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dual-output, low-power

thermostat

036

10V max.

can switch up to 5 mA (400 mV drop at 50 µA). If it is required to

switch a higher current, a buffer stage is needed at the output. The

outputs are enabled when the values set with R 1 , R 2 , and R 3 are

exceeded—see the timing diagram. Resistors R 4 and R 5 are pull-

up components.

The output voltage of the temperature sensor is 6.2 mV

IC1

R4

R5

V

REF

1

1.250V

8

REFERENCE

R3

70

C

C –1

plus a fixed offset voltage of 395 mV. When the component values

of potential divider R 1 -R 2 -R 3 are calculated, assume a load cur-

rent of 50 µA for V REF . This means that the total value of the three

resistors is about 27 k

°

LM56

V

T2

OUT1

2

7

R2

50

C

. If great accuracy is wanted, the bias current

of the comparators should be taken into account, although the typ-

ical value of these is only 150 nA, so that they do not make much

difference in practice. With values as specified in the circuit dia-

gram, switching takes place at 50

V

T1

OUT2

3

6

R1

C.

Capacitor C 2 serves to decouple any interference. If such inter-

°

C and 70

°

GND

6.2mV/

C + 395mV

4

5

TEMPERATURE

V

TEMP

SENSOR

C1

C2

100n

100n

V TEMP

V T2

994041 - 11

V T1

A National Semiconductor application

The LM56 from National Semiconductor is an accurate, dual-out-

put, low-power thermostat contained in an 8-pin SMD case. It con-

tains a 1.25 V bandgap voltage reference, two comparators with

5 °C hysteresis and a temperature sensor. The output voltage of

the sensor is internally linked to the comparators. The supply volt-

age may lie in the range 2.7–10 V.

There are several variants: the LM56CIM has an accuracy of

±4 °C and the LM56BIM ±3 °C, both over the temperature range

–40

OUT2

OUT1

V TEMP

V T2

V T1

C. Typical applications are temperature monitor-

ing in a variety of system, protection against low or high tempera-

tures, and control of alarm systems or fans.

Outputs OUT1 and OUT2 are open-collector outputs, which

C to +125

°

OUT1

OUT2

994041 - 12

Elektor Electronics

7-8/99

63

°

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ference persists, the value of the capacitor may be increased to

1 µF without any adverse effect on the speed of reaction.

The LM56 draws a current of not more than 230 µA, which

means that the total current drain, including the current through the

potential divider, will not exceed 400 µA. This makes the IC par-

ticularly useful for systems operating from 3 V or 5 V battery

packs.

[994041]

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