lt1719cs8-trpbf Linear Technology Corporation, lt1719cs8-trpbf Datasheet - Page 11

• Manufacturer Part Number: lt1719cs8-trpbf
• Description: 4.5ns Single/dual Supply 3v/5v Comparator With Rail-to-rail Output
• Manufacturer: Linear Technology Corporation

APPLICATIONS
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
with light 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
voltage swing of (+V
– 600mV.
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.
R3 = (R1 R2)(+V
Figure 4. Additional External Hysteresis
S
OH
V
and ground to within 200mV of the rails
REF
(output high voltage).
R2
R1
INPUT
U
S
S
– 0.6V)/(additional hysteresis)
INFORMATION
+
–
– 300mV) – (300mV) = +V
LT1719S8
U
R3
S
W
– 300mV, for a total
1719 F04
U
S
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.
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 LT1719. For 5% accuracy, the current must be at least
20 times the input current, more for higher accuracy. This
illustration used an LT1719S8; with an LT1719S6 the same
procedure is used with V
R2 = (V
Figure 5. Model for Additional Hysteresis Calculations
TH
as before:
REF
V
REF
TH
R2
R1
– V
= (V
V
TH
TH
REF
)/(V
+
–
LT1719S8
)(R1)/(R1 + R2). The new R2 is
TH
+
/R1 + [V
R3
substituted for +V
TH
V
AVERAGE
– (+V
1719 F05
=
S
+V
2
)/2]/R3)
LT1719
S
S
.
11
S
/2)