@Kybo_Ren, A resistor does limit current pull, but it also drops the voltage output and as explained earlier there is no source of 5volts inside the 770.
The battery puts out 3.7volts. This is most likely regulated to 3.3volts. The only reason the input voltage is 5volts is to allow for the regulator circuit to fast charge the battery. So if you did want to run external USB devices on the 770, you would have to built and install a 3.7volt to 5volt DC-DC converter.