I am still trying to wrap my brain around all the usb charging specs but usb 2.0 will probably not charge your phone. 500ma just isn't enough juice. Go to Amazon and search Mediabridge car chargers. Read the reviews and it explains a lot about iphone standards, low power standards, high power standards, data pin shorting and all the other complicated stuff. I still don't know the differences between usb 2.0 and 3.0 but there are differences. There are differences in data and charging cables was what I got out of it. The only way to get high power is short the data wires and I still don't know if computer usb will handle it. I'm not going to experiment either.
ATT N3