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

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From: George_Tang@Dell.com
Date: Tue May 23 2000 - 12:19:40 PDT


Barry,

The current can flow from the cap, but it will get to the IC at the wrong
time.

Regards,
George
george_tang@dell.com

-----Original Message-----
From: Barry Ma [mailto:barry_ma@altavista.com]
Sent: Tuesday, May 23, 2000 10:22 AM
To: George_Tang@exchange.dell.com
Cc: si-list@silab.eng.sun.com; emc-pstc@ieee.org
Subject: RE: Charge moving from decoupling capacitors

George,

I am impressed by your attitude to pursue the correctness, and glad to
discuss with you further on “How does a decoupling capacitor support an IC?”

Here is my two cents worth. The decap supplies necessary charge to the IC
during Tr through a transmission line. As you mentioned before: “The current
is an impulse function, although the voltage waveform is a step function.”
This impulse function, actually a bell-like function on Tr, happens every
time period T when the IC gate switches from low to high. The corresponding
frequency spectrum contains lots of frequencies. There must be some
frequencies making the transmission line a 1/4, 3/4, ... wavelength. It is
hard for me to be convinced that currents of those frequencies cannot flow
from the decap to the IC. ... Pleas correct me if misunderstood. Thanks.

Regards,
Barry Ma
bma@ANRITSU.com

--------
On Mon, 22 May 2000, George_Tang@Dell.com wrote:

>
> Barry,
>
> I need to make a correction. I was rushing to lunch on Thursday, so I did
> not read over what I wrote. Here is the correction for the 2nd comment
> below:
>
> At 1/4 wavelength, the charges are 180 degrees out of phase, so they are
> working against the IC current draw. 1/8 wavelength (90 degrees out of
> phase) is what I consider to be acceptable.
>
> Regards,
>
> George Tang
> george_tang@dell.com

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