Hi,
Just to be clear, the on-board regulators are there to prevent damage to any of the on board components by providing a clean, stable power supply, and their output is indeed fixed at 3.3v
The bottom end of the Vin range is 4.5v and this value is determined by the LD1117S33. We set it to be 4.5v because the regulator output is 3.3v and the regulators *worst case* drop out is 1.2v. You might find that you can still power your mbed below that.
The top end of the range is set by heat dissipation. At 9v, the regulator is dropping 5.7v. As the mbed could be drawing anything up to 220mA (with LPC1768 and interface devices active, current being drawn from GPIO pins, the Ethernet Phy and oscillator active, etc) the heat dissipated is 5.7v * 0.22A = 1.25w
There are many scenarios when it will be perfectly safe to go above 9v on Vin, but if you do, you need to consider the heat issues first.
Hope this helps!
Is is possible to change Vin and Vout range in software? This would avoid needing more board space for op-amps in my application. Can it be done?? Tnx...
Gary B