[SI-LIST] Return currents

  • From: "Pras venki" <venki.pras@xxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Sun, 4 Nov 2007 21:44:27 -0500

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.


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