Re: PCI card current loads

["Thomas Schutt" <Thomas_Schutt@splashtech.com>]

Kirk,

The worst case dynamic load for an add in card will most likely be found 
on a card with large amounts of memory, like a graphics or printer 
interface. Both of these types of cards use large amounts of RAM, some 
over 256 Mbytes. Most of the dynamic load comes from cycling the memory. 
In general this means the variation in current draw is highly dependant 
on the software.

I have seen add in cards with stand by currents in the 2-3 amp range 
that, depending on the software, could easily exceed the 5 amp limit. 
And we all know there are companies that bend the rules to get products 
out. For example I measured a card once that would sustain over 12 amps 
running a memory diagnostic. On paper, if just the DRAM chips were worst 
case it could draw 20 amps! The drivers were written to keep the card 
under 5 for normal user applications (read "the ones they tested").

For your power system I would assume a worst case add in card to be 
virtually 100% DRAM. And assume the designer put enough bypass on the 
card to keep the power rails with in the operating limits of the DRAM.
I think you will find the worst case dynamic load is getting worse with 
each new generation of DRAM. I believe it is possible to build a card 
using CMOS gate arrays and DRAM with a stand by current of less than 1 
amp that will step to a sustained 5 amps with PCI burst transfers. 

Designing a system for worst case power in all the PCI slots will most 
likely be very expensive. Most desk top systems do not do it. You may 
wish to review the types of add in cards you are planning to support and 
design for them.

- tom_schutt@splashtech.com
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