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

[Rick Collins - Arius <PCImail@arius.com>]

At 02:19 PM 3/28/00 , you wrote:
>The same reason it was stupid to not require 3.3V supply on PMC
>slots in the first place, 3.3 Volt regulators dump a lot of heat.
>Like, if you need 5A @ 3.3V (16.5W, well less than the 25W max)
>then the regulator will be dissipating 8.5W, then the total is
>25W. If the 3.3V can be used instead of regulating the 5V down
>that would be a tremendous thermal savings, increasing the range
>of operation and reliability greatly. Putting a switch on a PMC
>board to use 3.3V if available makes sense. Yes, it is a hassle
>and can have sequencing, etc. problems. Probably easier just to
>use a switching regulator, which still wastes heat, takes up
>board space, and makes noise, but given that one can not rely on
>3.3V supply on PMC slots, the 3.3V pins are pretty much worthless
>(except for AC bypassing ;-)

A 3.3v regulator only dumps 8.5 watts if you are using a linear regulator.
A switching regulator can be as high as 90% efficient dissipating as little
as 1.8 watts. So using a regulator is not a real burden. 

But the original question of why not use the regulator at all times is
valid in any case. If you have to use the regulator when you don't have
3.3v available and you can get away with the extra heat dissipation, then
why do you need to switch it out when you do have a 3.3v supply available?
You will have already designed the board to handle the higher dissipation,
so what will the lower power accomplish, longer battery life?  ;)



Rick Collins

rick.collins@arius.com


Arius - A Signal Processing Solutions Company
Specializing in DSP and FPGA design

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