Datasheet

LT6105
13
6105fa
a 20mΩ sense resistor to give a 100mV full-scale input
to the LT6105. Input offset voltage will limit resolution to
5mA. Neglecting contact resistance at solder joints, even
one square of PC board copper at each resistor end will
cause an error of 5%. This error will grow proportionately
higher as monitored current levels rise.
Gain Setting
The gain is set with three external resistors, R
IN1
, R
IN2
,
R
OUT
. The gain, R
OUT
/R
IN
, can be selected from 1V/V to
100V/V as long as the maximum current does not exceed
1mA. Select Gain = R
OUT
/R
IN2
for sense input voltage op-
eration greater than 1.6V. Select gain = R
OUT
/R
IN1
for sense
input voltage operation less than 1.6V. The overall system
error will depend on the resistor tolerance chosen for the
application. Set R
IN1
= R
IN2
for best accuracy across the
entire input range. The total error will be gain error of the
resistors plus the gain error of the LT6105 device.
Output Signal Range
The LT6105’s output signal is developed by current
through R
IN2
(44V > V
–IN
> 1.6V) or R
IN1
(0V < V
–IN
<
1.6V) conducted to the output resistor, R
OUT
. This current
is V
SENSE
/R
IN2
or V
SENSE
/R
IN1
. The sense amplifi er’s maxi-
mum output current before gain error begins to increase
APPLICATIONS INFORMATION
is 1mA. This allows low value output resistors to be used
which helps preserve signal accuracy when the output pin
is connected to other systems.
For zero V
SENSE
, the internal circuitry gain will force V
OUT
to V
O(MIN)
referred to V
–
. Depending on output currents,
V
OUT
may swing positive to within V
O(MAX)
referred to V
+
or a maximum of 36V, a limit set by internal junction break-
down. Within these constraints, an amplifi ed, level shifted
representation of R
SENSE
voltage is developed at V
OUT
. The
output is well behaved driving capacitive loads.
CM Input Signal Range
The LT6105 has high CMRR over the full input voltage
range. The minimum operation voltage of the sense ampli-
fi er inputs is 0V whether V
+
is at 2.7V or 36V. The output
remains accurate even when the sense inputs are driven
to 44V. The graph in Figure 2 shows that V
OS
changes very
slightly over a wide input range. Furthermore, either sense
inputs V
S
+
and V
S
–
can collapse to 0V without incurring any
damage to the device. The LT6105 can handle differential
sense voltages up to 44V. For example, V
S
+
= 44V and V
S
–
=
0V can be a valid condition in a current monitoring applica-
tion (Figure 3) when an overload protection fuse is blown
and V
S
–
voltage collapses to ground. Under this condition,
the output of the LT6105 goes to the positive rail, V
O(MAX)
.
Figure 2. Input Offset Voltage vs V
S
+
Input Voltage
Figure 3. Current Monitoring of a Fuse Protected Circuit
OUTPUT
OUT
6105 F03
R
SENSE
FUSE
LT6105
V
S
–
V
S
+
V
–
V
+
C2
0.1MF
C1
0.1MF
DC SOURCE
(≤ 44V)
5V
TO LOAD
–
+
+
–IN
+IN
R
IN2
R
OUT
R
IN1
V
S
+
INPUT VOLTAGE (V)
0
INPUT OFFSET VOLTAGE (mV)
0.20
0.40
0.60
35
0
–0.20
10 20
5
15 25
40
30 45
–0.80
–1.00
–0.40
0.80
–0.60
6105 F02
T
A
= –40°C
T
A
= 25°C
T
A
= 85°C
T
A
= 125°C
V
+
= 12V
V
SENSE
= 5mV
A
V
= 50V/V