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

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Fri May 12 2000 - 11:13:36 PDT


> 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.
 
The charge may be moving in the metal, but the energy (which makes the
charge keep moving) is primarily in the electro-magnetic field between the
planes, in the dielectric. The charge won't move unless there is an E-M
field to push it.

It is just like an ordinary transmission line such as stripline. The
propagation velocity of a trace is that of the dielectric, even though the
charge moves only in the metal trace and planes.

> (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.
 
That depends whether you need the capacitor to help during the rise time
interval itself.

If you had a single 1.0 Farad cap and attached it with 20 foot long jumper
cables to your chip, it would do nothing to help the chip during the rise
time interval. The jumper cable is a transmission line. If the voltage
sags at your chip, it takes many nanoseconds for the sag to reach the
capacitor. Until the sag reaches it, charge doesn't even start moving out
of it, i.e., the cap might as well not be there.

Now flatten the jumper cable into two planes. The planes are a fat
transmission line (really!). The voltage sag propagates outward from the
chip, consuming charge stored in the intrinsic capacitance of the planes bit
by bit (not all of it at once!), and eventually reaching external capacitors
which help hold up the voltage.

> 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. ......
 
Pwr/gnd planes are similar to a long pipe. At first they help hold up the
pressure, but without a tank, the pressure would disappear.

Even with a tank, the pressure does drop a little when you open the faucet.
Open a very large faucet, and the pressure immediately drops a lot ...
especially if you are the house at the end of the water main. Open a very
large faucet right at the tank or pumping station, and the drop in pressure
is much less. (This analogy is not very good, however, because water pipes
also have resistance.)

Regards,
Andy

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