LT1787CMS8-PBF 데이터 시트보기 (PDF) - Linear Technology
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LT1787CMS8-PBF Datasheet PDF : 16 Pages
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LT1787/LT1787HV
APPLICATIONS INFORMATION
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 VBIAS to VOUT 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 VOUT pin is held equal to the
reference voltage appearing at the op amp’s noninverting
input. This allows monitoring VS 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.
RSENSE
5V
1 FIL–
FIL+ 8
2 VS– LT1787 VS+ 7
R1
20k
3
DNC
VBIAS 6 IOUT
5%
4
VEE
ROUT 5
VOUT
R2
5k
5%
C1
1µF
+IN
VCC CS
LTC1286 CLK
–IN
VREF GND
DOUT
5V
TO µP
1787 F06
Figure 6. Unidirectional Output into A/D
with Fixed Supply at VS+
TO
CHARGER/
LOAD
RSENSE
ISENSE
1 FIL–
2 VS–
FIL+ 8
LT1787 VS+ 7
3
DNC
4
VEE
VBIAS 6
ROUT 5
VOUT
2.5V
1M
5%
C1
1µF
2.5V + VSENSE(MAX)
2.5V
C3
1000pF
–
A1
+ LT1495
LT1389-1.25
1787 F07
VOUT A
Figure 7. Single Supply 2.5V Bidirectional Operation
with External Voltage Reference and I/V Converter
Single Supply Unidirectional Operation
Figure 8 shows the simplest connection in which the
LT1787 may be used. The VBIAS pin is connected to
ground, and the VOUT 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 VBIAS 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 VBIAS and VOUT.
TO
LOAD
RSENSE
C
0.1µF
2.5V TO
60V
1 FIL–
FIL+ 8
2 VS– LT1787HV VS+ 7
3
DNC
4
VEE
VBIAS 6
ROUT 5
VOUT
1787 F08
VOUT
Figure 8. Unidirectional Current Sensing Mode
1787fc
12
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