[SI-LIST] : Inductance and Decoupling

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From: Itzhak Hirshtal ([email protected])
Date: Mon Mar 12 2001 - 06:32:47 PST

Hello, all

I've recently started to calculate the de-coupling needed for
efficiently supplying the spike currents needed by high-speed devices.
During this task, I've encountered several ambiguities and results that
I would like to share with you and perhaps hear some (useful) feedback
from you.

(1) I tried to evaluate the situation for one high-pin-count device with
several buses connected to it (essentially a bus bridge). Even
calculating for just one synchronous bus (with 144 bits overall) I
arrived to the result that a few Amps (maybe even 5) are drawn when all
or most of this bus bits change state. I wonder what will be the result
if I would calculate for an additional bus (assuming it's synchronous
with the first). And what about the internal changes? They might be
contributing even more than the external bus! (e.g., the Motorola
PowerPC HW manual states that 90% of the power consumption of this
device is drawn internally, not externally).

(2) I've also tried to calculate the inductance of the decoupling
capacitors connections to the device. Even assuming a 40-mil wide 50-mil
long trace right above a reference plane for the connection I have app.
L=150-200pH. If I can't connect at least one of the capacitor pads so
short I might have to do a direct connection via to a reference plane. I
calculated this to have more than L=1nH!

(3) I assumed the calculated peak currents change at a rate equivalent
to the rise time of the device's output buffers. I don't know if it's
true, but this seems to me the most logical thing to do. Even if I take
it to be 2ns (1 ns is closer to worst-case, I believe), I get the
result that I need 40 to 50 low-ESL decoupling capacitors for the case
where L=1nH. Only if I succeed to connect the capacitors directly and
close enough to both GND and VDD pins (L=150-200pH) do I get the result
that it is sufficient to use 4-6 decoupling capacitors.

(4) While calculating vias inductance, I've encountered 2 similar but
different equations for this parameter. One is given by Mr. H. Johnson
in his famous book (page 259), as follows:


The other is given by Mr. Bogatin in one of his articles, and is:


Can somwone explain the reason for the difference, or who is right? The
difference starts to be quite critical when dealing with u-Vias!

Thanks for anyone who makes the effort to read this email.

Itzhak Hirshtal
Elta Electronics
POB 330 Ashdod
Israel 77102
Tel: 972-8-8572841
Fax: 972-8-8572978
email: [email protected]

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