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LTC1531CSW

Self-Powered Isolated Comparator

Linear Technology 

LTC1531

APPLICATIONS INFORMATION

The Remote Light-Controlled Switch (Figure 7) is similar

to the Isolated Thermistor Temperature Controller. The

thermistor is replaced with a Cadmium Light Sensor.

The Isolated Switch Control (Figure 8) is also similar,

where a low voltage switch is isolated from the AC power

control. Here, a charge pump using the 1µF nonpolar

capacitor and diodes are used for powering the LTC1531.

The Isolated Voltage Sense circuit (Figure 9) uses the

three-state CMPOUT pin in a delta-sigma configuration.

Here, the time constant of R1C1 is increased by the

effective duty cycle of CMPOUT ON to OFF time. At a 300Hz

sample rate and a typical ON time of 108µs, the time

constant is:

(1M • 0.22µF)/(300Hz • 108µs) ≈ 6.6sec

The input range is 0V to 2.5V set by the VREG output

voltage. The output is recovered using a rail-to-rail op

amp, LT1490, averaging circuit with a 10sec time con-

stant. The output range is 0V to VCC output for a 0V to VREG

input range.

The Isolated Potentiometer Transducer Sense circuit

(Figure 10) uses the same principle as the Isolated Voltage

Sense circuit to provide a 0V to VCC output proportional to

the potentiometer sensor input.

The Isolated Thermocouple Voltage circuit (Figure 11)

again uses the delta-sigma approach to translate a ther-

mocouple temperature into a 0V to VCC output. Addition-

ally, a micropower op amp, the LT1495, is used to provide

a continuous voltage amplification of the thermocouple.

The LT1389 with the thermistor bridge provides cold

junction compensation over a 0°C to 60°C temperature

range within ±0.5°C. The op amp gain is set to give the K-

type thermocouple a 0°C to 200°C range which translates

to a 0V to VCC output signal. Reducing R3 will increase the

temperature sensing range.

The Over Temperature Detect circuit (Figure 12) uses the

same continuous micropower cold junction compensa-

tion circuit as in the Isolated Thermocouple Voltage cir-

cuit. In this case, the comparator’s minus input is set to

1.25V, which corresponds to 100°C as set by the LT1495

op amp gain. When the thermocouple exceeds 100°C,

VTRIP goes high.

The Isolated Battery Cell Monitor circuit (Figure 13) uses

LTC1531 isolation to both float the individual grounds on

the isolated comparator and isolate the battery from the

logic outputs, CELL1, CELL2, ... In this application, R1 and

R2 (R3 and R4) divide the 2.5V reference down to 0.89V,

while the cell voltage is divided in half by connecting V1 to

the cell and V2 to 0V. Hence, when the cell voltage drops

below 1.786V, CELL1 goes high. Likewise for additional

cells with additional LTC1531s.

The Isolated Window Comparator circuit (Figure 14) uses

two LTC1531s and a logic gate to provide isolated window

comparisons. In this circuit, the first LTC1531, VHIGH,

does the comparison:

V1 – V3 > V4 – V2

or

(0V – X • VREG) > (VIN– – VIN+)

or

X • VREG < (VIN+ – VIN–)

where X = R2/(R2 + R1).

The second LTC1531, VLOW, does the comparison:

(–X • VREG) > (VIN+ – VIN–)

When (VIN+ – VIN–) is less than –X • VREG, VLOW goes high

and when (VIN+ – VIN–) is greater than X • VREG, VHIGH

goes high. In between –X • VREG and +X • VREG, VWINDOW

is high. Therefore, the window width is 2 • X • VREG.

The AC Line Overcurrent Detect circuit (Figure 15) uses

the micropower op amps, the quad LTC1496, to peak

detect the voltage across a sense resistor in series with an

AC load. The two amplifiers connected to RSENSE act as

half-wave rectifiers because their outputs cannot swing

below ground. The gain is set to trip when the voltage on

RSENSE exceeds 125mV and the minus comparator input

is set to 1.25V. The peak detector has a discharge resistor

of 1M plus the op amp input bias current.

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