Thermal Diodes

VO

V

+

GND

LM20

SNIS106Q – DECEMBER 1999 – REVISED JANUARY 2015

www.ti.com

7 Detailed Description

7.1 Overview

The LM20 device is a precision analog output CMOS integrated-circuit temperature sensor that operates over a

temperature range of

−55°C to 130°C. The power supply operating range is 2.4 V to 5.5 V. The transfer function

of LM20 is predominately linear, yet has a slight predictable parabolic curvature. The accuracy of the LM20 when

specified to a parabolic transfer function is typically ±1.5°C at an ambient temperature of 30°C. The temperature

error increases linearly and reaches a maximum of ±2.5°C at the temperature range extremes for the LM20. The

temperature range is affected by the power supply voltage. At a power supply voltage of 2.7 V to 5.5 V, the

temperature range extremes are 130°C and

−55°C. Decreasing the power supply voltage to 2.4 V changes the

negative extreme to

−30°C, while the positive remains at 130°C.

The LM20 quiescent current is less than 10

μA. Therefore, self-heating is less than 0.02°C in still air. Shutdown

capability for the LM20 is intrinsic because its inherent low power consumption allows it to be powered directly

from the output of many logic gates or, does not necessitate shutdown at all.

The temperature sensing element is comprised of a simple base emitter junction that is forward biased by a

current source. The temperature sensing element is then buffered by an amplifier and provided to the OUT pin.

The amplifier has a simple class A output stage thus providing a low impedance output that can source 16 µA

and sink 1 µA.

7.2 Functional Block Diagram

7.3 Feature Description

7.3.1 LM20 Transfer Function

The LM20 transfer function can be described in different ways with varying levels of precision. A simple linear

transfer function with good accuracy near 25°C is:

VO = −11.69 mV/°C × T + 1.8663 V

(1)

Over the full operating temperature range of

−55°C to 130°C, best accuracy can be obtained by using the

parabolic transfer function.

VO = (−3.88×10

−6×T2) + (−1.15×10−2×T) + 1.8639

(2)

Using Equation 2, the following temperature to voltage output characteristic table can be generated.

Table 1. Temperature to Voltage Output Characteristic Table

TEMP

VOUT

TEMP

VOUT

TEMP

VOUT

TEMP

VOUT

TEMP

VOUT

TEMP

VOUT

TEMP

VOUT

(°C)

(V)

(°C)

(V)

(°C)

(V)

(°C)

(V)

(°C)

(V)

(°C)

(V)

(°C)

(V)

-55

2.4847

-28

2.1829

-1

1.8754

26

1.5623

53

1.2435

80

0.9191

107

0.5890

-54

2.4736

-27

2.1716

0

1.8639

27

1.5506

54

1.2316

81

0.9069

108

0.5766

-53

2.4625

-26

2.1603

1

1.8524

28

1.5389

55

1.2197

82

0.8948

109

0.5643

-52

2.4514

-25

2.1490

2

1.8409

29

1.5271

56

1.2077

83

0.8827

110

0.5520

-51

2.4403

-24

2.1377

3

1.8294

30

1.5154

57

1.1958

84

0.8705

111

0.5396

-50

2.4292

-23

2.1263

4

1.8178

31

1.5037

58

1.1838

85

0.8584

112

0.5272

8

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