Datasheet
11
LT1787/LT1787HV
1787fc
APPLICATIONS INFORMATION
WUU
U
with absolute minimum swing at 30mV above V
EE
. V
OUT
may swing positive to within 0.75V of V
S
+
or a maximum
of 35V, a limit set by internal junction breakdown. Within
these contraints, an amplified, level shifted representation
of the R
SENSE
voltage is developed across R
OUT
.
Split Supply Bipolar Output Swing
Figure 2 shows the LT1787 used with split power supplies.
The V
BIAS
pin is connected to ground, and the output
signal appears at the V
OUT
pin. Bidirectional input currents
can be monitored with the output swinging positive for
current flow from V
S
+
and V
S
–
. Input currents in the
opposite direction cause V
OUT
to swing below ground.
Figure 2 shows an optional output capacitor connected
from V
OUT
to ground. This capacitor may be used to filter
the output signal before it is processed by other
circuitry.Figure 3 shows the voltage transfer function of
the LT1787 used in this configuration.
Single Supply with Shifted V
BIAS
Figure 4 shows the LT1787 used in a single supply mode
with the V
BIAS
pin shifted positive using an external
LT1634 voltage reference. The V
OUT
output signal can
swing above and below V
BIAS
to allow monitoring of
positive or negative currents through the sense resistor,
as shown in Figure 5. The choice of reference voltage is not
critical except for the precaution that adequate headroom
must be provided for V
OUT
to swing without saturating the
internal circuitry. The component values shown in Figure 4
allow operation with V
S
supplies as low as 3.1V.
Figure 2. Split Supply Operation
*OPTIONAL
C2
1µF
–5V
1787 F02
OUTPUT
C3*
1000pF
C1
1µF
R
SENSE
15V
TO
CHARGER/
LOAD
1
2
3
4
8
7
6
5
LT1787
FIL
+
FIL
–
V
BIAS
V
OUT
V
S
–
V
S
+
DNC
V
EE
R
OUT
Figure 3. Split Supply Output Voltage
SENSE VOLTAGE (V
S
+
– V
S
–
) (mV)
OUTPUT VOLTAGE – OUTPUT BIAS VOLTAGE (V)
1787 F05
1.5
1.0
0.5
0
–0.5
–1.0
–1.5
–128
–64
0
32
–96 –32
64
96
128
V
S
= 3.3V TO 60V
T
A
= –40°C TO 85°C
Figure 5. Single Supply Output Voltage
with V
BIAS
= 1.25V
Figure 4. Charge/Discharge Current Monitor on
Single Supply with V
BIAS
= 1.25V
C2
1µF
20k
5%
1787 F04
3.3V
LT1634-1.25
*OPTIONAL OUTPUT
C1
1µF
R
SENSE
3.3V
TO
60V
TO
CHARGER/
LOAD
1
2
3
4
8
7
6
5
LT1787HV
FIL
+
FIL
–
V
BIAS
V
OUT
V
S
–
V
S
+
DNC
V
EE
C3*
1000pF
R
OUT
SENSE VOLTAGE (V
S
+
– V
S
–
) (mV)
OUTPUT VOLTAGE (V)
1787 F03
1.5
1.0
0.5
0
–0.5
–1.0
–1.5
–128
–64
0
32
–96 –32
64
96
128
V
S
= 3.3V TO 60V
T
A
= –40°C TO 85°C