LT1719 Linear Technology, LT1719 Datasheet - Page 9

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

APPLICATIONS
Hysteresis
The LT1719 includes internal hysteresis, which makes it
easier to use than many other similar speed comparators.
The input-output transfer characteristic is illustrated in
Figure 3 showing the definitions of V
upon the two measurable trip points. The hysteresis band
makes the LT1719 well behaved, even with slowly moving
inputs.
The exact amount of hysteresis will vary from part to part
as indicated in the specifications table. The hysteresis
level will also vary slightly with changes in supply voltage
and common mode voltage. A key advantage of the
LT1719 is the significant reduction in these effects, which
is important whenever an LT1719 is used to detect a
threshold crossing in one direction only. In such a case,
the relevant trip point will be all that matters, and a stable
offset voltage with an unpredictable level of hysteresis, as
seen in competing comparators, is useless. The LT1719 is
many times better than prior comparators in these re-
gards. In fact, the CMRR and PSRR tests are performed by
checking for changes in either trip point to the limits
indicated in the specifications table. Because the offset
voltage is the average of the trip points, the CMRR and
PSRR of the offset voltage is therefore guaranteed to be at
least as good as those limits. This more stringent test also
puts a limit on the common mode and power supply
dependence of the hysteresis voltage.
V
OL
Figure 3. Hysteresis I/O Characteristics
V
TRIP
V
OS
–
=
0
V
(= V
TRIP
U
TRIP
+
+ V
2
V
HYST
+
INFORMATION
TRIP
– V
U
TRIP
–
V
HYST
–
)
/2
V
TRIP
W
OS
+
and V
V
IN
V
OH
= V
1719 F03
IN
HYST
+
U
– V
IN
based
–
Additional hysteresis may be added externally. The rail-to-
rail outputs of the LT1719 make this more predictable than
with TTL output comparators due to the LT1719’s small
variability of V
To add additional hysteresis, set up positive feedback by
adding additional external resistor R3 as shown in Figure
4. Resistor R3 adds a portion of the output to the threshold
set by the resistor string. The LT1719 pulls the outputs to
+ V
loads, and to within 400mV with heavy loads. For the load
of most circuits, a good model for the voltage on the right
side of R3 is 300mV or +V
swing of (+V
With this in mind, calculation of the resistor values needed
is a two-step process. First, calculate the value of R3 based
on the additional hysteresis desired, the output voltage
swing and the impedance of the primary bias string:
Additional hysteresis is the desired overall hysteresis less
the internal 4mV hysteresis.
The second step is to recalculate R2 to set the same
average threshold as before. The average threshold before
was set at V
calculated based on the average output voltage (+V
and the simplified circuit model in Figure 5. To assure that
the comparator’s noninverting input is, on average, the
same V
For additional hysteresis of 10mV or less, it is not uncom-
mon for R2 to be the same as R2 within 1% resistor
tolerances.
R3 = (R1 R2)(+V
R2 = (V
S
and ground to within 200mV of the rails with light
TH
as before:
Figure 4. Additional External Hysteresis
REF
TH
S
OH
– 300mV) – (300mV) = +V
– V
V
REF
= (V
(output high voltage).
R2
R1
TH
INPUT
REF
)/(V
S
– 0.6V)/(additional hysteresis)
)(R1)/(R1 + R2). The new R2 is
TH
+
–
LT1719
/R1 + (V
S
R3
– 300mV, for a total voltage
TH
1719 F04
– V
S
S
/2)/R3)
– 600mV.
LT1719
S
9
/2)