[SI-LIST] Re: Decoupling capacitors
- From: "Martin Euredjian" <martin@xxxxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Sun, 23 Jun 2002 15:57:43 -0700
Abe Riazi wrote:
...
> ... in some other cases it is preferable to use multiple values of
> capacitance (with low ESL) to satisfy a desired flat low PDS impedance
over
> a specified/broad frequency bandwidth.
...
I've modeled (and, yes, models can't always be trusted) alternatives with
hundreds of caps of different values and packages covering a range from low
frequency to high frequencies and in various combinations. My conclusion
from this experience is that you cannot achieve a "flat" impedance over a
frequency range (probably obvious) and that the "shotgun" approach of
peppering the board with a wide range of values may not produce a usable
performance change. This is due to the interplay of the RLC components in
any capacitor producing the familiar V shaped impedance curve. You can get
an impedance curve that has all these nicely spaced notches, but flat it
will not be.
And then the question is? does it really need to be flat? And super low?
I mean --oversimplifying-- once the PDS can can deliver the required current
within a given frequency range, why go any lower? Why introduce all these
notches with dozens of capacitor values?
The PDS I now have on paper consists of 16x 150uF, 0.1ohm tantalums and 73x
0.01uF, 0402 chip caps. On paper, and in theory, this PDS can deliver an
impedance of less-than 0.01ohms from about 20KHz to about 200MHz and
less-than 0.05ohms from 200MHz to about 1GHz. And, all else being adequate,
this would mean a theoretical current delivery capability of at least 60A at
3.3V from 20KHz to 1GHz ... with two capacitor values!
Of course, I know that this is far from reality. PCB trace, layout, package
effects, etc. getting in the way of perfection. Mother Nature always wins.
However, it does beg the question: What do you really need in order to
achieve the required frequency-current budget? My gut feeling is that a
couple of well-chosen capacitor values along with good layout/placement
should do for all but the most esoteric applications.
===============================
Martin Euredjian
eCinema Systems, Inc.
===============================
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