Digital Signal Processing Reference
In-Depth Information
The power plane can either be connected to the same supply powering the I/O
driver, or it could be an unrelated supply that powers the ASIC core or other logic
on the board. As we will see, the return paths are very different in these situations.
6.6.2 What Is the Behavior When One Return Path Is a Related Power Plane?
An ASIC driving a stripline is shown in Figure 6.7. The I/O driver drives the line to
a logic high by closing the switch and connecting the resistance (which represents
the on resistance of the driver) to the ASIC I/O power supply.
The stripline is perfectly centered between a ground plane and a power plane
that connects to the ASIC I/O driver. This makes the capacitance (and inductance
not shown) from the trace to the ground plane identical to the capacitance (and
inductance) from the trace to the power plane.
This is not necessarily the case in practice. For example, with offset stripline
the trace is closer to one plane than the other, increasing the coupling to that plane
and decreasing it to the other plane. We can develop our explanation with that
configuration, but to simplify things, we will assume the capacitances (and the
inductances) to each plane are equal.
We start by assuming that the driver output has been at 0V long enough to fully
discharge all the capacitors between the trace and ground. Because the line is at 0V,
the capacitors formed with the power plane are all charged to the plane voltage.
From Figure 6.7 we see that once the driver switches to a logic high it launches
1.5V down the 50
transmission line. Solving for
I
i
with (6.3), we find that the
launched current is 30 mA.
Since the capacitances are equal, the same amount of return current flows in
the two planes. As expected, 15 mA flows in the ground plane back to the driver.
Figure 6.7 shows that the power plane current is actually the result of the driver
discharging the upper capacitors, which were charged when the line was held at
0V. The discharge path is through the ASIC power rail to the pull-up in the driver
and out to the line.
Ω
15 mA
15 mA
Power plane
30 mA
I/O
supply
15 m
A
I
= 30 mA
i
=
50
signal
trace
Ω
V
i
= 1.5V
15 mA
15 mA
15 mA
Ground plane
Figure 6.7
Return path current when stripline is formed between a power plane connected to the
I/O driver and a ground plane.
