LT1720 Linear Technology, LT1720 Datasheet - Page 9

• Manufacturer Part Number: LT1720
• Description: 4ns/ 150MHz Dual Comparator with Independent Input/Output Supplies
• Manufacturer: Linear Technology

APPLICATIONS
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 LT1720/
LT1721 is the significant reduction in these effects, which
is important whenever an LT1720/LT1721 is used to de-
tect 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 hyster-
esis, as seen in competing comparators, is of little value.
The LT1720/LT1721 are many times better than prior com-
parators in these regards. 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.
Additional hysteresis may be added externally. The rail-to-
rail outputs of the LT1720/LT1721 make this more pre-
dictable than with TTL output comparators due to the
LT1720/LT1721’s small variability of V
voltage).
To add additional hysteresis, set up positive feedback by
adding additional external resistor R3 as shown in Figure
3. Resistor R3 adds a portion of the output to the threshold
set by the resistor string. The LT1720/LT1721 pulls the
outputs to the supply rail and ground to within 200mV of
the rails with light loads, and to within 400mV with heavy
loads. For the load of most circuits, a good model for the
Figure 3. Additional External Hysteresis
V
REF
R2
R1
INPUT
U
INFORMATION
+
–
1/2 LT1720
U
R3
W
1720/21 F03
OH
(output high
U
voltage on the right side of R3 is 300mV or V
for a total voltage swing of (V
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 3.5mV 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 4. 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.
This method will work for additional hysteresis of up to a
few hundred millivolts. Beyond that, the impedance of R3
is low enough to effect the bias string, and adjustment of
R1 may also be required. Note that the currents through
the R1/R2 bias string should be many times the input
currents of the LT1720/LT1721. For 5% accuracy, the
current must be at least 120 A(6 A I
higher accuracy.
CC
R3 = (R1 R2)(V
R2 = (V
– 600mV.
Figure 4. Model for Additional Hysteresis Calculations
TH
as before:
REF
V
TH
REF
R2
R1
– V
= (V
V
TH
TH
CC
REF
)/(V
– 0.6V)/(additional hysteresis)
–
+
1/2 LT1720
)(R1)/(R1 + R2). The new R2 is
TH
/R1 + (V
R3
LT1720/LT1721
CC
– 300mV) – 300mV =
V
TH
AVERAGE
B
– V
1720/21 F04
=
CC
0.05); more for
V
2
CC
/2)/R3)
CC
– 300mV,
CC
9
/2)