LT6118
16
6118f
For more information www.linear.com/LT6118
The comparator has one input available externally. The
other comparator input is connected internally to the
400mV precision reference. The input threshold (the
voltage which causes the output to transition from high
to low) is designed to be equal to that of the reference.
The reference voltage is established with respect to the
device V
–
connection.
Comparator Input
The comparator input can swing from V
–
to 60V regardless
of the supply voltage used. The input current for inputs
well above the threshold is just a few pAs. With decreas
-
ing input voltage, a small bias current begins to be drawn
out of the input near the threshold, reaching 50nA max
when at ground potential. Note that this change in input
bias current can cause a small nonlinearity in the OUTA
transfer function if the comparator input is coupled to
the amplifier output with a voltage divider. For example,
if the maximum comparator input current is 50nA, and
the resistance seen looking out of the comparator input is
1k, then a change in output voltage of 50µV will be seen
on the analog output when the comparator input voltage
passes through its threshold.
applicaTions inForMaTion
Setting Comparator Threshold
The comparator has an internal 400mV precision reference.
In order to set the trip point of the LT6118 comparator as
configured in Figure 12, the input sense voltage at which
the comparator will trip, V
SENSE(TRIP)
must be calculated:
V
SENSE(TRIP)
= I
SENSE(TRIP)
• R
SENSE
The selection of R
IN
is discussed in the Selection of Exter-
nal Input Gain Resistor R
IN
section. Once R
IN
is selected,
R
OUT
can be calculated:
R
OUT
= R
IN
V
SENSE(TRIP)
Since the amplifier output is connected directly to the
comparator input, the gain from V
SENSE
to V
OUT
is:
A
V
=
V
SENSE(TRIP)
As shown in Figure 13, R2 can be used to increase the
gain from V
SENSE
to V
OUT
without changing V
SENSE(TRIP)
.
As before, R1 can be easily calculated:
R1= R
IN
V
SENSE(TRIP)
OUTA
I
OUTA
–
+
V
+
C1
SENSEHI
INC
6
7
8
5
6118 F12
V
+
V
+
V
–
LT6118
SENSELO
LE
OUTC
1
2
3
4
V
LE
R
C
V
PULLUP
LOAD
SUPPLY
V
SENSE
SENSE
OVERCURRENT
FLAG
R
IN
+
–
V
–
I
SENSE
=
V
SENSE
R
SENSE
R
OUT
C
L
V
400mV
REFERENCE
C
LC
–
+
Figure 12. Basic Comparator Configuration
