U. Glaeser

FIGURE 31.5
to handle large voltages across the gate oxide or source/drain junction must be appropriately protected
by cascoding and well-biasing. This section begins with discussing these large output-swing transmitters.
The section follows with low-swing transmitters, which are more common for high-performance designs.
Noise is the primary challenge. Techniques that reduce noise such as impedance matching, swing
control, and slew-rate control are described next. The last part addresses techniques that can be used to
reduce intersymbol interference due to a band-limiting transmission channel.
Large-Swing Output Drivers
A simple push-pull architecture, as shown in Fig. 31.5, can drive a signal as large as the voltage provided
for the I/O, . When driving a transmission line, the initial output voltage is the result of a voltage division
V
s
d o
where Rdrv
is the on-resistance of the driving device. The initial voltage is also the final voltage if the line is
terminated appropriately at the receiver. In which case, the driver draws continuous current even with the
absence of signal transitions. With only source termination ( Rdrv
equal to Ro),
the line voltage settles to Vs.
The power dissipation is less since no current flows when the signal is constant. If the line is unterminated
on either end, the signal will reflect several times before settling to Vs.
Because the bit period must be long
enough for the signal to settle, high-performance links avoid this penalty.
Impedance matching at the transmitter is challenging because (1) process, voltage, and temperature
(PVT) varies, and (2) the impedance changes significantly as the device is switched from on to off.
To
minimize the net variation, designers over-design the size of the device for an impedance much lower
than Ro.
And then, by adding an external but constant resistance Rext
= Ro
− Rdrv,
the net impedance varies
within an acceptable tolerance.
Many chips are required to interface with chips that operate at different power supply voltages. As onchip
supplies lower with CMOS technology scaling, the disparity between on-chip and off-chip voltages
increases. Unfortunately, for high reliability, the on-chip devices cannot tolerate excessive over-voltage.
Catastrophic breakdown of gate oxide occurs at 12 MV/cm of oxide thickness.
Device technologists address the issue by providing transistors that are slower but high-voltage tolerant.
One of the tasks of the pre-driver is to shift the level of the input so that the output-driver devices are
fully turned off.
Figure 31.5 illustrates an example of level-shifting using cross-coupled PMOS devices in
the pre-driver.
To avoid over-voltage, circuit designers add a cascode transistor in series with the output switch to
reduce the voltage drop [39]. Figure 31.6(a) shows a bottom device that switches with the data. The
upper cascoding device uses a constant high gate voltage that is commonly the core Vdd.
As long as the
output voltage does not exceed + , the gate oxide is preserved. remains below − ,
© 2002 by CRC Press LLC
V dd
Push-pull I/O driver with level shifting pre-drivers.
V
dd
Gnd
V o
=
V
oxide(max)
V s
V dd
Ro --------------------
Rdrv + Ro V S
R drv
Rext Rdrv off-chip V ss
⎛ ⎞
⎝ ⎠
V
x
Z o
V
dd
VT(eff)