I have an unbranded USB charger set, which I bought from a department store here in Finland. By default it charges my 770 really slowly and my cell phones won't even start charging. However, the charger package included an adapter, which is supposed to change the USB port output from 5V to 6V. I haven't tested this with a multimeter, but when plugging this adapter between an USB port and the charger cable, both my phones and 770 are charging normally.