[SI-LIST] Re: Return currents

Pras,
Image current in this example just so happens to flow on the VDD plane when the 
drvr switches state.  The current loop is completed through local decoupling 
capacitors.  The issue is that these caps cannot supply the switching rate 
currents required creating power/ground bounce.
-Jory

----- Original Message ----
From: Pras venki <venki.pras@xxxxxxxxx>
To: si-list@xxxxxxxxxxxxx
Sent: Sunday, November 4, 2007 8:44:27 PM
Subject: [SI-LIST] Return currents


Hello Guys,
I have this confusion regarding "*Return currents*"-

1) In the paper "SSN & power plane bounce in CMOS technology" by Larry
 Smith
( http://www.csee.umbc.edu/vlsi/reports/ssn_pwr_planes.pdf
<http://www.csee.umbc.edu/vlsi/reports/ssn_pwr_planes.pdf+>).  This is
available online for free, so i m pasting it here. (I hope i can)

In the following excerpt-

"Suppose the transition is from low to high and the cross-section of
 the
package has the transmission line located above a Vdd plane as shown in
figure 1.The driver connects the Vdd plane to the transmission line
 through
a low impedance.Current flows from the Vdd plane onto the transmission
 line
which is low, say ground potential.As the wave front propagates down
 the
transmission line, charge flows into the capacitance between the trace
 and
the Vdd plane, raising the potential on the trace up to Vdd. The
 current
path is complete because charge from the Vdd plane flows in a complete
 loop
from the Vdd plane, through the driver and onto the transmission line
 that
is referenced to the Vdd plane. If there is a ground plane underneath
 the
Vdd plane, it is not disturbed because it is not part of the current
 loop."

Where does the return current flow? Does the return current flow
 through the
inter-plane capacitance? Doesn't it need to flow thru a reference
 plane?

If it can't flow thru the Gnd plane, is it possible for it to flow thru
 the
same power plane which is supplying the current (i hope not coz it will
totally screw my fundamentals on current flow).

 2)What if the package does not have an explicit power or ground plane
i.e. Power
& Gnd are normal, thin traces (like other signal traces), distributed
sporadically along with other I/Os, signal, clock traces etc. (Although
 the
power & ground traces are de-coupled inside the chip & if the I/O
 traces r
driven using a CMOS buffer)

How is the return current going to flow now, when the I/O traces r
 driven
using the buffer?

Will the return current still look to flow through the nearest Power or
 Gnd
reference trace it finds, given they are randomly routed in the package
 like
ordinary traces?

I'd really appreciate if somebody can clear this nagging doubt.

Thanx in advance.

Regards,
pras


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