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TMP35GRT View Datasheet(PDF) - Analog Devices

Part Name
Description
Manufacturer
TMP35GRT
ADI
Analog Devices ADI
TMP35GRT Datasheet PDF : 16 Pages
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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 useful in microprocessor applications. With
the popularity of fast 486 and Pentium® laptop computers, the
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 kand R2 is set to 1 M. An expression for this circuit’s
hysteresis is given by:
( ) VHYS =
R1
 R2
VLOGIC SWING, CMP402
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.
3.3V
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
sets R5 equal to 1.22 M. Since the closest 1% value for R5 is
1.21 M, a 50 kpotentiometer 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
0.1F
VS
VOUT
TMP35
GND
R5
100k
2
R1A
10k
0.1F
6
REF191
3
4
R1B
10k
CL
1000pF
0.1F
3
6
4
C1
2
5
14
R3
16k
13
VREF
<80؇C
1F
R4
10k
0.1F
INTERRUPT
>80؇C
C1 =
1
4
CMP402
Figure 7. Pentium Overtemperature Interrupt Generator
Pentium is a registered trademark of Intel Corporation.
–10–
REV. C
 

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