My hardware needs to be able to be powered from both USB (during configuration with a PC) and from a power pack (out in a field). The power pack will use a 5V regulator to power the mbed.

I also have an external IC that needs a 5V power supply and I want it to work regardless of the power source. It draws 2mA maximum.

Is there any reason why I cannot combine the external 5V supply with the USB 5V supply at the VU pin using two schottky zener diodes? Of course I need to take into account the voltage drop across the diodes.

thanks, Andy

If you use a pair of 1A-rated schottkys, it should work fine, but I would use a 220uF or more cap at the output of these two.

Panasonic FC, or other low impedance electrolytic.

This is to ensure that if the two input voltages are both present, and nearly equal, they don't swap from one source to the other as load changes - actually, they may still swap, but slower, and it's fast swapping that's worth avoiding for noise reasons.