FreeLists / si-list / [SI-LIST] Antwort: Re: Placement of Decoupling Caps

[Andreas_Lenkisch@xxxxxxxxxx]

Ray,

I agree with your numbers of 1/10 or 1/8 of wavelength and term "sphere of
influence", but I would relate this number to the highest frequency of
interest (approx. 0,3xx/trise), not to the self resonant frequency (SRF).
In a very simple approach, at higher frequencies you bypass anyway with the
inductive part of the cap, the capacitive part is only used to block DC.
Two different caps of different capacitance but the same package will have
different SRF's, but (nearly) the same body inductance. Above the SRF they
behave in the same manner. Looking from the point of application, I would
make no difference in placing, but I would choose in the highest
capacitance in the smallest package (due to lowest inductance)

regards
Andreas Lenkisch



                                                                                
                              
                      Ray Anderson                                              
                              
                      <raymond.anderson        An:       si-list 
<si-list@xxxxxxxxxxxxx>                      
                      @sun.com>                Kopie:    Rodel Regucera 
<RodelRegucera@xxxxxxxxxxxxxxx>       
                      Gesendet von:            Thema:    [SI-LIST] Re: 
Placement of Decoupling Caps           
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                      02.08.2002 07:17                                          
                              
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                      raymond.anderson                                          
                              
                                                                                
                              
                                                                                
                              







Rodel Regucera wrote:
>
> Ray,
>
> Can you explain to me your time delay equation with an example. I thought
> this as only needed in the high-speed board. I am thinking of how  can I
> apply it in a high power switching supply with a pwm freq of around 80
khz.
>
> Thank you in advance.
>
> Rodel Regucera


Rodel-

When you place decoupling caps on a board they have a "sphere of influence"
or should I say a "disk of influence" that has a radius of about 1/10 the
wavelength of the decaps series resonant frequency. This means that current
sinks that are within the 1/10 wavelength distance from the decap in
question will be effectively bypassed. The 1/10 number isn't a hard number.
1/8 wavelength might be OK too, the point is that the decap needs to be
close
in relation to the frequency you are trying to bypass. Also the distance is
the distance on the board not in free space.

Say you are using a large bulk cap to bypass something in your 80 khz
switching power supply. We'll assume its series resonant frequency is
somewhere near your switching frequency. 1/10 wavelength at 100khz
is around 100 meters in air or 50 meters on FR4. So you could place
you decap anywhere on the board and it would be just as effective.

Now assume you are decoupling a high frequency processor running at
1 GHz. Say you chose a decap that resonates at 1GHz. 1/10 wavelength
at 1GHz is about .03 meters in air or .015 meters on FR4. So
you would need to place your decap within about 1.5 cm of the current
consumer for it to be effective.

These examples are described in the frequency domain, but if you
change gears and start thinking about time domain you will be able
to see that the same arguments hold except that instead of talking
about distances you will be talking about time delays.

One commercial tool that is useful for power distribution design
is Cadence Specctraquest Power Integrity. When you place decaps
on the power planes with that tool, a circle of influence is
automatically drawn based on the 1/10 (or 1/8) wavelength criteria
to assist the user in visualizing where to place the caps.

As others have mentioned, the 1/10 wavelenth criteria is just one
of many things that need to be considered when placing decaps.

Ray Anderson
Sun Microsystems Inc.
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