[SI-LIST] Re: decoupling
- From: "Ken Cantrell" <Ken.Cantrell@xxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Thu, 12 Jul 2001 13:12:36 -0600
Ray,
Yes, delta I. I pulled it from HJ's book.
Great explanation by the way.
Ken
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Ray Anderson
Sent: Thursday, July 12, 2001 12:25 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: decoupling
Mark-
I'm sure Ken will reply, but since I'm familiar with what's going on
I'll respond for now:
1) the .02 in the equation in Ken's message stems from the fact
that he is deriving a Target Impedance based on a 2% variation
in voltage. It could have just as well been 5%. The percentage
of allowable ripple is determined by the designer based on
how much noise he/she thinks the circuit can tolerate.
2) Not sure if Ken really means di/dt or not. In the equation
published in our paper we mean (delta I) for the current
and don't assign any time dependency to it. (Ken can you
comment?)
2) A similar equation was first widely published in a paper in the
IEEE Transactions on Advanced Packaging August, 1999
pp 284-291 entitled "Power Distribution System Design
Methodology and Capacitor Selection for Modern CMOS
Technology" by L. Smith, R. Anderson, D. Forehand,
T. Pelc, and T. Roy.
(paper downloadable at:
http://www.qsl.net/wb6tpu/si_documents/decap_whitepaper.pdf )
3) The equation is based on good old Ohm's law (R=E/I)
(Vdd * % ripple)
Ztarget = ----------------
dynamic current
For example:
suppose your system runs on 1.8 volts DC and can tolerate 2% ripple
The maximum dynamic current swing is from 15 to 20 amps
then: (1.8 * .02) 36 mV
Zt = ---------- = --------- = 7.2 mOhms
(20 - 15) 5 amps
The equation (at least the one published in our paper
in 1999) assumed a constant target Z from DC to daylight. Our
view on that has changed since the original publication, but that
is a different conversation....
-Ray
>
>Ken,
>
>I've played around with your equation below, but I'm having trouble getting
>anything meaningful out of it. Did this come from a book somewhere that
has an
>explanation, or is this your own? If it's the latter, do you have a
>derivation?
>
>A couple of specific questions:
>
>is 'powersupply' your VCC voltage?
>where did 0.02 come from? Is this 5% variation divided by 2?
>
>Thanks a lot,
>mark
>
>
>Ken Cantrell wrote:
>
>> I use Ztarget = (powersupply*0.02)/di/dt, and have stopped using COGs
(ESL
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