TMP35/TMP36/TMP37

Rev. F | Page 13 of 20

MICROPROCESSOR INTERRUPT GENERATOR

Because 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 determines the amount of comparator hysteresis. Using a

3.3 V supply, the output logic voltage swing of the CMP402 is

2.6 V; therefore, for a hysteresis of 5°C (50 mV @ 10 mV/°C),

R1 is set to 20 kΩ, and R2 is set to 1 MΩ. An expression for the

hysteresis of this circuit is given by

These inexpensive temperature sensors can be used with a

voltage reference and an analog comparator to configure an

interrupt generator for microprocessor applications. With the

popularity of fast microprocessors, the need to indicate a

microprocessor overtemperature condition has grown

tremendously. The circuit in Figure 30 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 is 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 to R4 voltage

divider of the REF191 output voltage. Because the output of the

TMP35 is scaled by 10 mV/°C, the voltage at the inverting

terminal of the CMP402 is set to 0.8 V.

(

)

CMP402

SWING

LOGIC

HYS

V

R2

R1

V

,

⎟

⎠

⎞

⎜

⎝

⎛

=

Because this circuit is probably used in close proximity to high

speed digital circuits, R1 is split into equal values and a 1000 pF

capacitor 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.

R2

1MΩ

3

4

TMP35

0.1µF

GND

0.1µF

CMP402

INTERRUPT

<80°C

>80°C

REF191

R1A

10kΩ

R1B

10kΩ

3.3V

2

6

1000pF

R3

16kΩ

1µF

R4

10kΩ

0.1µF

0.1µF

C1 =

CMP402

4

1

2

4

3

14

13

5

6

R5

100kΩ

+

–

+

Figure 30. Microprocessor Overtemperature Interrupt Generator