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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.

usb-charging-circuit-with step down IC AP5002

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  • \$\begingroup\$ To help other people answer your question, could you ask a specific question? electronics.stackexchange.com/help/how-to-ask "What else am I missing here" is a very broad question. A better question would focus on your exact problem. \$\endgroup\$
    – Voltage Spike ♦
    Commented May 13, 2016 at 17:28

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USB chargers usually aren't chargers in the sense that they control the charging of the device. They really just supply current at a specific voltage, and the device does all of the complicated parts of actually charging the battery. So, all the so called "USB chargers" are really just 5 volt powersupplies.

USB does have one wrinkle, though. USB is only allowed to provide a very low current until the device and the host have negotiated over the data bus for a higher current to be supplied. Well behaved USB devices therefore will not draw a high current from USB port.

Since your powersupply can't negotatiate with the USB device, the devices draw only low current from the supply - it takes a long time to charge or doesn't charge at all.

The good news is that there are standards that make it possible for a USB device to recognize a "dumb" USB host that can provide high current.

The simplest is to just short D- and D+ together. The USB device will then recognize that you have a powersupply that can deliver high current with out negotiation.

There are other standards (there's 200 ohm from D- to D+ and there's a slightly more complicated Apple standard) but many devices work with the shorted D- D+.

This site goes into some detail on implementing the various standards.

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  • \$\begingroup\$ Thanks for the answer. I will then try to add some controller IC to do the negotiation. Shorting D+/D- is not an option for me. \$\endgroup\$
    – Zhiyong Li
    Commented May 13, 2016 at 16:41

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