# Re: [SI-LIST] : deCoupling caps and there placement

From: [email protected]
Date: Fri Nov 17 2000 - 13:11:38 PST

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
[email protected]
972-569-8317

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