Datasheet

LT6109-1/LT6109-2
18
610912fa
APPLICATIONS INFORMATION
The desired trip points and full-scale analog output voltage
for the circuit in Figure 13 can be achieved as follows:
R1=
400mV
I
OVER
R2 =
V
MAX
I
MAX
R1
( )
I
MAX
R3 =
400mV –I
UNDER
R1+ R2
( )
I
UNDER
Trip points for the LT6109-2 can be set by replacing I
UNDER
with a second overcurrent, I
OVER2
.
Hysteresis
Each comparator has a typical built-in hysteresis of 10mV
to simplify design, ensure stable operation in the pres-
ence of noise at the inputs, 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 12.
External positive feedback circuitry can be employed
to increase the effective hysteresis if desired, but such
circuitry will have an effect on both the rising and fall-
ing input thresholds, V
TH
(the actual internal threshold
remains unaffected).
Figure 13 shows how to add additional hysteresis to a
noninverting comparator.
R6 can be calculated from the extra hysteresis being added,
V
HYS(EXTRA)
and the amplifier output current which you
want to cause the comparator output to trip, I
UNDER
. Note
that the hysteresis being added, V
HYS(EXTRA)
, is in addition
to the typical 10mV of built-in hysteresis.
R6 =
400mV V
HYS(EXTRA)
I
UNDER
R1 should be chosen such that R1 >> R6 so that V
OUTA
does not change significantly when the comparator trips.
Figure 12. Comparator Output Transfer Characteristics
Figure 13. Noninverting Comparator with Added Hysteresis
V
HYS
OUTC1
(LT6109-1/LT6109-2)
OUTC2
(LT6109-2)
OUTC2
(LT6109-1)
V
HYS
V
TH
INCREASING
V
INC1,2
610912 F12
+
V
+
V
+
V
INC2
V
5
610912 F13
OUTA
7
8
V
+
V
+
SENSEHI
9
10
1
3
LT6109-1
R
IN
R
SENSE
I
LOAD
R3
V
+
SENSELO
OUTC2
400mV
REFERENCE
R5
R6
R1
VTH
R2
+