RE: [SI-LIST] : Query

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From: Larry Miller (ldmiller@rhapsodynetworks.com)
Date: Fri Feb 23 2001 - 05:06:26 PST


The return current AC components follow the paths of least INDUCTANCE
(Maxwell's Laws), while the DC or low-frequency return current components
follow the paths of least RESISTANCE (Ohm's Law). Thus, the return current
can divide between multiple paths, depending upon frequency content. None of
this is a binary situation-- the currents flow in accordance with 1/R and
1/L. If the Vcc plane presents the lowest inductance to an edge wave, then
that's where most of the current will flow.

Ideally, Vcc and ground are shorted together for AC. This is getting more
difficult as edge rates get faster and decoupling component parasitics
become more significant. Nonetheless, the above paths are what are followed,
like it or not.

Larry Miller

-----Original Message-----
From: owner-si-list@silab.eng.sun.com
[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of kuriakose anand
Sent: Friday, February 23, 2001 2:08 AM
To: si-list@silab.eng.sun.com
Subject: [SI-LIST] : Query

I have a question regarding the return currents
flowing through the nearest reference plane in a
multilayer PCB stackup. I dont understand the concept
of return current flowing through the power(VCC) plane
if it happens to be the nearest plane to the current
carrying trace. When the driver makes a low to high
transition current flows from the vcc plane(rather the
bypass cap) into the Vcc pin of the driver and out of
the I/O pin and into the receiver(load) and then
finally into the ground and again re-enters the Vcc
pin of the driver making a current loop. In the above
scenario how does the return current flow through the
Vcc plane if i happens to be the nearest ref plane and
not the ground plane itself as i have put in the
current loop above. Thanks in advance.

Anand Kuriakose

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