USB-powered lithium-ion battery charger

With a maximum power rating of 5.25V/500 mA, the USB power bus is an excellent source for charging single-cell lithium-ion batteries. The circuit in Figure 1 illustrates how to build a USB-powered single-cell lithium-ion battery charger using National Semiconductor's LM3622 lithium-ion battery charge controller.

The battery charger circuit is designed to operate as a high-power USB function. To comply with the USB specification (version 1.1), the high-power function must not draw more than 500 mA from the bus during normal operation. The LM3622 uses a 0.25 ohm current-limiting resistor R1 to set the maximum charging current to 400 mA. This leaves a 100 mA margin for supplying the USB control circuitry and other functions within the device.

High-power USB functionality also includes additional current limiting, applicable during system startup or when a device is first connected to the active bus. Until the device is correctly configured by the USB system, it may not draw more than 100 mA of current from the bus. In the design described above, National Semiconductor's LM3525 USB power switch isolates the battery charging circuitry from the bus during startup to prevent charging current from overloading the bus. When the port is correctly enumerated, a USB control signal enables the LM3525 switch, connecting USB power (VBUS) to the charging circuitry. In addition to switching, the LM3525 also provides overcurrent and undervoltage protection for the design.