[SI-LIST] Re: [!! SPAM] Re: 6 layers stackup
- From: Istvan Novak <istvan.novak@xxxxxxxxxxx>
- To: steve weir <weirsi@xxxxxxxxxx>
- Date: Tue, 11 Mar 2008 09:04:29 -0400
Joel,
Just a few late comments to the thread (I was on vacation):
- setting the resistance to dampen resonances is a useful thing to
reduce worst-case transient noise and EMI risk. For regular MLCCs this
usually requires ESR values higher than what we get with a typical
manufacturing process today.
- in ceramic capacitors the required resistance can be created either by
a smart internal construction (see the TDK announcement pointed out on
the thread), or by adding resistance externally such as printed
resistors (such as Sanmina's annual ring resistor) or by adding discrete
resistor pieces
- as it was mentioned, inductance is crucial: whether the resistance is
added inside the MLCC or externally, we should not add too much
inductance as it would defeat the purpose. Fortunately each of the
above three approaches can be implemented with sufficiently low
inductance.
- for implementation options and simulated measured results you can see
some of the published papers on the topic. See for instance:
"Overview of Some Options to Create Low-Q Controlled-ESR Bypass
Capacitors" and "History of Controlled-ESR Capacitors at SUN" in
TecForum TF-MP3 "Controlled-ESR Bypass Capacitors Have Arrived" on
http://home.att.net/~istvan.novak/papers.html
Finally another comment on inductance: instead of setting the resistance
to the optimum value, sufficiently low overall inductance of the bypass
capacitors can also be used to reduce/eliminate resonances. This is the
rational (knowingly or unknowingly) behind the approach Charles
mentioned (sprinkling the board with a lot of capacitors) and this is
the reason why lower inductance parts are always a welcome help.
However, from a design point of view we need to keep in mind that using
a low cumulative inductance instead of the optimum resistance comes with
a price: either the low-inductance parts are more expensive, or the
low-inductance parts require more vias, or simply we need more parts.
Of course setting the resistance to the proper value has its own price
tag as well, so eventually the design engineer has to make the decision
which solution fits bets his/her requirements. Typically in low-density
and low-cost systems adding more capacitors works well; in high-density
and/or high-performance systems selecting PDN components with the proper
resistance becomes more attractive.
Regards,
Istvan novak
SUN Microsystems
steve weir wrote:
> Joel, controlled / high ESR caps are rare birds. Istvan has been
> championing them for years nd some parts have been made. If you are
> prepared to buy 100,000,000 at a shot you might get somewhere w/ the
> manufacturers. But right now, most are running their lines to capacity
> w/ commodity parts.
>
> Regards,
>
>
> Steve.
> Joel Brown wrote:
>
>> While we are all talking about bypass caps, I just got done reading a book
>> by Bruce Archambeault called "PCB Design for Real-World EMI Control". In the
>> book there is a chapter on bypassing and it shows measurements from actual
>> test boards. In one case, a certain percentage of the caps are "high ESR"
>> which seems to result in substantial damping (reduction) of the resonances.
>> The high ESR caps are described as ceramic capacitors with a series resistor
>> built into the package, I have been unable to find such a part. Has anybody
>> found a benefit to this approach and a source for high ESR capacitors?
>>
>> Thanks - Joel
>>
>>
>>
>>
>>
>>
>
>
>
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