Hi Senthil, Basically what i am doing is substituting VL/Lo in place of di/dt in the equation, where VL is the smallest of the voltages across the output inductor and Lo is the output inductor value. And this is what the switching regulator's datahseet also says and this puts us in a safe Cmin. The design is for the chipset and the chipset manufacturer hasnt specified the di/dt specification. Regards, Anand. From: Senthil S Velmurugan <vel_ss@xxxxxxxxx> on 08/02/2002 01:00 AM Please respond to vel_ss@xxxxxxxxx@SMTP@Exchange To: si-list@xxxxxxxxxxxxx@SMTP@Exchange cc: Subject: [SI-LIST] Re: Output Capacitor of a switching Regulato Hi Anand, Can you let me know how you have arrived at di/dt value for the equation ? I see, say, some CPU's requirement for X-Amp/uS How much you want to have VRM or switcher bulk cap to supply those X-Amp/uS ?. I think the X-Amps is supplied by a combination of package, plane and decoupling caps and bulk cap follows to fill those, At what rate is my question in other way. Hi Larry, I think this need to be clarified to calculate Cmin, but I do not see any info on this subject other than using a tool(ex,. Ansoft)to model the complete switcher with package parasitic of CPU which you may not know unless you use TDR to extract those parameters, these models are not given out by any CPU vendor. Tools to reduce plane impedance are being used to reduce the plane impedance below Z target by choosing an huge array of caps. Caveat being, What is the roll up of impedance curve at what frequency depends on the processor's working pattern and freq'y, this pattern needs to be known to reduce the amount of caps suggested by the tool. Can you through some more light on this and a suggest an optimum way to ensure right Bulk cap for a on board regulator in the first time without doing any simulation or without building the board and measure it approach. VRM modules have the luxury of holding 10 caps on a vertical board but not on a small form factor embedded mother boards is another challenge and no room to have liberal approach. Thanks Senthil __________________________________________________ Do You Yahoo!? Yahoo! Health - Feel better, live better http://health.yahoo.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: //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