Published by Eric Bogatin on 01 Dec 2009
11/29/09 Answer to last month’s pop quiz: “The total amount of bulk decoupling capacitance for a PDN rail should be selected based on the target impedance and…what else?”
Next No Myths Allowed Webinar, “Selecting Capacitor Values for the PDN”, Dec 9 2009, 1 pm EDT. Signup now.
This question is actually one of the topics covered in our upcoming webinar, mentioned above. Of course, the bulk capacitors are used to provide low impedance at a frequency where the VRM is no longer able to keep the impedance of the PDN below the target impedance.
How much bulk capacitance is required depends on the target impedance and this frequency. When the VRM impedance exceeds the target impedance, the impedance of the bulk capacitor (shown in red) had better be there to keep the PDN below the target impedance. This figure is from our Essential Principles of SI (EPSI) class.
Of the possible options, the correct answer is the second one, “the frequency where the VRM impedance is too high.”
This was a pretty easy question as most folks got it correct. It is interesting that 25% selected the third answer, that the bulk capacitance depends on the loop inductance of the bulk capacitors.
The loop inductance of the bulk capacitors is important, as it will influence the parallel resonance with the ceramic capacitors, but it by itself will not determine how much bulk capacitance is required.
This whole topic of selecting capacitors- the number and their value- is very confusing, depends on a large number of system features and details and is difficult to generalize from what worked on one design to the next design.
That’s why we decided to cover this topic in the next webinar. I’ve collaborated with Larry Smith on this project, since he is one of my PDN gurus and I’ve learned a lot about PDN design from him. I hope you will join us on Wed, Dec 9, 2009. If you miss this live event, the webinar will be recorded and posted on our web site.
If you would like to get additional details on PDN design, you can also read the new chapter I wrote in my book, Signal and Power Integrity- Simplified.
And, if you are looking for a new challenge, check out the latest pop quiz we added to the web site!
I hope to see you in cyberspace!