Datasheet

LT6108-1/LT6108-2
20
610812fa
Figure 14. Comparator Output Transfer Characteristics
This gain equation can be easily solved for R1:
R1 = A
V
• R
IN
The value of R2 can be calculated:
R2=
400mV •R
IN
– V
SENSE(TRIP)
• R1
V
SENSE(TRIP)
Hysteresis
The comparator has a typical built-in hysteresis of 10mV
to simplify design, ensure stable operation in the pres-
ence of noise at the input, and to reject supply noise that
might be induced by state change load transients. The
hysteresis is designed such that the threshold voltage is
altered when the output is transitioning from low to high
as is shown in Figure 14.
External positive feedback circuitry can be employed
to increase the effective hysteresis if desired, but such
APPLICATIONS INFORMATION
circuitry will have an effect on both the rising and fall-
ing input thresholds, V
TH
(the actual internal threshold
remains unaffected).
Figure 15 shows how to add additional hysteresis to the
comparator.
R5 can be calculated from the amplifier output current which
is required to cause the comparator output to trip, I
OVER
.
R5=
400mV
I
OVER
, Assuming R1+R2
( )
>> R5
To ensure (R1 + R2) >> R5, R1 should be chosen such
that R1 >> R5 so that V
OUTA
does not change significantly
when the comparator trips.
R3 should be chosen to allow sufficient V
OL
and compara-
tor output rise time due to capacitive loading.
R2 can be calculated:
R2 = R1•
V
DD
– 390mV
V
HYS(EXTRA)






Note that the hysteresis being added, V
HYS(EXTRA)
, is in
addition to the typical 10mV of built-in hysteresis. For very
large values of R2 PCB related leakage may become an
issue. A tee network can be implemented to reduce the
required resistor values.
Figure 15. Inverting Comparator with Added Hysteresis
–
+
V
+
V
+
V
–
INC
V
–
4
610812 F15
OUTA 6
7
5
V
+
V
+
SENSEHI
LT6108-1
R
IN
R
SENSE
I
LOAD
V
+
SENSELO
OUTC3
1
8
400mV
REFERENCE
R3
R4
R5
R1
VTH
R2
V
DD
–
+
V
HYS
OUTC
V
TH
INCREASING
V
INC
610812 F14