The Universal Serial Bus (USB) has become one of the most widespread and convenient ways to connect electronic devices to the PC. Countless modern portable products with built-in USB connectors readily use the USB data bus to transmit and receive data to and from PCs, but many of these battery-powered units still use a separate power supply for battery charging (often a charging cradle or a simple AC/DC converter). Sometimes overshadowed by its data bus partner, each USB connection also contains a power bus.

With a maximum power rating 5.25V/500 mA, the USB power bus is a great source for charging a single-cell Lithium-Ion battery. The circuit in Figure 1 shows how to build a USB-powered single-cell Li-Ion battery charger using National Semiconductor’s LM3622 Li-Ion Battery Charger Controller.

Circuit uses existing USB power-bus to charge a single-cell Li-Ion battery.

USB-Powered Lithium-Ion Battery Charger

Circuit for USB-Powered Lithium-Ion Battery Charger. Credit: National Semiconductor

The battery-charger circuit is designed to operate as a high power USB function. To be compliant with USB Specifications (Rev. 1.1), a high-power function must not draw more than 500 mA from the bus during normal operation. The LM3622 uses the 0.25Ω current-limit resistor R1 to set a 400 mA maximum charging current. This leaves a 100 mA surplus that can be used to supply USB control circuitry and other functions in the device.

There are additional current restraints on a high-power USB function that apply during system start-up or when a device is initially connected to an active bus. Until a device is properly configured by the USB system, the device may not draw more than 100 mA from the bus. In the above design, National Semiconductor’s LM3525 USB power switch keeps the battery charger circuit isolated from the bus during start-up so that the charge current does not overload the bus.

When the port is properly enumerated, a USB control signal enables the LM3525 switch, connecting USB power (VBUS) to the charger circuit. In addition to on-and-off switching, the LM3525 provides over-current and under-voltage protection to the design.

The circuit designed with minimum voltage drop will allow the charger to function at the low USB power bus voltage.

Thus, the circuit presented is able, under most conditions, to utilize the power supplying capabilities of the USB bus to provide easy and convenient Lithium-Ion battery charging.

Source: National Semiconductor Application Note “A USB-Powered Lithium-Ion Battery Charger” by Brian Conner.