Just looked at the mbed schematic and spec and it looks like VU output can support 5V/500mA only when USB cable inserted. Power from VOUT is at 3.3V fed from VIN via LT1117S33 regulator which is a 800mA rated part (http://cds.linear.com/docs/Datasheet/1117fd.pdf) however, with Digital I/O max at 400mA, this leaves only a potential 400mA.

Possible issue is that looking back through mbed spec a question was posted as below:

which is the max load I can drive with the 3.3 regulated output? My GPS application consumes around 120mA but this seems to be to much for the LPC1768. The datasheet of the used LD1117S33 promised up to 800mA, so I was hopeful to drive my app directly, but now I need obviously an external power supply :-(

Technical Reference

Power •Powered by USB or 4.5v - 9.0v appiled to VIN •<200mA (100mA with Ethernet disabled) •Real-time clock battery backup input VB •1.8v - 3.3v Keeps Real-time clock running •Requires 27uA, can be supplied by a coin cell •3.3v regulated output on VOUT to power peripherals •5.0v from USB available on VU (only available when USB is connected!) •Current limited to 500mA •Digital IO pins are 3.3v, 4mA each, 400mA max total

Mbed has two 3V3 regulators. One is for Vout, the other provides power to the processor and other onboard stuff. Max current is often not the limitation. The problem is max dissipation in the regulator. This depends on current x voltage drop across the regulator.