Datasheet

LT1715
12
1715fa
Figure 6. Additional External Hysteresis
–
+
1/2 LT1715
INPUT
1715 F06
R2
V
REF
R3
R1
APPLICATIONS INFORMATION
Unused Comparators
If a comparator is unused, its output should be left fl oa-
tingto minimize load current. The unused inputs can be
tied off to the rails and power consumption can be further
minimized if the inputs are connected to the power rails
to induce an output low. Connecting the inverting input
to V
CC
and the noninverting input to V
EE
will likely be the
easiest method.
Hysteresis
The LT1715 includes internal hysteresis, which makes it
easier to use than many other similar speed comparators.
The input-output transfer characteristic is illustrated in
Figure 5 showing the defi nitions of V
OS
and V
HYST
based
upon the two measurable trip points. The hysteresis band
makes the LT1715 well behaved, even with slowly moving
inputs.
The exact amount of hysteresis will vary from part to part
as indicated in the specifi cations table. The hysteresis level
will also vary slightly with changes in supply voltage and
common mode voltage. A key advantage of the LT1715
is the signifi cant reduction in these effects, which is im-
portant whenever an LT1715 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 volt-
age with an unpredictable level of hysteresis, as seen in
competing comparators, is useless. The LT1715 is many
times better than prior generation comparators 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 specifi cations 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 LT1715 make this more predictable
than with TTL output comparators due to the LT1715’s
small variability of V
OH
(output high voltage).
To add additional hysteresis, set up positive feedback
by adding additional external resistor R3 as shown in
Figure 6. Resistor R3 adds a portion of the output to the
threshold set by the resistor string. The LT1715 pulls the
outputs to +V
S
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 volt-
age on the right side of R3 is 300mV or +V
S
– 300mV,
for a total voltage swing of (+V
S
– 300mV) – (300mV) =
+V
S
– 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:
R3 = (R1||R2)(+V
S
– 0.6V)/(additional hysteresis)
Additional hysteresis is the desired overall hysteresis less
the internal 4mV hysteresis.
Figure 5. Hysteresis I/O Characteristics
V
HYST
(= V
TRIP
+
– V
TRIP
–
)
V
HYST
/2
V
OL
1715 F05
V
OH
V
TRIP
–
V
TRIP
+
ΔV
IN
= V
IN
+
– V
IN
–
V
TRIP
+
+ V
TRIP
–
2
V
OS
=
V
OUT
0
Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.