Why are we basing engineering decisions on opinion? This isn't politics, it's engineering and is based on science. Lee Ritchey leeritchey@xxxxxxxxxxxxx Why Wait? Move to EarthLink. > [Original Message] > From: Raj Raghuram <raj.raghuram@xxxxxxxxxxxxxxx> > To: <si-list@xxxxxxxxxxxxx> > Date: 8/4/2003 8:02:35 AM > Subject: [SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator > > Another opininon on this I have heard is that at higher frequencies, it > is the decoupling on the package and IC that really matter. The package > decap can have lower inductance and the IC decap almost negligible > inductance. Unfortunately, the system level designer is often not privy > to information on these decaps, especially the on-chip decap. All he has > is the IBIS model which is made assuming ideal supplies. IC > manufacturers need to provide some information about on-chip decoupling. > Some opinions I have heard from designers on this are: > > 1. A profile of the current drawn by the IC is really needed for > effective PDS design. > > 2. If there is on-chip cap deliberately added, this needs to be > mentioned somewhere in the data or IBIS model. > > 3. Often there are a number of gates not switching. Each one has some > capacitance between power and ground. The sum of these can be a > significant source of decoupling, whether intended as such or not. > > 4. Issues such as whether on chip core supply is the same as the IO, > etc. become important if on-chip decoupling is to considered. > > Ray Anderson wrote: > > >With all due respect to the UMR authors, I think the location > >of a decap on a set of power planes DOES matter. > > > >There is an inherent time delay associated with the current flow > >from the location where a decap is placed to the location where the > >chip to bypassed is located. > > > >The decaps job is to provide current to the load quickly enough > >that a voltage collapse on the planes doesn't occur. If the decap > >is too far away from the current consuming load it is possible > >that load's need for a pulse of current will be over before the > >current can get there. > > > >At low frequencies you can place a decap just about anywhere on a plane > >and it will function just fine. At higher frequencies the decap must > >be closer to the device to be bypassed. The concept of "effective bypassing > >radius" says that a decap must be within say 1/6 to 1/10 wavelength > >at the frequency of interest to be effective. (the exact number is > >debatable). The frequency of interest is the resonant frequency of > >the mounted decap (where the caps ESR is lowest), and the distance is > >a function of the speed of propagation with a given dielectric between > >the planes (around 180ps/inch for FR4). > > > >You need to consider the role each component of a PDS (power distribution > >system) plays in maintaining a low impedance across a wide bandwidth. > >A typical PDS is composed of a VRM, bulk capacitors, ceramic decaps, and > >the power planes and last but not least the package housing the silicon. > > > >The VRM provides a low impedance from DC to perhaps a few hundred kHz, > >the bulk capacitors contribute their impedance profile from a few > >hundred kHz up to about a MHz. From a Mhz to maybe 120MHz or so the > >ceramic decoupling caps provide low impedance. Above that frequency > >the distributed capacitance provided by the planes provides the requisite > >low impedance to the PDS. When the individual impedance profiles of > >each of the constituent parts are superimposed, the resulting composite > >profile provides a low impedance over the full frequency range that the > >PDS was designed for. Note that above the package resonance frequency > >(which usually is from about 50 to 100 MHz for typical packages) decoupling > >on the board is ineffective and the necessary decoupling must be provided > >for either in the package or on the silicon. However, at frequencies > >above the package resonance frequency the discrete decaps and distributed > >plane capacitance can be most useful for EMI purposes. > > > >To summarize, at low frequencies, the decoupling capacitors can be placed > >almost anywhere withing reason and will be effective. As the frequency > >of interest increases, the allowable distance decreases. As an example, > >at 1 MHz the wavelength (on FR4) is around 214 meters. So lambda/10 is > >around 21 meters. Hence, if your decap which is effective at 1 MHz is > >within 21 meters of the consuming device, then the decoupling will be > >effective. Conversely, if you are trying to decouple at device with > >a decap that is effective at say 100 MHz then the wavelength of interest > >is about 1.4 meters. Lambda/10 is then .14 meters (approx 5.5 inches). > >In this case if the decap is appreciably more than 5.5 inches from the > >currewnt consumer then the delay will be such that the decap will not be able to > >provide current quickly enough to be effective. > > > >-Ray Anderson > >Sun Microsystems Inc. > > > > > >Lee Ritchey wrote: > > > > > >>>If you remember the UMR paper on power bus decoupling, it made a clear case > >>>that the location of the capacitors is relatively unimportant. > >>> > >>> > > > > > >Abe Riazi wrote: > > > > > >>To be fair, it should be added that different recommendations ( from above) > >> > >> > >exist in literature > > > > > >>regarding optimum location of the decoupling caps. > >> > >>Two examples follow: > >> > >>1. Stephen H.Hall et al., " High-Speed Digital System Design........", on page > >> > >> > >247 it is stated: > > > > > >>" Figure 10.10 suggests not only that the local decoupling capacitor value > >> > >> > >should be chosen > > > > > >>appropriately but that the capacitor is placed physically close to the > >> > >> > >component to minimize > > > > > >>the loop area ". > >> > >>2. A technical paper by R. Chen, "Where to Place Decoupling Capacitors?..." > >> > >> > >available on > > > > > >>www.sigrity.com web site, implies that the best location for decoupling caps > >> > >> > >needs to > > > > > >>be ascertained with aid of electromagnetic field simulation. > >> > >>Best Regards, > >> > >>Abe Riazi > >>ServerWorks > >> > >> > > > >------------------------------------------------------------------ > >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: > >//www.freelists.org/webpage/si-list > > > >For help: > >si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > > >List archives are viewable at: > > //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 > > > > > > > > > > -- > Raj Raghuram > Berkeley Design Automation (http://berkeley-da.com) > 2902 Stender Way, > Santa Clara, CA-95054 > PH: (408)-496-6600 ext.203 > Cell: (408)-390-7614 > EMAIL: raj.raghuram@xxxxxxxxxxxxxxx > > > > ------------------------------------------------------------------ > 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: > //www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > List archives are viewable at: > //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: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //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