LT1719 Linear Technology, LT1719 Datasheet - Page 13

• Manufacturer Part Number: LT1719
• Description: 4ns/ 150MHz Dual Comparator with Independent Input/Output Supplies
• Manufacturer: Linear Technology

APPLICATIONS
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 fre-
quency. The output transistors are idled in a low power
state once V
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 LT1719 is several hundred picoseconds faster when
V
to the internal speed limit; the gain stage operates between
V
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
providing 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.
EE
EE
= – 5V, relative to single supply operation. This is due
and + V
S
OH
, and it is faster with higher reverse voltage
or V
OL
U
is reached, by detecting the Schottky
INFORMATION
U
W
U
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 distur-
bances 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 propaga-
tion delay, without regard to these external influences
that can cause trouble in nonhysteretic comparators.
The input trip points test circuit uses a 1kHz triangle wave
to repeatedly trip the comparator being tested. The LT1719
output is used to trigger switched capacitor sampling of
the triangle wave, with a sampler for each direction. Be-
cause the triangle wave is attenuated 1000:1 and fed to the
LT1719’s differential input, the sampled voltages are there-
fore 1000 times the input trip voltages. The hysteresis and
offset are computed from the trip points as shown.
Crystal Oscillator
A simple crystal oscillator using an LT1719 is shown on
the first page of this data sheet. The 2k-620 resistor pair
set a bias point at the comparator’s noninverting input.
The 2k-1.8k-0.1 F path sets the inverting input node at an
appropriate DC average level based on the output. The
crystal’s path provides resonant positive feedback and
stable oscillation occurs. Although the LT1719 will give
the correct logic output when one input is outside the
common mode range, additional delays may occur when
it is so operated, opening the possibility of spurious
operating modes. Therefore, the DC bias voltages at the
inputs are set near the center of the LT1719’s common
mode range and the 220 resistor attenuates the feed-
back to the noninverting input. The circuit will operate with
any AT-cut crystal from 1MHz to 10MHz over a 2.7V to 6V
supply range. As the power is applied, the circuit remains
off until the LT1719 bias circuits activate, at a typical V
of 2V to 2.2V (25 C), at which point the desired frequency
output is generated.
V
TRIP
Test Circuit
LT1719
13
CC