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'[EE]: Decoupling Caps'
2003\10\12@230407 by Josh Koffman

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Ok, I need some help. Do decoupling caps have to be electrolytic? Is it
better to have some smaller value caps, rather than none at all? Do they
need to be polarized? I'm trying to design a super small circuit, and
the large electrolytics I usually use are too large. I have some
salvaged SMT ceramics that would be cool.

Josh
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2003\10\12@231445 by David Duffy

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Josh Koffman wrote:

>Ok, I need some help. Do decoupling caps have to be electrolytic? Is it
>better to have some smaller value caps, rather than none at all? Do they
>need to be polarized? I'm trying to design a super small circuit, and
>the large electrolytics I usually use are too large. I have some
>salvaged SMT ceramics that would be cool.
>

I usually have a 100n (0805) and a 47u 5mm (?) can for surface mount boards.
David...

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2003\10\13@002215 by Ken Pergola

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Hi Josh,

I'm speaking in general terms here since we don't know much about the
specific application:

If these capacitors will be used as local IC decoupling capacitors, it would
be rare that you would see electrolytic capacitors used. Usually you will
see ceramic (most likely) or tantalum capacitors for local IC decoupling
capacitors.
(Keep them as close to the ICs as possible -- keep the lead lengths or PCB
traces as short as possible).
Common capacitor values for decoupling ICs are 0.01 uF and 0.1 uF.

If you do happen to use polarized capacitors, you definitely want to observe
the polarity.

I'm sure you will get all sorts of answers, but just a few additional things
to think about:

1) If you use tantalum capacitors, just remember that there have been
tantalum shortages in the past which can significantly drive up the cost of
your product if you use a lot of tantalum capacitors and the industry
happens to be in the middle of a tantalum shortage. Remember Horatio Alger?
:)

2) Observe the working voltage rating of the capacitor -- do not violate
it -- good engineering practice is to perform derating (for example: use 10V
WVDC capacitors (instead of say 6.3V WVDC) if you use them for the 5V supply
to ICs).

3) There will be instances in which you will see the physical size of the
capacitor significantly increase as the DC working voltage rating (WVDC)
increases. If you are trying to keep things small, keep this in mind:
there's usually no need using a capacitor with a 100V WVDC rating if you
have access to a smaller size 10V WVDC capacitor *as long as you follow #2
above*.

A lot of companies should have application notes on using decoupling caps --
a few minutes web-searching should yield a treasure chest.

Try this for starters (not much meat on the bones, but at least it's a
start):

http://www.eettaiwan.com/ARTICLES/2001MAY/PDF1/2001MAY03_AMD_DSP_AN1148.PDF

Note: Cut and paste URL if it is word-wrapped.


Good luck!

Best regards,

Ken Pergola

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2003\10\13@033453 by

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Ken Pergola wrote:

> Try this for starters (not much meat on the bones, but at least it's a
> start):
>
> http://www.eettaiwan.com/ARTICLES/2001MAY/PDF1/2001MAY03_AMD_DSP_AN1148.PDF


Slightly of-topic, but..

That is an 7 page PDF file, the page contents are :

page 1 : Title page and TI logotype.
page 2 : Disclaimer.
page 3 : Trademark notice.
page 4 : Contact information (4 lines)
page 5 : Contents page (3 lines)
page 6 : Abstract (1 sentence of 12 words)
page 7 : (called "page 8" !) Aha ! Some decoupling info !

(The info in the doc is OK, b.t.w...)

:-) :-)

Jan-Erik.

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2003\10\13@033906 by

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Josh Koffman wrote:
> Do decoupling caps have to be electrolytic? Is it
> better to have some smaller value caps, rather than none at all?
> Do they need to be polarized? I'm trying to design a super small circuit, and
> the large electrolytics I usually use are too large. I have some
> salvaged SMT ceramics that would be cool.


As an "rule of thumb", the higher the frequency
the smaller the cap value. Higher value
electrolytics for mains "hum", and smaller
ceramics for high frequency "swithing" spikes.

Electrolytics are, by design, bad decouplers for
high frequency signals.

Jan-Erik.

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2003\10\13@084306 by Micro Eng
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Decouplers.....just back from XFEST2002

Need a good variety of small to large.  Most of the smaller caps are
typically called out to be X7R or X5R MLC types, and the larger ones at
least one ought to be a tantalum for the flatter frequency response.

Just watch the total ESR and the package sizes and the layers going thru.
Sometimes you will need to put the really smallers ones on the top so they
are closer to the power plane.

Then of course you need to watch the total ESR of the system.........

Back in the day.......when it was simple.......just a decoupler on every
chip.  These days with muliple layers and power planes......its an
art...that needs to be done right else the system might not work as well as
you designed it to.




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2003\10\13@143324 by Josh Koffman

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Thanks for the advice everyone! I see that I had mixed up terminology
between power supply filter caps, and decoupling caps.

Thanks!

Josh
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