Power Management Reference Design for a Wearable Device with Wireless Charging using the bq51003 and bq25120
Trademarks
All trademarks are the property of their respective owners.
Power Management Reference Design for a Wearable Device with Wireless Charging Using the bq51003 and bq25120
Wearable devices require advanced power management to achieve long battery run times with always-on functionality. Additionally, the devices need to use small rechargeable batteries and enable small footprint designs. This application note shows the implementation of a scalable power management solution for wearables that can be tailored for activity monitors, watches, and more. The design provides a wireless charging input, highly configurable battery management solution with Li-Ion battery charger and low quiescent current (Iq) DC/DC buck, boost converter for PMOLED display, boost converter for Heart Rate Monitor (HRM), and low Iq DC/DC buck.
Figure 1. PCB Top Assembly 
Figure 2. Block Diagram of Wearable Power Management 1 Wearable Power Design
Table 1. Wearable Power Requirements
| DESCRIPTION | VALUE |
|---|---|
| Input Voltage (VIN) | 5 V USB input or Qi Wireless Transmitter |
| Input Current | Up to 500 mA |
| Output Voltage for Li-Ion Battery | 3.6 V to 4.65 V |
| Fast Charge Current For Li-Ion Battery | 5 mA to 300 mA |
| Termination Current for Li-Ion Battery | 500 µA to 37 mA |
| Output Voltage for MCU (SYS rail) | 1.8 V nominal (Adjustable from 1.1 V to 3.3 V) |
| Output Current for MCU (SYS rail) | Up to 300 mA |
| Output Voltage for Second Buck Rail | 1.8 V |
| Output Current for Second Buck Rail | Up to 300 mA |
| Output Voltage for PMOLED Display | 12 V |
| Output Current for PMOLED Display | Up to 100 mA |
| Output Voltage for Heart Rate Monitor | 5 V |
| Output Current for Heart Rate Monitor | Up to 300 mA |
| Output Voltage for Sensors or Radio (LDO) | 0.8 V to 3.3 V |
| Output Current for Sensors or Radio (LDO) | Up to 100 mA |
1.1 Wireless Charging Input
A large number of low-power wearable devices such as smart watches, fitness wrist bands and headphones are adopting wireless charging. The BQ51003 is an advanced, integrated receiver tailored for wearable applications. A standard Qi-compliant design will deliver 5W with a 50-mm coil. Figure 3 is modified from a Qi-compliant design with a smaller 30-mm coil and adjustable 500 mW to 1500 mW capabilities. When used with a Qi-compliant wireless transmitter, the RX_OUT supplies the input to a Li-Ion charger, in this case the bq25120. This better matches the wearable form factor and battery requirements, and is optimized for the device to stay cooler during power transfer.
