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

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From: sweir (weirsp@atdial.net)
Date: Wed May 17 2000 - 14:47:36 PDT


Barry,

I will certainly agree that a bulk decoupling capacitor may be placed
anywhere on the board, because the frequency response limit is so
low. However, you should be a little bit more careful about how you treat
the smaller value decoupling capacitors. There is frequently some gain to
be had in maintaining those devices close to the loads, as the plane
inductance and propagation time, albeit small, has to be overcome before
any charge from the decoupling capacitors comes to the aid of charge stored
in the device.

There is another benefit of placing decoupling capacitors near device I/O.
The capacitor ground via, ( multiple ground layers and the signal is
reflecting against a plane which is not the same as the device I/O ground
plane ), or the capacitor itself, ( the signal is reflecting against a
power plane that the capacitor decouples ) provides a path for return
currents into the the device.

Regards,

Steve.
At 09:15 AM 5/17/00 -0700, you wrote:
>Steve,
>
>Thanks a lot for the very nice hierarchy description below.
>
>If there's a 10 mil or less spacing between pwr and gnd planes, the plane
>cap is available. The plane cap and the decaps are complementary in whole
>frequency range. Plane cap takes care of high end, and decaps cover low
>portion. Then the location of decaps are not critical. And then decaps can
>be shared by other chips, according to the excellent research conducted by
>EMC lab at UMR.
>
>Please allow me to modify a bit your description as follows.
>
>The capacitance inside the device supports the chip first, but usually not
>enough.
>Charge from the planes also supports the chip and replenishes the device
>capacitance,
>Decaps replenish the plane on low frequency portion, while plane cap
>responds itself on HF end,
>Bulk capacitors replenish decaps and plane cap,
>The voltage regulator replenishes the bulk capacitors.
>
>Please correct me. Thanks.
>
>Regards,
>Barry
>bma@ANRITSU.com
>-----------------
>From: "sweir" <weirsp@atdial.net>, on 5/11/00 9:28 PM:
>....
>
>The capacitance inside the device supports the chip,
>Charge from the planes replenishes the device capacitance,
>HF capacitors on the board replenish the planes,
>Bulk capacitors replenish the HF capacitors,
>The voltage regulator replenishes the bulk capacitors.
>
>....[edited by bm]
>
>
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