Istvan and Ray, Think Viking, the one that makes me famous/infamous in your current/previous employer. That's all I want to say and that's all we should say. Chris -----Original Message----- From: Istvan NOVAK [mailto:istvan.novak@xxxxxxxxxxxxxxxx] Sent: Sunday, July 18, 2004 9:28 AM To: Chris.Cheng@xxxxxxxxxxxx; Larry.Smith@xxxxxxx; si-list@xxxxxxxxxxxxx Subject: Re: [SI-LIST] Re: Is Impedance Enough for Describing the PDS? Chris, You make a very good and relevant point about regulators today. As Ray pointed out, however, with multi-phase converters, we already have effective switching frequencies above 10MHz available. As Steve commented, raising the switching frequency per phase has efficiency and cost penalties. Fifteen years ago I had the fun the work on experimental DC-DC converters with 200MHz switching frequency and above. It is true that only a few nF output capacitance was needed, but the material cost of the converter at that time was $200. This is still the limiting factor today. I think the real problem today is not switching frequency. I fully agree when you complain about the need for a lot of extra capacitance, but this is primarily due to a constant drive for lower cost and smaller size. Commercially available single-phase DC-DC converters six years ago, running at 300kHz, were able to produce a stable output impedance of less than five milliohms up to 70kHz, without any external capacitor (1.5V output voltage, with 30A rating, cost was about $60). Today's popular converters with similar ratings are much smaller and much cheaper, and they run typically with the same switching frequency, but the output impedance goes above five milliohms at a few hundred Hz. If for sake of simplicity we pick 700Hz, where the output impedance goes beyond five milliohms, this will require HUNDRED times more capacitance at the output (assuming we want to maintain the same, say five milliohms impedance). It turns out that adding this much capacitance still may yield a cheaper overall system solution, but the overall space needed is undoubtedly higher. So I think today the converters are optimized for overall lowest cost, which may be a good target for some users. People who need smaller overall size, now have to rely on custom converters. Regards, Istvan Novak SUN Microsystems ----- Original Message ----- From: "Chris Cheng" <Chris.Cheng@xxxxxxxxxxxx> To: <Larry.Smith@xxxxxxx>; <si-list@xxxxxxxxxxxxx> Sent: Friday, July 16, 2004 3:50 PM Subject: [SI-LIST] Re: Is Impedance Enough for Describing the PDS? > I think the power supply response is just as important as the impedance. If > you can get your DC/DC to beat faster, the amount of bulk caps needed to > hold the low end of the impedance can be minimized. One of these days, some > very smart people will come out with a 10-100MHz DC/DC. Things will get very > interesting. > > -----Original Message----- > From: Larry Smith [mailto:Larry.Smith@xxxxxxx] > Sent: Friday, July 16, 2004 10:41 AM > To: si-list@xxxxxxxxxxxxx > Subject: [SI-LIST] Re: Is Impedance Enough for Describing the PDS? > > > Scott and Steve have both had some good comments on the importance of > mounting inductance for decoupling capacitors. The mounting > inductance is almost inversely proportional to the number of vias used > to attach the capacitor to the power planes and proportional to the > length of the vias (depth to the first plane). The number of high > frequency capacitors required to meet a PDS target impedance is nearly > proportional to the mounting inductance over the frequency range that > we care the most about. So in a sense, the number of vias is > important, the number of capacitors is not. > > Ten years ago, the typical mounting inductance for decaps was 3 to 5 > nH as we used traces to vias; to power planes; that were buried in the > middle of our boards; usually with 14 mils or more separation between > planes. We sometimes shared vias between 2 or more caps! As > mentioned by Scott, it is now possible to attach capacitors with a > total mounting inductance of 200pH or less. So the number of > capacitors necessary to meet a target impedance a decade ago was 20 > times more than what it is today. Actually, the number of capacitors > has stayed about the same and the target impedance has dropped by > about a factor of 20. > > When you think about it, all we really need is a single ideal 1 uF > capacitor connected to our PDS. The impedance of a 1 uF capacitor at > 1 MHz is 159 mOhms. It is 15.9 mOhms at 10 MHz, 1.59 mOhms at 100 MHz > and 0.159 mOhms at 1 GHz. Imagine that, a PDS with much less than 1 > mOhm at a GHz! And all you need is one ideal capacitor! The moral of > this story is that a low impedance PDS at high frequency has very > little to do with capacitance and is all about managing the parasitic > inductance. > > Small power islands work great for core power. Just put enough > capacitors (and vias) in parallel that you achieve the target > impedance up to the desired corner frequency. It does not take too > many well-mounted capacitors in parallel to do this. The biggest > problem is that the impedance of the parallel combination of all caps > mounted on the plane becomes less than the impedance of the power > planes used to bring the current into the chip. This is where the > thin power plane dielectric (BC) comes in. Once again, it is not > about capacitance but is all about spreading inductance. If the > impedance of the conduit is higher than that of the parallel caps, > don't bother putting any more caps on the board.. > > The subject of this thread is, "Is Impedance Enough for Describing the > PDS?" For the purpose of power integrity (getting the power to the > consumer when it is required and stop delivering power to the consumer > when it is not required), the answer is a resounding YES! If the chip > circuits look out and see their target impedance up to a frequency > that is related to the transient rise time, they are happy. If the > chips are happy, they don't make EMI noise. EMI noise is just a > result of chips demanding current from a PDS that is too high in > impedance. Current drawn from a high impedance implies power injected > into the PDS and that is trouble. Manage the PDS impedance at the > power consumer and (PDS related) EMI problems will be greatly reduced > or go away completely. > > regards, > Larry Smith > (still at) Sun Microsystems > > PS - Boy, I sure miss Ray! > > steve weir wrote: > > > > Zhangkun, > > > > As I stated, the mounted inductance of IDC's and X2Y's is less than 1/3 > the > > mounted inductance of ordinary 0603's using optimized two via mounts. The > > inductance of the IDC / X2Y capacitors by themselves is even less, but as > > you note the attachment inductance interferes. The reason that IDCs do > > very well is that it takes four times as many via holes as a regular > > capacitor. Done correctly, there is an almost proportionate decrease in > > the attachment inductance. So, an IDC can be viewed as four 0603s, in > > parallel but with only one part to mount. > > > > The physics of the X2Y are a bit different. In cooperation with X2Y, I > > have developed optimized mounts for X2Ys that use six vias and get results > > essentially identical to the IDC with optimal mounts. This has been > > verified by both simulation and measured results. The IDC's require 33% > > more vias, and a much higher component cost to do the same decoupling job > > as X2Ys. > > > > Using either X2Y or IDCs we can cut component count by 3:1 or > > better. Using X2Ys we reduce the BOM cost as well. Both Teraspeed and I > > offer services to optimize PDS design to suite needs whether it is > density, > > cost or some combination. There are probably other consultants available > > who do as well. > > > > Regards, > > > > Scott McMorrow wrote: > > > > Zhangkun, > > > > Zhangkun wrote: > > > > >How about the value of the inductance of these kind of capacitors? When > the cap is soldered on the PCB, there is leading inductance of about 1nH. > This could not be eliminated by better caps. > > > > > Actually, better designed capacitors and better mounting structures do > > help. Steve and I both have developed mounting structures for some of > > these capacitors that are down in the 200 pH range, with a total mounted > > device inductance below 400 pH. It's all a matter of physics and good > > design practices. With X2Y and IDC capacitors, we can obtain between a > > 3 and 4-to-1 part count reduction over 0603 capacitors with the best > > designed mounting solution. > > > > regards, > > > > scott > > > > -- > > Scott McMorrow > > Teraspeed Consulting Group LLC > > 121 North River Drive > > Narragansett, RI 02882 > > (401) 284-1827 Business > > (401) 284-1840 Fax > > (503) 750-6481 Cellular > > http://www.teraspeed.com > > > > Teraspeed is the registered service mark of > > Teraspeed Consulting Group LLC > > > ------------------------------------------------------------------ > 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 FAQ wiki page is located at: > http://si-list.org/wiki/wiki.pl?Si-List_FAQ > > List technical documents are available at: > http://www.si-list.org > > 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 FAQ wiki page is located at: > http://si-list.org/wiki/wiki.pl?Si-List_FAQ > > List technical documents are available at: > http://www.si-list.org > > 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org 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