear charger bq25100 on the same PCB with a total solution size of 5 mm x 15 mm. It is smaller than a fingertip and can be easily placed inside any wearable devices on the market. Paired with TI’s wireless transmitter IC bq500212A, it is a complete charging solution that you can apply to your device.
Is there a wearable device on your wish list this year? Please leave a comment and let me know!
- For more details about our design for wearables, visit ti.com/Wearables
- Order samples or evaluation module of the bq25100 linear charger
- Order samples or evaluation module of the bq51003 wireless power receiver
- Visit ti.com/wirelesspower for other wireless charging solutions that you can considered for low power applications