# Re: [SI-LIST] : Questions abt Power Distribution System

From: Istvan NOVAK ([email protected])
Date: Fri Jan 28 2000 - 04:06:29 PST

Mr. Chang,

I am sure Ray will answer all of your questions, but since it is probably a
couple of more hours until he sees your message, let me offer my thoughts on
questions 1 and 2.

You can calculate the impedance of a real-life capacitance with a series
RLC, for bypass applications we usually neglect the parallel resistance.
The impedance of the series RLC is Z(jw)=R+j(wL-1/(wC)). If you have two
capacitors, you can add the admittances and take the inverse of it
Z=1/(1/Z1+1/Z2), where Z1 and z2 are the two Z(jw) functions as above. These
are complex numbers, and you can calculate/plot the impedances/admittances
either with SPICE, or in this simple case of having just a few parallel
capacitors, you can use a spreadsheet or MATLAB or something similar to do
spreadsheet for this purpose from http://home.att.net/~istvan.novak/
More detailed spreadsheets with up to 10 parallel capacitors and with
automatic statistical tolerance calculation is also available, let me know
if you want it.

Istvan Novak
SUN Microsystems

----- Original Message -----
From: Chang, Isaac Yew Beng <[email protected]>
To: <[email protected]>
Sent: Friday, January 28, 2000 2:50 AM
Subject: [SI-LIST] : Questions abt Power Distribution System

> Raymond Anderson,
>
> I've read your paper on Power Distribution System Design Methodology and
> Capacitor selection for Modern CMOS technology. I found it excellent and
> well written. I hv some questions here regarding to the topic.
>
> 1. From figure 12(showing the anti-resonance of parallel caps), how do you
> add up two Z-Freq response graphs to get the anti-resonance?
> 2. Say I got the R and X values of a cap for various freq(100, 1K, 10K,
> 100K, 1MHz), say, how can I plot out the Z vs Freq graph for 2 caps(same
> question as above, but it's actually asking how to calculate it and put it
> on a graph)?
> 3. For the formula in estimating the needed decoupling capacitance for a
> PDS, ie. C=I(dt/dv), how do you estimate I and dt here? I is the transient
> current, is it the Imax in the system? dt is the VRM respond time, how do
> you get dt? Is it from the VRM spec or somewhere else?
> 4. Do you do a 'anti-resonance' and Ztarget check at then end of your
> simulation for PDS?
>
> Isaac Chang
>
>
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