[SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator

  • From: Larry Smith <Larry.Smith@xxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Mon, 4 Aug 2003 09:10:14 -0700 (PDT)

Robert - Let me address the subject of capacitor placement with a quick
war story.  Several years ago, we were having an EMI problem at about
900 MHz.  We determined the capacitor value that made a low impedance
at that frequency while mounted on our power planes, I think it was
about 27pF or so.  We placed several of them on the PCB just a few
inches away from our microprocessor in the decoupling pad sites that
were available.  The result was that EMI at 900 MHz got worse!  Some
simple calculations showed why.  We had placed relatively low impedance
capacitors 1/4 wavelength from the source.  Because of transmission
line phenomenon, a low impedance becomes a high impedance 1/4
wavelength away.  This was in the very early days of power plane
modeling.  Our primitive power plane models predicted this effect quite
nicely.  That convinced us that we had to use PDS tools to place caps
that resonate at frequencies that translate to wavelengths that are
comparable to the PCB size.

That was back in the days when we used power and ground planes spaced
about 14 mils apart with two signal layers in-between. Since then, we
have learned the value of thin power plane dielectrics and always place
power and ground planes next to each other in the stackup, somewhere
between 2 and 4 mils apart.  This has a number of advantages including
increased capacitance between planes, lower spreading inductance, lower
power plane impedance, increased noise damping and better return path
for high frequency signals traveling in reference to those planes.
Several papers have been written from authors at Sun which fully
describe this.  Many EMI problems have been solved by thin power plane
dielectrics.

With a good power plane stackup, it may not be necessary to use many
(or any..) capacitors below 1000pF.  As Ray mentioned below, the power
integrity problem is usually limited to less than 100 MHz by the chip
package inductance.  If it is still necessary to use low valued
capacitors for the signal integrity (return current) or EMI problems,
then position on the power plane does become an important issue.

regards,
Larry Smith
Sun Microsystems

> Delivered-To: si-list@xxxxxxxxxxxxx
> From: "Robert Sefton" <rsefton@xxxxxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Subject: [SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator
> Date: Sun, 3 Aug 2003 10:27:54 -0700
> MIME-Version: 1.0
> Content-Transfer-Encoding: 8bit
> X-Priority: 3
> X-MSMail-Priority: Normal
> X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1165
> X-archive-position: 7707
> X-ecartis-version: Ecartis v1.0.0
> X-original-sender: rsefton@xxxxxxxxxxxxx
> X-list: si-list
> 
> Ray -
> 
> 5.5" is a pretty loose requirement. That seems consistent with
> Lee's (and UMR's) statement that "location of the capacitors is
> relatively unimportant" for decoupling. But what about EMI? Does
> placing the decaps closer help with EMI, even if it doesn't
> improve the PDS performance? Doesn't seem like it would.
> 
> Thanks,
> Robert
> 
> ----- Original Message ----- 
> From: "Ray Anderson" <Raymond.Anderson@xxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Sent: Friday, August 01, 2003 12:50 PM
> Subject: [SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator
> 
> 
> > With all due respect to the UMR authors, I think the location
> > of a decap on a set of power planes DOES matter.
> >
> > There is an inherent time delay associated with the current flow
> > from the location where a decap is placed to the location where
> the
> > chip to bypassed is located.
> >
> > The decaps job is to provide current to the load quickly enough
> > that a voltage collapse on the planes doesn't occur. If the
> decap
> > is too far away from the current consuming load it is possible
> > that load's need for a pulse of current will be over before the
> > current can get there.
> >
> > At low frequencies you can place a decap just about anywhere on
> a plane
> > and it will function just fine. At higher frequencies the decap
> must
> > be closer to the device to be bypassed. The concept of
> "effective bypassing
> > radius" says that a decap must be within say 1/6 to 1/10
> wavelength
> > at the frequency of interest to be effective. (the exact number
> is
> > debatable). The frequency of interest is the resonant frequency
> of
> > the mounted decap (where the caps ESR is lowest), and the
> distance is
> > a function of the speed of propagation with a given dielectric
> between
> > the planes (around 180ps/inch for FR4).
> >
> > You need to consider the role each component of a PDS (power
> distribution
> > system) plays in maintaining a low impedance across a wide
> bandwidth.
> > A typical PDS is composed of a VRM, bulk capacitors, ceramic
> decaps, and
> > the power planes and last but not least the package housing the
> silicon.
> >
> > The VRM provides a low impedance from DC to perhaps a few
> hundred kHz,
> > the bulk capacitors contribute their impedance profile from a
> few
> > hundred kHz up to about a MHz. From a Mhz to maybe 120MHz or so
> the
> > ceramic decoupling caps provide low impedance. Above that
> frequency
> > the distributed capacitance provided by the planes provides the
> requisite
> > low impedance to the PDS. When the individual impedance profiles
> of
> > each of the constituent parts are superimposed, the resulting
> composite
> > profile provides a low impedance over the full frequency range
> that the
> > PDS was designed for. Note that above the package resonance
> frequency
> > (which usually is from about 50 to 100 MHz for typical packages)
> decoupling
> > on the board is ineffective and the necessary decoupling must be
> provided
> > for either in the package or on the silicon. However, at
> frequencies
> > above the package resonance frequency the discrete decaps and
> distributed
> > plane capacitance can be most useful for EMI purposes.
> >
> > To summarize, at low frequencies, the decoupling capacitors can
> be placed
> > almost anywhere withing reason and will be effective. As the
> frequency
> > of interest increases, the allowable distance decreases. As an
> example,
> > at 1 MHz the wavelength (on FR4) is around 214 meters. So
> lambda/10 is
> > around 21 meters. Hence, if your decap which is effective at 1
> MHz is
> > within 21 meters of the consuming device, then the decoupling
> will be
> > effective. Conversely, if you are trying to decouple at device
> with
> > a decap that is effective at say 100 MHz then the wavelength of
> interest
> > is about 1.4 meters. Lambda/10 is then .14 meters (approx 5.5
> inches).
> > In this case if the decap is appreciably more than 5.5 inches
> from the
> > currewnt consumer then the delay will be such that the decap
> will not be able to
> > provide current quickly enough to be effective.
> >
> > -Ray Anderson
> > Sun Microsystems Inc.
> >
> >
> > Lee Ritchey wrote:
> > >> If you remember the UMR paper on power bus decoupling, it
> made a clear case
> > >> that the location of the capacitors is relatively
> unimportant.
> >
> 
> 
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> 
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
> 
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> 
> List archives are viewable at:     
>               //www.freelists.org/archives/si-list
> or at our remote archives:
>               http://groups.yahoo.com/group/si-list/messages 
> Old (prior to June 6, 2001) list archives are viewable at:
>               http://www.qsl.net/wb6tpu
>   
> 

------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:     
                //www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages 
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
  

Other related posts: