LT1787CS8 datasheet(12/16 Pages) AD

12 / 16 page

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 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.

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.

APPLICATIONS INFORMATION

Figure 8. Unidirectional Current Sensing Mode

1787 F08

0.1

µF

RSENSE

2.5V TO

60V

VOUT

LOAD

LT1787HV

FIL+

FIL–

VBIAS

VOUT

–

DNC

VEE

ROUT

Figure 7. Single Supply 2.5V Bidirectional Operation

with External Voltage Reference and I/V Converter

2.5V

µF

RSENSE

ISENSE

2.5V + VSENSE(MAX)

CHARGER/

LOAD

VOUT A

1787 F07

LT1495

1000pF

LT1389-1.25

2.5V

–

LT1787

FIL+

FIL–

VBIAS

VOUT

–

DNC

VEE

ROUT

Figure 6. Unidirectional Output into A/D

with Fixed Supply at VS+

1787 F06

IOUT

µF

VREF

VCC

GND

LTC1286

CLK

DOUT

+IN

–IN

µP

RSENSE

LT1787

FIL+

FIL–

VBIAS

VOUT

–

DNC

VEE

20k

ROUT

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