FreeLists / si-list / [SI-LIST] Re: Return current -- retransmit because of corruption.

[Neeraj Pendse <cnepsc@xxxxxxxxxxxxxxx>]

Hi Gautam:

When you say "stripline" configuration, I am assuming that you mean a
physical
stackup "G-S-P". (My belief is that) you can call it a stripline only if
both the
planes are carrying return current.

For a CMOS driver, assuming no decoupling capacitance between the
planes, for a L->
H transition, the return path will be the power plane. ANd for an H-> L
transition,
it will be the ground plane. You can easily see this if you draw
yourself the
output stage of a CMOS driver ... approximated like a simple CMOS NOT
gate.

Now the argument is that there is "sufficient" capacitance between the
two planes
(by virtue of being exposed to each other because they are on the same
board and by
intentional power decoupling capacitors added. Since capacitors are a
short circuit
in AC, for the edges, the power and ground planes will appear to be the
"same",
and so the return current with flow in both the planes. There are
numerous
references which show how and how much decouplig capacitance to use for
what
frequencies - and the one I recall is Dr. Johnson's book.

To determine the fractions of currents flowing on the two planes, I
don't have a
back of the envelope calculation. But it will depend upon all the mutual
interactions between all the structures (i.e., the traces, the planes
and the
de-caps). I suggest if you build a model for a short
stripline-configuration, you
will be easily able to see it. You can use lumped RLC or distributed RLC
model with
partial inductances to achieve this.

Hope that helps,

- Neeraj.

National Semiconductor Corporation
2900 Semiconductor Drive, Mail Stop 19-100
Santa Clara, California 95051
http://www.national.com/, NYSE: NSM



Goutham.S@xxxxxxxxxx wrote:

> Hi,
>
> In a stripline environment, with the signal trace equidistant from a ground
> plane and a power plane, what will the return current path be for a H-L and 
> L-H
> transition be?  Also what percentage of the return current will flow thro the
> ground and power planes, assuming the return current will be ditributed 
> between
> the two ref planes. Also the two ref planes are assumed to have equal 
> thickness.
>
> Any answers or comments........
>
> Regards
> Goutham Sabavat
> Force Computers india Pvt Ltd
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> http://www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
> List archives are viewable at:
>                http://www.freelists.org/archives/si-list
> or at our remote archives:
>                http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
>                http://www.qsl.net/wb6tpu
>
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages 
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu