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

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From: Nick Dietz (nick@apptel.com)
Date: Tue Apr 11 2000 - 13:44:34 PDT


I'm fairly new to this, but I'll give this one a shot. Somebody shoot me
down if I'm off target. :-)

I worked on a design that required very low noise on a 5V plane that was
powered from a potentially noisy power supply. I placed the bulk capacitors
on the plane side of the inductor and added additional bulk capacitors on
the power supply side. You don't seem to have any bulk capacitance on the
plane side; I'd be careful about that.

I am told that the most important thing to check is that the inductor is
rated for the amount of current you will be pulling through it. This is
especially true for ferrites since they do not work correctly if they are
saturated.

It seems that unless you expect a lot of high frequency noise coming in on
your power supply, you should just scrap the inductor. The low frequency
impedance it adds could hurt you, and potential resonances could also hurt
you. If keeping the inductor, consider adding more caps on the plane side
of the inductor.

Nick.

At 01:58 PM 4/11/00 -0500, you wrote:
>Please note L202 series inductor inserted between the bulk capacitance and
>the connection to the power plane. This worries me most. There seems to
>be no valid reason to stuff series inductance between the bulk and the
>plane. Further, all the local ceramic IC pin bypass will also have to
>suck current through that same inductor. Seems like a terrible idea to
>me. Comments welcome.
>
>Matt Stanik

---
Nicholas Dietz
Applied Telecom, Inc.
3060 Ogden Avenue, Suite 300
Lisle, IL 60532-1690
(630) 357-9290 x 219

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