[SI-LIST] Re: [!! SPAM] Re: 6 layers stackup
- From: "Joel Brown" <joel@xxxxxxxxxx>
- To: "'Istvan Novak'" <istvan.novak@xxxxxxxxxxx>
- Date: Tue, 11 Mar 2008 10:57:40 -0700
Istvan,
Just a few qusetions regarding your comments:
By adding discrete resistors do you mean add them in series with normal low
ESR ceramic capacitors?
If 0402 packages are used for both would the inductance be sufficiently low?
I assume the higher ESR bypassing should be done as percentage of the total
bypassing and should not be used to bypass local IC power pins?
Thanks - Joel
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Istvan Novak
Sent: Tuesday, March 11, 2008 6:04 AM
To: steve weir
Cc: Joel Brown; leeritchey@xxxxxxxxxxxxx; 'Scott McMorrow'; 'QU Perry';
si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: [!! SPAM] Re: 6 layers stackup
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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