RE: [SI-LIST] : App note assumptions

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From: Mark Geddes (mark@AudeSi.com)
Date: Mon Apr 17 2000 - 07:53:57 PDT


My main point was:

1) Cypress wrote a good article on the effects of using 2 different values
of decoupling capacitors.
2) They then listed a bunch of recommendations, and sample layouts, based on
nothing in the article.

http://www.cypress.com/pub/appnotes/decouple.pdf

Read the article yourself & you will notice it "falls apart" half way
through the "Recommendations" section on page 2.

There's a popular expression, "What's that got to do with the price of eggs
in China?". In other means, what does the effect created by using two
different values of decoupling caps have to do with the routing & placement
"assumptions" listed? Cypress may have valid reasons for some of their
recommendations, but they didn't justify most of them in this article.

I'm not singling out Cypress, but app notes in general. The more of them I
read the less "real" information I find. That's why I joined this forum. I
was hoping to get "the experts" opinions.

Mark...

> Mark,
>
> I'm afraid I've thought about this some and can't get to your point.
> Would you please explain?
>
> -Eric
>
> >
> > Has anyone else noticed in this "app note" (see below) that
> Cypress has a
> > very good
> > scientific explanation about why using several values of decoupling
> > capacitors can cause problems, and then they go on to
> recommend a bunch of
> > general routing rules that they haven't substantiated?
> >
> > This is how false information is spread.
> >
> > 1+1=2, because if I have one cow in a field, & then put
> another cow in that
> > field, I will have two cows in the field. Therefore use one
> cap per power
> > pin, 1+1=2, place caps close to the IC & on the same side,
> and keep leads
> > below 6mm. Can anyone pick out the assumptions I made after
> giving an
> > example why 1+1=2?
>

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