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RE: How to power a PCI card from either 3.3V OR 5V..

>  On the board there is going to be a 5V to 3.3V regulator, but if the
>host is providing 3.3V power in the system, then I want to turn off the
>output of the 5V-3.3V regulator and power the board from the hosts 3.3V

Why not just use your on-board regulator all the time?

There is some advantage to using a common supply if your card will be in the
3.3V signaling environment, since both output and input threshold levels
track the supply voltage in that environment.  However, not in the 5V
signaling environment.

A common supply also avoids power sequencing problems, but if your card is
going to sense and switch supply voltages, it may have the same problems