Datasheet
12
LT1787/LT1787HV
1787fc
Operation with A/D Converter
Figure 6 shows the LT1787 operating with the LTC1286
A/D converter. This low cost circuit is capable of 12-bit
resolution of unipolar currents. The –IN pin of the A/D
converter is biased at 1V by the resistor divider R1 and R2.
This voltage increases as sense current increases, with the
amplified sense voltage appearing between the A/D con-
verters –IN and +IN terminals. The front page of the data
sheet shows a similar circuit which uses a voltage refer-
ence for improved accuracy and signal range. The LTC1286
converter uses sequential sampling of its –IN and +IN
inputs. Accuracy is degraded if the inputs move between
sampling intervals. A filter capacitor from FIL
+
to FIL
–
as
well as a filter capacitor from V
BIAS
to V
OUT
may be
necessary if the sensed current changes more than 1LSB
within a conversion cycle.
Buffered Output Operation
Figure 7 shows the LT1787’s outputs buffered by an
operational amplifier configured as an I/V converter. This
configuration is ideal for monitoring very low voltage
supplies. The LT1787’s V
OUT
pin is held equal to the
reference voltage appearing at the op amp’s noninverting
input. This allows monitoring V
S
supplies as low as 2.5V.
The op amp’s output may swing from ground to its positive
supply voltage. The low impedance output of the op amp
may drive following circuitry more effectively than the high
output impedance of the LT1787. The I/V converter configu-
ration also works well with split supply voltages.
Single Supply Unidirectional Operation
Figure 8 shows the simplest connection in which the
LT1787 may be used. The V
BIAS
pin is connected to
ground, and the V
OUT
pin swings positive with increasing
sense current. The LT1787’s outputs can swing as low as
30mV as shown in Figure 9. Accuracy is sacrificed at small
output levels, but this is not a limitation in protection
circuit applications or where sensed currents do not vary
greatly. Increased low level accuracy can be obtained by
level shifting V
BIAS
above ground. The level shifting may be
done with resistor dividers, voltage references or a simple
diode. Accuracy is ensured if the output signal is sensed
differentially between V
BIAS
and V
OUT
.
APPLICATIONS INFORMATION
WUU
U
Figure 8. Unidirectional Current Sensing Mode
1787 F08
C
0.1µF
R
SENSE
2.5V TO
60V
V
OUT
TO
LOAD
1
2
3
4
8
7
6
5
LT1787HV
FIL
+
FIL
–
V
BIAS
V
OUT
V
S
–
V
S
+
DNC
V
EE
R
OUT
Figure 7. Single Supply 2.5V Bidirectional Operation
with External Voltage Reference and I/V Converter
2.5V
C1
1µF
R
SENSE
I
SENSE
2.5V + V
SENSE(MAX)
TO
CHARGER/
LOAD
V
OUT A
1M
5%
1787 F07
LT1495
C3
1000pF
LT1389-1.25
2.5V
+
–
A1
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 6. Unidirectional Output into A/D
with Fixed Supply at V
S
+
R2
5k
5%
1787 F06
I
OUT
C1
1µF
5V
V
REF
V
CC
GND
LTC1286
CS
CLK
D
OUT
+IN
–IN
TO µP
R
SENSE
5V
1
2
3
4
8
7
6
5
LT1787
FIL
+
FIL
–
V
BIAS
V
OUT
V
S
–
V
S
+
DNC
V
EE
R1
20k
5%
R
OUT