I once wrote an article about wearable devices and how to make them more powerful and smaller. Well, due to innovations in battery charging, low quiescent current operation, smart power management, and high integration, wearables will become smaller, more feature-rich, and run longer.
The battery charging of wearable devices is a challenging problem because of the necessity of using small, low-capacity batteries as the power source. For example, when you use batteries of different capacities, such as 40mAh, 100mAh, or 200mAh, and you want to charge the battery at 0.5C, 1C, or 2C, in order to achieve the goal of fast charge and charge cycle life, The charging current will change greatly. The key is to include a programmable fast charge current in your design. The bq25120 battery management solution offers a 5mA to 300mA charge current setting to support multiple battery and charging system configurations.
Although it is important to be able to charge the battery quickly, it is also important to transfer the power as much as possible into the battery. In order to achieve this goal, the termination current must be very accurate and can reliably terminate the charging below 1 mA or less.
The battery must power the microcontroller (MCU), radio, and sensor between charges. To maximize the battery life, you need to pay more attention to components with low power consumption and very low power consumption at turn-off. Since the buck converter is the MCU's power supply and must be running at all times, it is the most important device for achieving low quiescent current (Iq) operation. Consider using a product such as the bq25120 that integrates a very low current DC/DC converter. This product is connected to a 1.8V voltage rail, and when the MCU is powered without a load, 700nA Iq can be achieved. If your wearable device requires an extra low Iq buck, the TPS62743 is a good choice.
Some sensors and radios are not always used and can be turned off completely, requiring a low leakage shutdown mode. The bq25120 integrates a load switch that turns the component off when it is not in use and can be configured as a regulated LDO output when needed.
Some wearable devices have a display or heart rate monitor (HRM) that requires a boost converter. Different display flat-to-voltage requirements are not the same, so the most flexible solution is to use discrete components to achieve the boost operation. If the display is an organic light-emitting diode (OLED), consider using a device similar to TPS61046, which provides 12V and is small and has a low Iq. The TPS61240 is ideal for providing 5V if the display is an LCD, electronic ink display, or heart rate monitor (HRM). The HRM requires 5V power to power the LEDs, and the TPS61240 has a very low leakage disconnect switch that can be completely turned off when the LED is not in use.
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