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 0.25 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.
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