Re: [SI-LIST] : Charge moving from decoupling capacitors

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From: sweir (weirsp@atdial.net)
Date: Thu May 11 2000 - 21:28:29 PDT


Barry,

To stay with your fluid analogy, the water will flow from the pipe only as
a result of the accelerating force, and limit by the constrictions. If you
abruptly open a pipe then the local pressure, ( voltage analog ), initially
drops and propagates backwards through the pipes as a sound wave. Have you
flushed the toilet while someone is taking a shower?

Charge only accelerates in response to a change in the local
potential. This is no different than the fluid analogy, except that
instead of molecules bouncing around, we have EM fields. The displacement
problem is so severe with modern edge rates that the decoupling capacitors
on the board do little or nothing for the leading edge. The capacitance
inside the chip, ( hopefully by design ) is the only charge reservoir close
enough to support the device voltage.

The capacitance inside the device supports the chip,
Charge from the planes replenishes the device capacitance,
HF capacitors on the board replenish the planes,
Bulk capacitors replenish the HF capacitors,
The voltage regulator replenishes the bulk capacitors.

The further each element in the hierarchy is from the element above it, the
more charge the higher element must hold locally, or face local collapse of
the power supply.

Regards,

Steve.

At 03:49 PM 5/11/00 -0700, you wrote:
>Hi,
>
>As the speed of digital signals gets faster and faster, people begin being
>concerned with the distance for electric charge to move on power and
>ground planes of multilayer PCB during the signal rise time from a
>decoupling capacitor (cap) to a chip it serves. I would like to raise two
>questions.
>
>(1) The charge is moving in a metalic plane, not inside the dielectric
>between pwr and gnd planes. Please let me know why you have to use the
>propagation velocity in the dielectric, instead of that in the metal.
>
>(2) The second question is regarding distance between the cap and the
>chip. Do we really have to limit the distance letting the charge have
>enough time to move from the cap to the chip during the rise time
>interval? I doubt it.
>
>Take the running water system for example. When we open, then close the
>water faucet within one second, does the water we've got in basin come
>from water tower (or water station, or reservoir)? No, it is the water
>that resides in the pipe. As a matter of fact, we have a very large pipe -
>pwr/gnd planes. Well, of cause you know, I did not mean we don't need
>water tower - the cap. ......
>
>Regards,
>Barry Ma
>bma@ANRITSU.com
>Morgan Hill, CA 95037
>Tel. 408-778-2000
>
>
>_______________________________________________________________________
>
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