Power Management Reference Design for a Wearable Device with Wireless Charging using the bq51003 and bq25120

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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

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.

Figure 3. Schematic of Wireless Charging Receiver Using the bq51003

1.2 Battery Charger, MCU, Radio, and Sensor Power

The BQ25120 is a highly integrated battery charge management solution that integrates the most common functions for wearable devices: Linear charger, buck output, load switch or LDO, manual reset with timer, and battery voltage monitor. The integrated buck converter is a high efficiency, low Iq switcher using DCS control that extends light load efficiency down to 10 µA load currents. The low quiescent current during operation and shutdown enables maximum battery life. The BQ25120 has an I²C interface that allows configuration of key parameters including charge current, termination threshold, battery regulation voltage, DC/DC buck output voltage, load switch or LDO voltage, pushbutton timers and reset parameters, input current limit, battery undervoltage threshold, safety timer limit, battery monitor reads, and fault conditions. The design procedure for the BQ25120 can be found in the datasheet.

Figure 4. Schematic of Battery Charger, MCU, Radio, and Sensor Power

1.3 Second Buck Output for MCU, Radio or Sensor

While the bq25120 integrates a single, ultra-low power step-down converter for one rail, some systems, such as an MCU, radio or sensor, need a second high-efficiency rail with a different voltage. For these sub-systems, a discrete ultra-low power step-down converter with similar performance to the bq25120 converter is required. PMP11311 includes a TPS62743 which contains a user-selectable choice of 8 different output voltages from 1.2 V to 3.3 V.

If the more common 1.2-V or 1.8-V rail is needed, then the pin-to-pin compatible TPS62746 may be used instead to obtain the extra feature of an input voltage switch (VIN switch). The VIN switch allows a no-leakage measurement of the battery voltage by the host MCU. More details about the TPS62743 and TPS62746 and their implementation are found in the data sheets in the references. Either device requires a total solution size of less than 10 mm².

Figure 5. Schematic of Second Buck for MCU, Radio or Sensor Power

2 PMOLED Display Power Design

• A PMOLED display is often used in the wearable device because of its low power consumption and low cost. The TPS61046 is a perfect boost converter to power the PMOLED display because of its features as following:True Disconnection between Input and Output during Shutdown.
• Small package size of 0.80-mm × 1.20-mm WCSP
• Output Voltage Up to 28 V capability and Output Over-Voltage Protection.
• Output Short Circuit Protection

At fixed 12 V output voltage condition, the device only needs three external components, as in Figure 6. More details about TPS61046 pin function, characteristics and external component selection can be found its datasheet. The method of using the device to power a PMOLED display and the performance waveforms can be found in another reference design “PMP9775”.

Figure 6. Schematic of Boost for PMOLED Display Power

3 Heart Rate Monitor or e-Ink Power Design

The TPS61240 is a high efficiency boost converter optimized for lithium-ion battery input and fixed 5-V output application. it features 3.5 MHz switching frequency and only needs three small surface-mount external components as shown in Figure 7 with solution size smaller than 13 mm². The 5-V output can be used to power the heart rate monitor module or e-Ink display in a wearable device

The function, characteristics and external component selection are found in the datasheet

Figure 7. Schematic of Boost for Heart Rate Monitor Power

4 Layout Guidelines for Wearable Design

Size is key in a wearable design and it must be taken into account when the different components are placed. In order to follow the power flow the layout is started from the wireless receiver to the battery charger and finishing on the buck and boost for the different power rails provided.

4.1 Wireless Receiver (bq51003)

• In-via pads are required in this device. Via interconnect on GND is critical for thermal performance.
• Place the AC capacitors (C6, C7, C8) close to the coil connection keeping the trace thick to lower its resistance.
• Output and RECT capacitors should be placed close to the OUT and RECT pins in the IC.
• BOOT, COMM and CLAP capacitors should be placed close to the pins. If vias are required, it is recommended to shield the traces from sensing traces to avoid interferences.
• Preferably provide a ground copper area underneath the sensing traces, REC, ILIM, FOD to shield them from the power and noisy traces.

4.2 Linear Charger (bq25120)

• Input capacitor (C21) must be placed close to the IC input pin. It is recommended to place the BAT capacitor close to the pin. Therefore, in this design the input trace coming from the output of the wireless receiver is connected through a via in an inner layer in order to reduce the size of the solution.
Figure 8. bq25120 Capacitors Placement
• The inductor should be placed close to the SW pin to reduce the size of the switching node.
• The output capacitors for the power rails (SYS, PMID, LS/LDO) need to be placed close to the pins.

4.3 Buck Converter (TPS62743/6)

• The input capacitor must be placed close to the Vin pin of the IC.
• The switching node should be as short as possible.
• Connect the output cap with a trace –no via- away from the SW node and noisy signals.

Figure 9. TPS62743 Layout

4.4 Boost Converters (TPS61046 and TPS61240)

• The switching node should be as short as possible.
• It is recommended to place the input capacitor close not only to the VIN and GND pins.
• The output capacitor must be placed close to the IC and it is recommended to be close to the ground pin. If possible, the ground for the input and output capacitor should be on the same plane. In the TPS61046 it was not possible to follow this rule due to the placement for reduced size. Therefore, a solid ground with vias was provided on the next layer making sure the connection is adequate.

Figure 10. Capacitor Grounding TPS61046

Figure 11. TPS61240 Layout