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

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From: Ritchey Lee (leeritchey@earthlink.net)
Date: Mon May 15 2000 - 18:30:13 PDT


A\ndy,

Those are very good answers. Keep up the good work.

Lee

Ingraham, Andrew wrote:

> > 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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