[SI-LIST] Re: PDN question

  • From: wolfgang.maichen@xxxxxxxxxxxx
  • To: "Joel Brown" <joel@xxxxxxxxxx>
  • Date: Thu, 20 Aug 2009 13:33:33 -0700

Joel,
theoretically, if there really is no stimulus in the system, then of 
course any resonance of the PDN at that frequency is unimportant.

In practice though things usually aren't so clear-cut. Remember that if 
you have e.g. a digital data stream, that contains a virtual continuum of 
frequency components from DC all the way up to the knee frequency 
(0.5/Trise) and even beyond. That's virtually guaranteed to excite that 
resonance and thus create additional power supply noise. So even a a 
single clock frequency the power supply current profile will have 
frequency spectrum.

Also, unless you spend an extraordinary effort on simulation, modeling and 
measurement, chances are you can't predict where your resonances are with 
sufficient accuracy to guarantee they won't lie in a "bad" spot. Add to 
that manufacturing and component tolerances and you'll soon find it's 
easier to just do a clean PDN design (impedance well below target over the 
whole frequency range of interest) rather than  count on your luck.

Just my 2 cents

Wolfgang







"Joel Brown" <joel@xxxxxxxxxx> 
Sent by: si-list-bounce@xxxxxxxxxxxxx
08/20/2009 01:16 PM

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Subject
[SI-LIST] PDN question






Is it really necessary to have a PDN with impedance flat or below a target
over a wide range of frequencies?
If a resonance occurs is it really a problem if there are no switching
currents at that frequency?

I am assuming that switching frequencies are a function of the clock
frequencies in the system.

 

Thanks - Joel

 

 



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