# Re: [SI-LIST] : Number of power pads for a chip, looking for rule of thumb.

From: D. C. Sessions ([email protected])
Date: Tue Jan 23 2001 - 06:51:45 PST

On Monday 22 January 2001 08:40, Yehuda D. Yizraeli wrote:

# Looking for a rule of thumb for the number of power pads for a chip running
# 135Mhz, 32Bits, driving at slopes of 1nSec to 3.3V 25pf.

I don't see why you'd hang a 25 pf capacitor on the pin :-)

OK, the rule that I use goes like this:

Calculate your timing margin w/o regard to SSO pushout.
Using an IBIS model, determine the slow-case drive into your line impedance.
Translate this into an equivalent resistance (e.g., 3.0 volts/30 mA = 100 ohms)
Determine your equivalent supply inductance per pin. Take mutual inductance
into account only if you have a reliable model for it.

N = Tmargin*R/(2*Lpin)

So if you have 1400 ps of margin, a 100 ohm slow-case drive/line combo,
and 7 nH per supply pin you get 10:1:1 signal:power:ground, which is probably
higher than you would want to use for other reasons.

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