USB Charging Voltage/Current

[olsenn]

I have a device that outputs 5.2 volts under no load; however, drops down to 3 volts (or lower) by the time an amp or so of current is being drawn. My question is, for USB charging devices, how do they determine how much current to use?

Will my phone for, instance, use as much current as it can without bringing the charger voltage below 5 volts, or do these devices drain a fixed amount of current and expect the charger to be able to maintain 5 volts at that current?