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

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From: Larry Miller (ldmiller@rhapsodynetworks.com)
Date: Mon Jan 22 2001 - 12:50:28 PST


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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