Datasheet

Data Sheet TMP35/TMP36/TMP37
MICROPROCESSOR INTERRUPT GENERATOR
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.
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 at 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
( )
CMP402SWINGLOGICHYS
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
V
OUT
+V
S
TMP35
0.1µF
GND
0.1µF
CMP402
INTERRUPT
<80°C
>80°C
REF191
R1A
10kΩ
R1B
10kΩ
3.3V
2
6
C
L
1000pF
R3
16kΩ
1µF
R4
10kΩ
V
REF
0.1µF
0.1µF
C1 = CMP402
4
1
2
4
3
14
13
5
6
R5
100kΩ
+
–
+
00337-028
Figure 30. Microprocessor Overtemperature Interrupt Generator
Rev. H | Page 13 of 19
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