I am trying to design a circuit that can charge tablet/cell phone via micro-usb port. Attached is the circuit I put together. The basic idea is, take 6 ~ 12V DC from an AC-adapter, then step the voltage down to 5V with a buck converter (AP5002, datasheet)and feed it to VBUS on USB port. Somehow this design doesn't work. It doesn't charge either tablets/cell phones. In one case, an old nook color, it was able to charge to battery to ~65% then stopped. In other cases, the tablets just don't recognize the charging circuit.
What could be the problem? My thoughts are:
1) When this circuit is attached to the micro-usb port of a tablet, there is usually a 5.1V voltage on VBUS, which is higher than 5V, the output of the step-down IC AP5002. So I put a diode in the place of L2, this doesn't do the trick.
2) USB charger is more than simply applying a 5V voltage on VBUS. There should be some controller IC.
What else I am missing here? Please advise, and thanks in advance.