From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Sun Apr 02 2000 - 17:06:00 PDT
>My question is which configuration is the best for the return path --
>1. One power and one ground plane, or
>2. Two ground planes with plenty of bypass cap. or
It is almost impossible to avoid return currents going through bypass caps,
or spreading out through the board's internal capacitance.
But there are a few special cases.
If the driver is open-drain or open-collector, then ~all the switching
current through the output transistor goes through the driver IC's ground
pins and ~none through its power pins. In that case, using stripline
between two ground planes would allow the most direct path from return
currents in the planes, through ground pins, to the switching transistor.
In theory, you wouldn't even need bypass caps at this IC because no
switching current due to the output transistor would ever flow through it.
But the IC also has internal switching noise that does need a bypass cap.
(Also note the opposite situation exists at the pull-up at the far end of
the line; bypass caps are essential there unless the signal trace was
between power planes.)
If the driver is PECL, then ~all the switching current goes through the IC's
power (+Vcc) pins and ~none through the ground (-Vee) pins. In that case,
stripline between two power planes would be best. (Or with normal ECL, use
stripline between two ground planes. That's why ECL used a negative
With a push-pull driver, you will get different currents through the
driver's power and ground pins depending on whether it switches low-to-high
or high-to-low, and depending on the load and the previous voltage.
One special case is when the driver is symmetrical, and the load is
terminated on the far end to Vdd/2. Let's say it's a 3.3V CMOS driver and
it drives 50 ohm traces. A full rail-rail output would switch 66mA through
the signal pin. If terminated to Vdd/2, half of this delta-I, or 33mA, is
from the power pin and the other half is from the ground pin (one going from
0mA to 33mA, the other going from -33mA to 0mA, or vice-versa).
In this case, physically symmetrical stripline between power and ground
planes would be best, because the return currents (which are equal in the
two planes) would match the switching currents in the driver IC's power AND
ground pins. In theory, no switching current would flow through bypass
Driving a push-pull output into a terminating pull-up resistor, looks like
the first case above, because the P-channel or pull-up transistor switches
"no" current and might as well not be there. Well, at least in principle,
if you look at only the transistors driving the trace and load and there are
no parasitics to contend with, no crossover current, etc.
An unterminated push-pull output tends to have a transient current through
~only its power pins for the low-to-high edge, and ~only its ground pins for
the high-to-low edge. There is no way to marry this to stripline and avoid
currents through bypass caps. Stripline between power and ground planes is
a good compromise, since half of the return path is direct.
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