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

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From: Chris Padilla (cpad@cisco.com)
Date: Mon Jan 22 2001 - 15:54:26 PST


I think not because the planes create a distributed capacitance rather than
a discrete value. This is where the difference lies. Think about charge
density between the two and how it is flowing. In the distributed case, as
the charge flows, it is relatively "smooth" but now stick a huge 10,000 pF
capacitance at one point and now think about your charge flow.

Remember that capacitance isn't everything, we also have inductance
associated with those discrete caps--on the order of 1.5 nH per cap. The
distributed cap has at least a magnitude less than that I would think
(never measured nor calculated so I am guessing here).

Your stack-up and needs of the hardware engineer normally determine whether
or not you want/wish to do Pwr/Gnd or Pwr/Pwr sandwiches in your
stack-up. I often like to put 2 pwr planes inside to outer gnd
plane--especially if those power planes need hacked up for several
different voltage levels. Then you don't care about traces crossing cuts
in the plane.

----->Chris

>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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