RE: [SI-LIST] : Bulk Caps -> Inductor? -> Plane

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Tue Apr 11 2000 - 15:38:52 PDT


>I'm just not accustomed to seeing additional impedance in the main
>path of the power subsystem.

In the old days, and I'm thinking early enough to include tubes or valves,
it was pretty common to have a large choke or two in the supply filter. An
LC pi filter gave you more bang-for-the-buck than simply using a bigger
capacitor.

Voltage regulators changed that.

If you consider a PC-type power supply with a wiring harness to the
motherboard, then you have a fair amount of inductance (especially common
mode) in those wires between the supply's bulk capacitance and the board. I
hope nobody would consider passing IC switching current through those wires.
That interface should be strictly LF.

Earlier, you wrote:
>Further, all the local ceramic IC pin bypass will also have to
>suck current through that same inductor.
 
Only when the supply first ramps up. Thereafter, the IC switching current
goes through all the local ceramic bypass caps and doesn't need to /
shouldn't come through the inductor; in fact the inductor blocks it. The HF
switching current should be local anyway and not involve the bulk
capacitors, which aren't good at high frequencies anyway.

Andy

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