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 - 16:07:56 PST


I think it is a matter of proportion. If the extra PWR-PWR coupling has a
high enough corner frequency that it cannot pull the voltage regulator high
frequency open loop response into the trouble zone, then you are OK. If you
look at previous posts, the caveat was "if not done correctly". I think most
voltage regulators are designed with the idea that the servo loop
compensates for the lower frequency input and load variations, and then you
are expected to use output capacitors to handle higher frequency
disturbances.

I don't think that planes fall into this category (not enough C).

Anyway, it sounded as though at least some people had gotten into trouble
with it.

Larry Miller

-----Original Message-----
From: owner-si-list@silab.eng.sun.com
[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Mike Hughes
Sent: Monday, January 22, 2001 2:13 PM
To: si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : Decoupling between non-ground power rails, yes
or no??

If this is true, then isn't using adjacent power planes for decoupling also
a
bad idea?

(PWR-PWR as opposed to PWR-GND)

Both planes are effectively GND to high frequencies. The main difference in
the
2 decoupling schemes is the capacitance value. You would generally have more
capacitance using discrete components than you would get from the planes. So
if
you used small valued discrete caps between the voltages, wouldn't it be the
same as using PWR-PWR planes? Is there a preference for using PWR-GND over
PWR-PWR planes? I've never heard that before.

Thanks,
Mike

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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--
================================
Mike Hughes
Product/Test Engineer
Analog Devices, Inc.
Phone 781-937-2370
Fax 781-937-1011
================================

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