Modifying a USB-CLA to support DCP

Written 2013-02-21


Today I was on the highway for about three hours, trying to travel about 15 miles. While waiting for the road to clear up, I tried to charge my phone with a cigarette lighter adapter(CLA) to USB port I had purchased some time ago, but had never used - my phone refused to charge.

The device contained a few capacitors, a coil, and what appeared to be a switching regulator. When I traced the USB data pins on the PCB, I found two pull-down resistors. Each data line pulled low with a 15kOhm resistor is the normal configuration for a USB1.1 host.

My phone will charge from a PC USB port or from a USB dedicated charge port(like the wall-wart that comes with most phones, D+ and D- shorted together). However, if the host port is configured for communication, and the phone cannot enumerate, then it will not charge.

The fix for this ended up being very simple. After reassembling the CLA, I tore a gum wrapper down to a rectangle about 2.5 USB pins wide. This gum wrapper, inserted conductive-side towards D+ and D- of my USB cable, convinced my phone that the CLA was a real USB-DCP, and I was finally able to charge it.