RE: [SI-LIST] : Variability of Supply voltages

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Tue Oct 10 2000 - 18:07:44 PDT


> Can we assume that when one power supply is at a maximum the other two
> supplies
> will be at a maximum?
 
No, as already described. There might be some interrelationships, but in
general, one should start by assuming that the three supply voltages are
unrelated; then later prove how and to what degree they aren't.

> Should we assume that all three supplies can be
> independent and we need to perform a maxtrix of 9 combinations to do
> a proper worst case analysis?
 
Well, that depends. If you know for some reason that the only interesting
cases to look at, are when all the supplies are at minimum, or all at
maximum, then maybe you could skip the other combinations, or some subset of
them. It may be that the other combinations are uninteresting as far as the
overall worst-case results are concerned. Or maybe not. A lot depends on
what goes on around the boundaries between the supply voltages. Margining
one low and the other high might cause significant pulsewidth shrinkage, for
example, which you would otherwise miss. To cover all the bases that
actually happen with your real circuit, you should do them all.

Andy

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