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TMP17 Datasheet(PDF) 5 Page - Analog Devices

Part No. TMP17
Description  Low Cost, Current Output Temperature Transducer
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TMP17 Datasheet(HTML) 5 Page - Analog Devices

   
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TMP17
REV. 0
–5–
ment (
θ
JA). Self-heating error in
°C can be derived by multiply-
ing the power dissipation by
θ
JA. Because errors of this type can
vary widely for surroundings with different heat sinking capaci-
ties, it is necessary to specify
θ
JA under several conditions.
Table I shows how the magnitude of self-heating error varies
relative to the environment. In typical free air applications at
25
°C with a 5 V supply the magnitude of the error is 0.2°C or
less. A small glued-on heat sink will reduce the temperature
error in high temperature, large supply voltage situations.
Table I. Thermal Characteristics
Medium
θ
JA ( C/watt)
τ (sec)*
Still Air
158
52
Moving Air @ 500 FPM
60
10
Fluorinert Liquid
35
2
NOTES
*
τ is an average of one time constant (63.2% of final value). In cases where the
thermal response is not a simple exponential function, the actual thermal
response may be better than indicated.
Response of the TMP17 output to abrupt changes in ambient
temperature can be modeled by a single time constant
τ
exponential function. Figures 3 and 4 show typical response
time plots for media of interest.
The time constant,
τ, is dependent on θ
JA and the thermal
capacities of the chip and the package. Table I lists the effective
τ (time to reach 63.2% of the final value) for several different
media. Copper printed circuit board connections will sink or
conduct heat directly through the TMP17’s soldered leads.
When faster response is required a thermally conductive grease
or glue between the TMP17 and the surface temperature being
measured should be used.
MOUNTING CONSIDERATIONS
If the TMP17 is thermally attached and properly protected, it
can be used in any temperature measuring situation where the
maximum range of temperatures encountered is between
40
°C
and
105
°C. Thermally conductive epoxy or glue is recom-
mended under typical mounting conditions. In wet environ-
ments condensation at cold temperatures can cause leakage
current related errors and should be avoided by sealing the
device in nonconductive epoxy paint or conformal coating.
APPLICATIONS
Connecting several TMP17 devices in parallel adds the currents
through them and produces a reading proportional to the
average temperature. Series TMP17s will indicate the lowest
temperature because the coldest device limits the series current
flowing through the sensors. Both of these circuits are depicted
in Figure 13.
If greater accuracy is desired, initial calibration and scale factor
errors can be removed by using the TMP17 in the circuit of
Figure 11.
8.66k
R1
1k
97.6k
R2
5k
7.87k
OP196
VOUT = 100mV/
oC
+5V
REF43
V–
TMP17
Figure 11. Two Temperature Trim Circuit
With the transducer at 0
°C adjustment of R1 for a 0 V output
nulls the initial calibration error and shifts the output from K to
°C. Tweaking the gain of the circuit at an elevated temperature
by adjusting R2 trims out scale factor error. The only error
remaining over the temperature by adjusting R2 trims out scale
factor error. The only error remaining over the temperature
range being trimmed for its nonlinearity. A typical plot of two
trim accuracy is given in Figure 12.
SUPPLY VOLTAGE AND THERMAL ENVIRONMENT
EFFECTS
The power supply rejection characteristics of the TMP17
minimize errors due to voltage irregularity, ripple and noise. If a
supply is used other than 5 V (used in factory trimming), the
power supply error can be removed with a single temperature
trim. The PTAT nature of the TMP17 will remain unchanged.
The general insensitivity of the output allows the use of lower
cost unregulated supplies and means that a series resistance of
several hundred ohms (e.g., CMOS multiplexer, meter coil
resistance) will not degrade the overall performance.
2.0
1.0
25
105
25
0
1.0
2.0
TEMPERATURE – C
0
75
40
Figure 12. Typical Two Trim Accuracy
The thermal environment in which the TMP17 is used deter-
mines two performance traits: the effect of self-heating on
accuracy and the response time of the sensor to rapid changes in
temperature. In the first case, a rise in the IC junction tempera-
ture above the ambient temperature is a function of two
variables; the power consumption level of the circuit and the
thermal resistance between the chip and the ambient environ-


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