RE: [SI-LIST] : Inductance and Decoupling

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From: John Lipsius (jlipsius@JasmineNetworks.com)
Date: Wed Mar 14 2001 - 11:29:22 PST


Itzhak,

re: item 2, decap (decoupling cap) inductance. My understanding
from actual characterization I saw done and the si-list, is that
traces from decap to vdd pin is inherently inferior to just
taking vias to vdd plane (likewise for decap gnd).

Your results notwithstanding, by definition the L for the
decap w/ vias straight to planes must always be less than when
connecting to vdd pin, since the loop area is thus minimized.

When you instead connect decap to vdd pin then this theory says you put
much more L in series with the chips supply than with the via-to-plane
situation, defeating the broadband filtering intent if it's done for many
or all decaps.

> -----Original Message-----
> From: Itzhak Hirshtal [mailto:hirshtal@is.elta.co.il]
> Sent: Monday, March 12, 2001 6:33 AM
> To: si-list
> Subject: [SI-LIST] : Inductance and Decoupling
>
>
> 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:
>
> L=5d*{ln(2d/r)+1}nH.
>
> The other is given by Mr. Bogatin in one of his articles, and is:
>
> L=5d*{ln(2d/r)-3/4}nH.
>
> 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: hirshtal@is.elta.co.il
>
>

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