Mark- I'm sure Ken will reply, but since I'm familiar with what's going on I'll respond for now: 1) the .02 in the equation in Ken's message stems from the fact that he is deriving a Target Impedance based on a 2% variation in voltage. It could have just as well been 5%. The percentage of allowable ripple is determined by the designer based on how much noise he/she thinks the circuit can tolerate. 2) Not sure if Ken really means di/dt or not. In the equation published in our paper we mean (delta I) for the current and don't assign any time dependency to it. (Ken can you comment?) 2) A similar equation was first widely published in a paper in the IEEE Transactions on Advanced Packaging August, 1999 pp 284-291 entitled "Power Distribution System Design Methodology and Capacitor Selection for Modern CMOS Technology" by L. Smith, R. Anderson, D. Forehand, T. Pelc, and T. Roy. (paper downloadable at: http://www.qsl.net/wb6tpu/si_documents/decap_whitepaper.pdf ) 3) The equation is based on good old Ohm's law (R=E/I) (Vdd * % ripple) Ztarget = ---------------- dynamic current For example: suppose your system runs on 1.8 volts DC and can tolerate 2% ripple The maximum dynamic current swing is from 15 to 20 amps then: (1.8 * .02) 36 mV Zt = ---------- = --------- = 7.2 mOhms (20 - 15) 5 amps The equation (at least the one published in our paper in 1999) assumed a constant target Z from DC to daylight. Our view on that has changed since the original publication, but that is a different conversation.... -Ray > >Ken, > >I've played around with your equation below, but I'm having trouble getting >anything meaningful out of it. Did this come from a book somewhere that has an >explanation, or is this your own? If it's the latter, do you have a >derivation? > >A couple of specific questions: > >is 'powersupply' your VCC voltage? >where did 0.02 come from? Is this 5% variation divided by 2? > >Thanks a lot, >mark > > >Ken Cantrell wrote: > >> I use Ztarget = (powersupply*0.02)/di/dt, and have stopped using COGs (ESL ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field 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