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

[Jesse Chamberlain <JChamberlain@mapletree.com>]

Hello, 

I am developing a PCI card that uses all 3.3V parts, but since some PCI
systems don't have 3.3V power, I need some way of sensing what voltage is
there.  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
power.  If there is no 3.3V power from the host, only 5V, then I want to
power the board off of the 5V-3.3V regulator.  

Has anyone done this, or have any ideas on how to accomplish this?  I was
thinking about doing a comparator driving 2 FET's opening or closing the
power paths.

Thanks alot for your time.

-------------------------
Jesse Chamberlain
Hardware Design Engineer
Mapletree Networks Inc.
email:  jchamberlain@mapletree.com
Phone: (781) 751-2412
   Fax:  (781) 751-2470