U. Glaeser

FIGURE 31.6
FIGURE 31.7
hence avoiding source/drain punchthrough of N1.
To avoid a source/drain punchthrough of N2
during
an output high-low transition, the size of the cascode device needs to be large enough so that Vx
does
8
not fall too quickly.
PMOS devices for the pull-up pose an additional challenge. In a half-duplex configuration, the system
tri-states the transmitter by pulling the gate of the driving device to the I/O supply voltage, Vs;
however,
with reflections and inductive ringing, line voltages can exceed Vs.
To avoid forward biasing the drain–well
junction, designers leave the well floating, as shown in Fig. 31.6(b) [8]. Transistors P1
and P2
allow the
well to be charged up to either the pad voltage or Vs
depending on which is higher. To avoid conduction
of the driving device when pad voltage is high,
Pdrv,
transistor P3
pulls the gate input to the pad voltage.
Small-Swing Output Drivers
© 2002 by CRC Press LLC
Cascoding (a) and well-biasing (b) to protect driving devices.
Low-swing, push-pull driver with supply bypassing.
7
I/O standards are migrating toward smaller output voltage swings due to several advantages. There is
less concern regarding over-voltages on I/O devices. Using smaller devices and fewer over-voltage protection
devices reduces output capacitance and improves bandwidth. The device stays in a single region
of operation (either in triode or saturation) reducing impedance mismatches. The transmitter also dissipates
less power because of the lower-swing and smaller drive devices; however, reducing signal swing
9
directly reduces the SNR making the designs more sensitive to noise. The following describes two commonly
used driver architectures: low-impedance and high-impedance drivers.
A simple extension of the large-swing push-pull driver to low-swing is shown in Fig. 31.7, where
is a low voltage that determines the signal swing. The transistors operate in the linear region of their I-V
curve appearing as a low-impedance signal source. With signal swings under 1 V, a smaller NMOS device
can have the same pull-up resistance as PMOS devices. The impedance matching is better than the large
signal driver because the device impedance varies less with Vds
[19].
However, with low-impedance drivers, power-supply noise appears directly on the signal. By connecting
the power supply as the signal’s return connection, the noise would appear as common-mode. Unfortunately,
7
Feedthrough from output to the Vgate
of N2
can dynamically elevate Vx
so N2
cannot be excessively large.
N2
can often be a size 4×
larger than N1.
Fortunately, many noise sources are proportional to the signal swing, so the SNR degradation is not overly severe.
8
9
d o
V dd
Pull up
Vx
N 2
N 1
Pad
d o
Pull down
(a) (b)
Pre-driver
do dout do V s
R ext
P 3
P drv
Z o
P 2
P 1
V well
Pad
V
s