From: Doug Brooks ([email protected])
Date: Wed Nov 10 1999 - 12:23:18 PST
At 12:16 PM 11/9/99 -0500, you wrote:
>When using power/ground planes as capacitors for decoupling, decreasing the
>distance between them increases the capacitance. Is there any reason why you
>should not use the minimum possible spacing? In other words, is it possible to
>have too much capacitance? How low can resonance occur? (Disregard, cost and
>manufacturing issues.)
Absent fab cost, it's hard to imagine "too much" capacitance can be made
available in a practical sense. I wish we all could ignore fab cost!
>
>What is the effective area for this type of capacitance? If you have 10"x10"
>planes with one device in the middle, how much of that 100sq" will help
>decouple
>the device? What if the device is at the edge or corner of the planes?
Let me offer one way to look at this. Assume a linear rise time of, say, 1
ns. And assume the propagation time is 6"/ns. At first glance, this would
mean that all the stored electrons would have to be within 6". But you will
need 50% of them in the first half ns. So half of them must be within 3".
Similarly, 1/4 of them must be within 1.5". And any that are more than 6"
away simply aren't going to participate in the rise time. Assuming the
charge is distributed evenly across the plane capacitance, that gives an
idea of how much of the plane capacitance might be available for decoupling.
Doug Brooks
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