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

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From: Zabinski, Patrick J. (zabinski.patrick@mayo.edu)
Date: Tue Oct 10 2000 - 11:06:11 PDT


> Can we assume that when one power supply is at a maximum the
> other two supplies
> will be at a maximum?

No.

> 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?
>

Absolutely.

The details of the answer lie in:
        1) the circuit topology, architecture, and technology; and
        2) packaging.

To determine how Item 1 affects the supplies takes a considerable
amount of effort, but in general, the core (1.8 or 2.5V)
supplies draw currents at different times and in
different directions than the I/O supplies (3.3V), so they
are not correlated.

In addition, as an IC designer, it is likely that you will
know what packaging environment the chip will go into
(note: I think IC designers *should* know, but it's rare
that they do). As such, you are likely not to have
a practical way of accurately predicting how well the
supplies are coupled (or uncoupled).

In short, if you know all the details of the IC and package,
you can then figure out the answer to your question on
a case-by-case basis. However, in the most general
case, the current draw from the various supplies are
unrelated, and you should assume all combinations.

My $0.02 worth (i.e., not much),
Pat

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