**From:** *RMELLISON@aol.com*

**Date:** Fri Nov 17 2000 - 13:11:38 PST

**Next message:**Ken Cantrell: "RE: [SI-LIST] : Capacitor dielectric type"**Previous message:**RMELLISON@aol.com: "Re: [SI-LIST] : Capacitor dielectric type"**Maybe in reply to:**Kowal, Keith: "[SI-LIST] : deCoupling caps and there placement"

Jun,

I agree with what you say, but keep in mind that the current is being

supplied from all the capacitance surrounding an IC power pin, which includes

the current stored in the dielectric material of the power-ground sandwich.

The propagation time of the sandwich is a function of the dielectric--this is

the same for either a stripline or a power plane. If the current requirement

(switching time) is slower than the prop time to the nearest decoupling cap,

the cap keeps a voltage hit from occuring at the IC. If the di/dt is faster

than the prop time to the cap, a voltage hit will occur at the IC. This

assumes that the current stored in the sandwich is insufficient to supply the

requirements of the IC.

I've analyzed the transient response of power planes to fast current changes

by considering the power plane sandwich to be providing current from all

directions (the pin is at the center of a circle) and locating the capacitors

at different locations to see their effects. An easy way to use Spice on

this is to consider concentric circles about 1/4 inch apart, and calculating

the value of capacitance for each circle. Since propagation time is constant

for the dielectric used, and is equal to sqrt(LC), the inductance and

impedance of each section can be calculated and a transmission line model

established. This gives a linear representation of the circular effect.

Pspice can handle a transient analysis on this circuit quite easily.

Richard Ellison, consultant

RMEllison@aol.com

972-569-8317

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**Next message:**Ken Cantrell: "RE: [SI-LIST] : Capacitor dielectric type"**Previous message:**RMELLISON@aol.com: "Re: [SI-LIST] : Capacitor dielectric type"**Maybe in reply to:**Kowal, Keith: "[SI-LIST] : deCoupling caps and there placement"

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