REV. C

TMP35/TMP36/TMP37

–10–

Microprocessor Interrupt Generator

These inexpensive temperature sensors can be used with a

voltage reference and an analog comparator to configure an

interrupt generator useful in microprocessor applications. With

the popularity of fast 486 and Pentium

need to indicate a microprocessor overtemperature condition

has grown tremendously. The circuit illustrated in Figure 7

demonstrates one way to generate an interrupt using a TMP35,

a CMP402 analog comparator, and a REF191, a 2 V precision

voltage reference.

The circuit has been designed to produce a logic HIGH interrupt

signal if the microprocessor temperature exceeds 80

°C. This

80

°C trip point was arbitrarily chosen (final value set by the

microprocessor thermal reference design) and is set using an

R3–R4 voltage divider of the REF191’s output voltage. Since

the output of the TMP35 is scaled by 10 mV/

°C, the voltage at

the CMP402’s inverting terminal is set to 0.8 V.

Since temperature is a slowly moving quantity, the possibility

for comparator chatter exists. To avoid this condition, hysteresis

is used around the comparator. In this application, a hysteresis

of 5

°C about the trip point was arbitrarily chosen; the ultimate

value for hysteresis should be determined by the end application.

The output logic voltage swing of the comparator with R1 and

R2 determine the amount of comparator hysteresis. Using a 3.3 V

supply, the output logic voltage swing of the CMP402 is 2.6 V;

thus, for a hysteresis of 5

°C (50 mV @ 10 mV/°C), R1 is set to

20 k

Ω and R2 is set to 1 MΩ. An expression for this circuit’s

hysteresis is given by:

=

R1

R2









()

Because of the likelihood that this circuit would be used in

close proximity to high speed digital circuits, R1 is split into

equal values and a 1000 pF is used to form a low-pass filter

on the output of the TMP35. Furthermore, to prevent high

frequency noise from contaminating the comparator trip point,

a 0.1

µF capacitor is used across R4.

Thermocouple Signal Conditioning with Cold-Junction

Compensation

The circuit in Figure 8 conditions the output of a Type K

thermocouple, while providing cold-junction compensation for

temperatures between 0

°C and 250°C. The circuit operates

from single 3.3 V to 5.5 V supplies and has been designed to

produce an output voltage transfer characteristic of 10 mV/

°C.

A Type K thermocouple exhibits a Seebeck coefficient of

approximately 41

µV/°C; therefore, at the cold junction, the

TMP35, with a temperature coefficient of 10 mV/

°C, is

used with R1 and R2 to introduce an opposing cold-junction

temperature coefficient of –41

µV/°C. This prevents the

isothermal, cold-junction connection between the circuit’s PCB

tracks and the thermocouple’s wires from introducing an error

in the measured temperature. This compensation works extremely

well for circuit ambient temperatures in the range of 20

°C to

50

°C. Over a 250°C measurement temperature range, the

thermocouple produces an output voltage change of 10.151 mV.

Since the required circuit’s output full-scale voltage is 2.5 V, the

gain of the circuit is set to 246.3. Choosing R4 equal to 4.99 k

Ω

sets R5 equal to 1.22 M

Ω. Since the closest 1% value for R5 is

1.21 M

Ω, a 50 kΩ potentiometer is used with R5 for fine trim of

the full-scale output voltage. Although the OP193 is a superior

single-supply, micropower operational amplifier, its output stage

is not rail-to-rail; as such, the 0

°C output voltage level is 0.1 V.

If this circuit were to be digitized by a single-supply ADC, the

ADC’s common should be adjusted to 0.1 V accordingly.

Using TMP3x Sensors in Remote Locations

In many industrial environments, sensors are required to oper-

ate in the presence of high ambient noise. These noise sources

take on many forms; for example, SCR transients, relays, radio

transmitters, arc welders, ac motors, and so on. They may also

be used at considerable distances from the signal conditioning

circuitry. These high noise environments are very typically in the

form of electric fields, so the voltage output of the tempera-

ture sensor can be susceptible to contamination from these

noise sources.

R2

1M

3

4

TMP35

0.1 F

GND

0.1 F

C1

INTERRUPT

<80 C

>80 C

REF191

R1A

10k

R1B

10k

3.3V

2

6

1000pF

R3

16k

1 F

R4

10k

0.1 F

0.1 F

C1 =

CMP402

4

1

2

4

3

14

13

5

6

R5

100k

Figure 7. Pentium Overtemperature Interrupt Generator

Pentium is a registered trademark of Intel Corporation.