[SI-LIST] : Réf. : [SI-LIST] : Query

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From: luca.giacotto@transport.alstom.com
Date: Fri Feb 23 2001 - 04:21:54 PST


The point is that you should consider the AC current. For the AC current, a well
decoupled power system looks short circuited. I mean that ideally in the AC
domain the cap is a short circuit, and so the AC current sees the GND plane and
the Power plane as "equipotential".

You can also see this from an electromagnetic point of view: the current
propagating down your line creates a magnetic field around the line itself
(Ampere's law, if I remember well...). This field is bounded by the metal
surfaces around, and on the nearest plane the strength of the field is bigger
then on a further one, and the induced current on this plane is also bigger. The
model is a little simplistic, but it should be good enough to get the feeling of
what happens.

Finally, remember that during propagation the EM wave doesn't know what is there
at the end of a line (an open circuit? A load referenced to Vcc or to GND?).

Hope this helps.
Luca

kuriakose anand <anand_kuriakose@yahoo.com> le 23/02/2001 11:08:16

Veuillez répondre à kuriakose anand <anand_kuriakose@yahoo.com>

Pour : si-list@silab.eng.sun.com
cc : (ccc : Luca GIACOTTO/TRV/DTR/GECALSTHOM)

Objet : [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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