Re: [SI-LIST] : Decoupling between non-ground power rails, yes or no??

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From: Vinu Arumugham (vinu@cisco.com)
Date: Mon Jan 22 2001 - 13:43:33 PST


If there are multiple voltage rails on a board that are properly decoupled to
the same ground, there is a significant amount of capacitance between the
voltage rails, even without explicitly adding caps between them. Why does that
not cause a problem?

Thanks,
Vinu

Larry Miller wrote:

> This instability is not a surprise. Putting caps between the voltages
> introduces a lead term (treble boost) into the voltage regulator feedback
> loop which can raise havoc by pushing the loop bandwidth out farther than it
> was designed for.
>
> Offhand, I would say that this is overall a Bad Idea.
>
> Larry Miller
>
> -----Original Message-----
> From: owner-si-list@silab.eng.sun.com
> [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Zabinski, Patrick
> J.
> Sent: Monday, January 22, 2001 9:04 AM
> To: si-list@silab.eng.sun.com
> Subject: RE: [SI-LIST] : Decoupling between non-ground power rails, yes
> or no??
>
> Michael,
>
> Not exactly answering your question
> directly, but I did want to bring up one cautionary
> note.
>
> In at least two past board designs in our group, folks
> have decoupled power planes directly to one another
> (e.g., +5 to +3.3) in addition to decoupling them to
> ground. In both of these board designs, oscillations
> were found in the board. When we removed one set of
> decoupling capacitors (i.e., breaking the GND-to-3.3-to-5-to-GND
> loop), the oscillations stopped.
>
> We did not have the time to dig into the root cause,
> but I have since made it a rule to avoid having strong
> decoupling between power supplies.
>
> Pat
>
> > Hi All,
> >
> > I recently saw something on a design that I
> > felt was questionable and wanted to see what
> > your thoughts were, especially as there appears
> > to be a number of folks that have a focus on
> > PDS and decoupling.
> >
> > While in the process of performing a PDS analysis
> > of a new board, I noticed that there was a signi-
> > ficant amount of decoupling between the power
> > rails. For instance, there were caps between 5
> > and 3.3, 3.3 and 2.5, etc. From discussions,
> > it appears that this is there to facilitate
> > return currents.
> >
> > So the obvious question is, Is this accomplishing
> > what it is believed to be and is there in fact a
> > better way to do this. Is a properly decoupled PDS
> > (below target impedance through frequency range)
> > all that is required. For the sake of argument,
> > assume that all the planes are whole.
> >
> > Thanks for your opinions
> >
> > Best Regards,
> >
> > Michael C. Greim Sonus Networks
> >
>
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