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

About this list Date view Thread view Subject view Author view

From: Doug McKean (dmckean@gte.net)
Date: Thu May 11 2000 - 22:05:57 PDT


Barry Ma 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.

Simple. The power/ground plane construction is
a 2-dimensional transmission line. Just as you
determine the velocity of propagation in a
1-dimensional transmission line, the dielectric
is part of the equation for the capacitive
component. And so it follows with the planar
construction.

> (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.

Depends. Distance can mean inductance.

Since, v(t) = L di/dt and with L in nano-henries (10^-9)
and suppose dt in pico-seconds (10^-12), we're already
up in the 10^3 range ... - Doug McKean

**** To unsubscribe from si-list or si-list-digest: send e-mail to
majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE
si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP.
si-list archives are accessible at http://www.qsl.net/wb6tpu
****


About this list Date view Thread view Subject view Author view

This archive was generated by hypermail 2b29 : Wed Nov 22 2000 - 10:50:18 PST