[SI-LIST] Re: Placement of Decoupling Caps
- From: Ray Anderson <raymond.anderson@xxxxxxx>
- To: si-list <si-list@xxxxxxxxxxxxx>
- Date: Thu, 01 Aug 2002 22:17:12 -0700
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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