# [SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator

*From*: Rod Barman <rodb@xxxxxxxxxx>*To*: si-list@xxxxxxxxxxxxx*Date*: Mon, 11 Aug 2003 23:13:45 -0700

Dear Istvan, > Such > a spreadsheet will be available for free download in the near future: send > me an off-line > mail if you are interested. Notification is sent out only to those, who ask > for the > e-mail notification. I would appreciate receiving an email notification when this becomes available. Thank you very much. --rod. Istvan NOVAK wrote: > Fred, Peter, > > You are correct, as always, it depends. For people who might be familiar > from their experience mostly with only one or the other extreme scenario, > let > me summarize briefly the situation. > > Though it was not specifically mentioned, everyone on this thread assumed > (or I think so) that the bypass capacitors are connected to planes (as > opposed > to the smallest possible power patch, when we cant speak about distance, > because there is just an active device with a few capacitors around it on > the > patch). It is usually agreed that at low frequencies, the location of > capacitor > does not matter much. So bulk capacitors could be excluded from the > discussion, though it is easy to show that we can always find an exception > if we want to: imagine the bulk capacitors that have their series > resonance frequencies in the 100kHz-1MHz range. The wavelength is very long > compared to any practical PCB size, and still, it may be a good idea to put > the > bulk capacitors close to the output of the DC-DC converter, otherwise the > switching ripple current (in the other of a few hundred kHz) will have to > travel > all over the board and would spread the switching ripple. > > At higher frequencies, the location of capacitors MAY matter. It depends on > the > impedance ratio of planes and capacitors. Similar to a matching resistor > for a > trace, when the impedance values agree or are close, the setup becomes > increasingly > insensitive to the geometry and relative locations. If we terminate the end > of a trace > properly, the input impedance on a low-loss trace is always the > characteristic > impedance, regardless where we are along the trace. Same applies in two > dimensions > to planes: if the impedance of the planes is close to our required PDN > impedance, > component locations matter very little as long as we provide a matched > termination to > the planes. > > The capacitor location increasingly matters as soon as we cannot or > do not want to use matched planes: either the impedance requirement might be > so > low that it cannot be easily provided by (matched) planes, or we do not have > the > proper components to provide the plane matching. As the ratio of plane > impedance > and capacitor ESR increase, there is an increasing sensitivity to component > location. > The typical scenario is that the plane impedance is high and the ESR is low. > This is > similar to having a shorted trace: the input impedance is a strong function > of distance > even at very low frequencies. If we keep the proper distance from the active > device, our impedance target still can be met. Capacitor-plane resonance > will build > up, but as long as we dont excite them, it is OK. > > Dependent on system considerations (determining our particular impedance > target), > available components, cost and manufacturing constraints, we may have one or > the > other extreme, or anything in between. All of the comments leaning one way > or > the other on this thread are correct within their validity range. We have > designs > embracing the full range explained above, we built and measured them. > > This thread started out with the posting about the UltraCAD tool. It is a > matter of > preference to use expression solvers (like the UltraCAD tool, or a > spreadsheet, or > a MATHCAD file) or a simulator (SPICE or alike). I use several different > options, > Berkeley SPICE, PSPICE, HSPICE, spreadsheets and scripts based on analytical > expressions. All have their particular sweet spot of usage. SPICE can run > fast, but > it is easier to use an expression solver to simulate a full impedance > surface, which > can be set to step through a range of frequencies and display the changing > impedance > surface of a plane with a bypass capacitor at an arbitrarily specified > location. Such > a spreadsheet will be available for free download in the near future: send > me an off-line > mail if you are interested. Notification is sent out only to those, who ask > for the > e-mail notification. > > Best regards, > > Istvan Novak > SUN Microsystems > > > ----- Original Message ----- > From: "Fred Balistreri" <fred@xxxxxxxxxxxxx> > To: <si-list@xxxxxxxxxxxxx> > Sent: Monday, August 11, 2003 6:21 PM > Subject: [SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator > > > >>I believe the condradictions start when we generalize. Every design >>is different and the solutions for one do not necessarily apply to >>another. But we should be able to agree on the basic concepts and >>come to understand what the assumptions are when statements are made. >> >>Best Regards, >> >>Peter Arnold wrote: >> >>>All, >>> >>>I am not sure there should be so much controversy over this placement > > thing. > >>>Among the conclusion in the UMR paper that I have a copy of [1] is this >>>assertion: "...all decoupling capacitors are shared in the frequency > > range > >>>where they are effective (typically below 200-300MHz), and the location > > of a > >>>decoupling capcitor on the board is relatively unimportant." >>> >>>The "unimportant placement" claim is made dependent on the condition > > that > >>>the caps, as mounted, are ineffective above 200MHz. At this frequency an >>>8"x9" test board is electrically small with respect to 200MHz wavelength > > in > >>>FR4 (30") and the paper reports placement not to be crucial in > > determining > >>>impedance. >>> >>>I think this electrical-smallness, and taking capacitor mounting > > inductance > >>>at 2-10nH to be >> plane inductance between parts, is what prompts them > > to > >>>simplify the power delivery model to a network connected between two > > spice > >>>nodes as if all components were physically at the same place. Analyzing > > this > >>>the authors identify a frequency "fa" lying between the last zero and > > the > >>>last pole of the impedance plot above which the decouplers are > > ineffective. > >>>If this frequency is low as in the examples given, then cap placement > > might > >>>well not matter. >>> >>>But designs today do not necessarily support the simplification given > > above. > >>>Modern techniques like via-in-pad, multiple fanouts, "broadside" case >>>connections etc. reduce mounted inductance to 1nH or less per part. This >>>should allow mounted capacitors to be effective at higher frequencies > > where > >>>wavelengths are only a few inches and the board is no longer > > electrically > >>>small. At this point the everything-between-two-nodes and >>>no-inductance-in-series-with-plane-capacitance simplification may not > > hold > >>>and a bedspring-type model would be needed. Decoupler placement may then >>>become important (as experience appears to confirm!) This does not >>>contradict the UMR authors' statement, which relates to a >>>high-mounted-inductance regime and frequencies below 200MHz on a small >>>board. >>> >>>Can anyone comment? >>> >>>[1] Hubing et al., "Power Bus decoupling on Multilayer Printed Circuit >>>Boards," in IEEE trans. EMC vol 37, no. 2, May 1995. >>> >>>Regards, >>>peter arnold. >>> >>>-----Original Message----- >>>From: si-list-bounce@xxxxxxxxxxxxx >>>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Ray Anderson >>>Sent: Monday, August 11, 2003 9:36 AM >>>To: si-list@xxxxxxxxxxxxx >>>Subject: [SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator >>> >>>Lee Ritchey wrote: >>> >>>>If the location of decoupling capacitors matters, perhaps some > > technical > >>>>demonstration would prove that. Short of such a demonstration, this > > is > >>>>speculation and not the sort of thing that should be used to make > > design > >>>>choices. >>>> >>>>It's time to do some good engineering on this subject and do away with >>>>opinion. The UMR paper is good engineering. Anyone who chooses to >>>>disagree with it has the burden of showing where it is wrong by using > > some > >>>>good science. >>>> >>>>Lee >>>> >>>>Lee Ritchey >>>>leeritchey@xxxxxxxxxxxxx >>>>Why Wait? Move to EarthLink. >>> >>>Or conversely, show that it doesn't matter. >>> >>>Take a large system board (say about 24" square) put a bunch of high > > current > >>>processors and ASICS on the left side of the board. Put all your decaps >>>on the right side of the board (since you maintain position doesn't > > matter). > >>>In this hypothetical case I can just about guarantee the board won't >>>function >>>properly or pass EMI. Admittedly, it is a contrived case, but I think it >>>illustrates the point. >>> >>>As far as the technical demonstration goes, we have indeed demonstrated > > to > >>>ourselves that position does matter. >>> >>>I'm not attempting to further an argument, but I do feel those who have >>>responded with viewpoints other than yours shouldn't be chastised and >>>accused >>>of spreading unfounded opinion. Don't discount the chance that an > > opinion > >>>that >>>differs from yours just might be correct. >>> >>>-Ray Anderson >>>Sun Microsystems Inc. >>> >>>------------------------------------------------------------------ >>>To unsubscribe from si-list: >>>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field >>> >>>or to administer your membership from a web page, go to: >>>http://www.freelists.org/webpage/si-list >>> >>>For help: >>>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field >>> >>>List archives are viewable at: >>> http://www.freelists.org/archives/si-list >>>or at our remote archives: >>> http://groups.yahoo.com/group/si-list/messages >>>Old (prior to June 6, 2001) list archives are viewable at: >>> http://www.qsl.net/wb6tpu >>> >>>------------------------------------------------------------------ >>>To unsubscribe from si-list: >>>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field >>> >>>or to administer your membership from a web page, go to: >>>http://www.freelists.org/webpage/si-list >>> >>>For help: >>>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field >>> >>>List archives are viewable at: >>> http://www.freelists.org/archives/si-list >>>or at our remote archives: >>> http://groups.yahoo.com/group/si-list/messages >>>Old (prior to June 6, 2001) list archives are viewable at: >>> http://www.qsl.net/wb6tpu >>> >> >>-- >>Fred Balistreri >>fred@xxxxxxxxxxxxx >> >>http://www.apsimtech.com >>------------------------------------------------------------------ >>To unsubscribe from si-list: >>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field >> >>or to administer your membership from a web page, go to: >>http://www.freelists.org/webpage/si-list >> >>For help: >>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field >> >>List archives are viewable at: >>http://www.freelists.org/archives/si-list >>or at our remote archives: >>http://groups.yahoo.com/group/si-list/messages >>Old (prior to June 6, 2001) list archives are viewable at: >> http://www.qsl.net/wb6tpu >> >> > > > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > > or to administer your membership from a web page, go to: > http://www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > List archives are viewable at: > http://www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: http://www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: http://www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu

**References**:**[SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator***From:*Peter Arnold

**[SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator***From:*Fred Balistreri

**[SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator***From:*Istvan NOVAK

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