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Automatic detection of USB port vs. wall adapter power

Posted: 29 Apr 2008 ?? ?Print Version ?Bookmark and Share

Keywords:Li-ion/poly battery charging? USB port? automatic detection? wall adapter? AnalogicTech application notes?

The AAT3685 is a versatile Li-ion/poly battery charge control IC capable of charging single-cell Li-ion/poly batteries at constant current rates up to 1A. The AAT3685 also utilizes an automatic charge reduction technique, allowing the device to safely operate from USB port power sources for charging batteries. In addition, this charger IC has the capability to be programmed for two discrete constant current charge levels. Collectively, these features make the AAT3685 a suitable solution for portable product applications that use both an AC wall adapter and a USB port for a charge source supply through a single input power connector.

Highly integrated portable products that use the USB port for both data communication and as a power source typically rely on their internal USB interface controller to detect the presence of a USB port connection. In this manner, a device can be set up so the battery charge controller will default to a fast charge constant current based on the capacity of a wall adapter when power is applied and no USB port host is detected. When a device is connected to an actual USB port, the system detects a USB host ID and can then set the battery charge control IC to a level at or below 500mA in order to match the current capacity available from most USB ports.

Using a USB controller to detect and set the charger IC for USB charging works well for devices that possess such capabilities. However, for low-cost products that do not contain a USB interface controller or do not have a requirement for data transfer, detecting the presence of a USB port versus a higher power wall adapter can be problematic. The AAT3685 can solve this dilemma with the addition of a simple external circuit using just two resistors and a general purpose NPN bi-polar transistor.

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