LT1719
17
1719fa
three categories: input speed limits, output speed limits,
and internal speed limits.
There are no significant input speed limits except the shunt
capacitance of the input nodes. If the 2pF typical input
nodes are driven, the LT1719 will respond.
The output speed is constrained by two mechanisms, the
fi rst of which is the slew currents available from the output
transistors. To maintain low power quiescent operation,
the LT1719 output transistors are sized to deliver 25mA
to 45mA typical slew currents. This is sufficient to drive
small capacitive loads and logic gate inputs at extremely
high speeds. But the slew rate will slow dramatically with
heavy capacitive loads. Because the propagation delay (t
PD
)
definition ends at the time the output voltage is halfway
between the supplies, the fixed slew current makes the
LT1719 faster at 3V than 5V with large capacitive loads
and suffi cient input overdrive.
Another manifestation of this output speed limit is skew,
the difference between t
PD
+
and t
PD
–
. The slew currents
of the LT1719 vary with the process variations of the
PNP and NPN transistors, for rising edges and falling
edges respectively. The typical 0.5ns skew can have either
polarity, rising edge or falling edge faster. Again, the skew
will increase dramatically with heavy capacitive loads.
A separate output speed limit is the clamp turnaround.
The LT1719 output is optimized for fast initial response,
with some loss of turnaround speed, limiting the toggle
frequency. The output transistors are idled in a low power
state once V
OH
or V
OL
is reached, by detecting the Schottky
clamp action. It is only when the output has slewed from
the old voltage to the new voltage, and the clamp circuitry
has settled, that the idle state is reached and the LT1719
is fully ready to toggle again. This is typically 8ns for each
direction, resulting in a maximum toggle frequency of
62.5MHz. With higher frequencies, dropout and runt pulses
can result. Increases in capacitive load will increase the time
needed for slewing due to the limited slew currents and
the maximum toggle frequency will decrease further. For
high toggle frequency applications, consider the LT1394,
whose linear output stage can toggle at 100MHz typical.
The internal speed limits manifest themselves as disper-
sion. All comparators have some degree of dispersion,
defined as a change in propagation delay versus input
overdrive. The propagation delay of the LT1719 will vary
with overdrive, from a typical of 4.5ns at 20mV overdrive
to 7ns at 5mV overdrive (typical). The LT1719’s primary
source of dispersion is the hysteresis stage. As a change
of polarity arrives at the gain stage, the positive feedback
of the hysteresis stage subtracts from the overdrive avail-
able. Only when enough time has elapsed for a signal to
propagate forward through the gain stage, backwards
through the hysteresis path and forward through the gain
stage again, will the output stage receive the same level
of overdrive that it would have received in the absence
of hysteresis.
The LT1719S8 is several hundred picoseconds faster when
V
EE
= –5V, relative to single supply operation. This is due
to the internal speed limit; the gain stage operates between
V
EE
and +V
S
, and it is faster with higher reverse voltage
bias due to reduced silicon junction capacitances.
In many applications, as shown in the following examples,
there is plenty of input overdrive. Even in applications pro-
viding low levels of overdrive, the LT1719 is fast enough
that the absolute dispersion of 2.5ns (= 7 – 4.5) is often
small enough to ignore.
The gain and hysteresis stage of the LT1719 is simple, short
and high speed to help prevent parasitic oscillations while
adding minimum dispersion. This internal “self-latch” can
be usefully exploited in many applications
because it occurs
early in the signal chain, in a low power, fully differential
stage. It is therefore highly immune to disturbances from
other parts of the circuit, such as the output, or on the
supply lines. Once a high speed signal trips the hysteresis,
the output will respond, after a fixed propagation delay,
without regard to these external influences that can cause
trouble in nonhysteretic comparators.
APPLICATIONS INFORMATION
