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

From: Vinu Arumugham ([email protected])
Date: Wed May 17 2000 - 17:11:12 PDT

Barry Ma wrote:

> Hi Yu,
>
> Let?s begin with the excellent description written by Andrew Ingraham:
>
> ?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.?
>
> When a chip gate opens, the V (and Q) sag on the metal plane propagates outward from the chip at circle front, consuming charges inside the circle. If the outgoing circle front meets a decap, charge would be moving from the decap to the chip. ...

As others have pointed out, this can be viewed as a transmission line. If the outgoing sag wave hits a decap, it will reflect back to the chip as a peak wave.

> What if the circle does not reach any decap before the end of Tr?

The decap should be much closer than 0.5 Tr to be of use. If there are no decaps, the sag wave will continue until it hits the edge of the board, see an open circuit, double up and travel back to the chip as a deeper sag. The propagation velocity will of course be determined by the dielectric between the two planes that form the transmission line. When decaps are present, the propagation will be slowed down just as in a signal transmission line with capacitive loads.

> The gate has closed, no current loop can be formed. That is the scenario I was interested in. Now an electrical potential imbalance happens in a metallic plane. Charge on the plane would flow toward the circle for regaining eqi-potential. ...
>
> Regards,
> Barry Ma
> ANRITSU company
> Morgan Hill, CA 95037
>
> -------------
> From: "Yu Wang" <[email protected]>, on 5/17/00 7:45 AM:
>
> Hi, Barry,
>
> I think we all do agree the 1st point you said.
>
> But on the 2nd point, I have another opinion. Say, you are right,"When an electrical potential imbalance happens in a metallic plane, a current would flow on the plane for regaining the equi-potential." But we regard the metallic plane as a reference plane,
> then, theoretically, there is no an electrical potential imbalance happens in a metallic plane. In fact, we can never get a ideal reference plane(even the earth is not IDEAL). Based on this, I would say "yes" on your question "Does it need EM field support from the dielectric?". Still but, the need of the support is usually slight if the electical size of the metallic plane is big enough. because we know the metallic plane is a equi-potential plane.
>
> regards,
> Yu Wang, Ph.D
> U.T. MD Anderson Cancer Center
> 1100 Holcombe Blvd., Box 217
> Houston, TX, 77030
> Tel:713-745-1671
>
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