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TMP01FS View Datasheet(PDF) - Analog Devices

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TMP01FS Datasheet PDF : 20 Pages
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TMP01
APPLICATIONS INFORMATION
SELF-HEATING EFFECTS
In some applications, the user should consider the effects of
self-heating due to the power dissipated by the open-collector
outputs, which are capable of sinking 20 mA continuously.
Under full load, the TMP01 open-collector output device is
dissipating
PDISS = 0.6 V × .020A = 12 mW
which in a surface-mount SOIC package accounts for a
temperature increase due to self-heating of
ΔT = PDISS × θJA = .012 W × 158°C/W = 1.9°C
This self-heating effect directly affects the accuracy of the
TMP01 and will, for example, cause the device to activate
the OVER output 2 degrees early.
Bonding the package to a moderate heat sink limits the self-
heating effect to approximately:
ΔT = PDISS × θJC = .012 W × 43°C/W = 0.52°C
which is a much more tolerable error in most systems. The
VREF and VPTAT outputs are also capable of delivering
sufficient current to contribute heating effects and should not
be ignored.
BUFFERING THE VOLTAGE REFERENCE
The reference output VREF is used to generate the temper-
ature setpoint programming voltages for the TMP01 and also
to determine the hysteresis temperature band by the reference
load current IVREF. The on-board output buffer amplifier is
typically capable of 500 μA output drive into as much as 50 pF
load (maximum). Exceeding this load affects the accuracy
of the reference voltage, could cause thermal sensing errors
due to dissipation, and may induce oscillations. Selection of
a low drift buffer functioning as a voltage follower with high
input impedance ensures optimal reference accuracy, and
does not affect the programmed hysteresis current. Amplifiers
which offer the low drift, low power consumption, and low cost
appropriate to this application include the OP295, and members
of the OP90, OP97, OP177 families, and others as shown in the
following applications circuits.
With excellent drift and noise characteristics, VREF offers a
good voltage reference for data acquisition and transducer
excitation applications as well. Output drift is typically better
than −10 ppm/°C, with 315 nV/√Hz (typ) noise spectral density
at 1 kHz.
PRESERVING ACCURACY OVER WIDE
TEMPERATURE RANGE OPERATION
The TMP01 is unique in offering both a wide range temper-
ature sensor and the associated detection circuitry needed
to implement a complete thermostatic control function in
one monolithic device. While the voltage reference, setpoint
comparators, and output buffer amplifiers have been carefully
compensated to maintain accuracy over the specified temper-
ature range, the user has an additional task in maintaining the
accuracy over wide operating temperature ranges in the
application.
Since the TMP01 is both sensor and control circuit, in many
applications it is possible that the external components used to
program and interface the device may be subjected to the same
temperature extremes. Thus, it may be necessary to locate
components in close thermal proximity to minimize large
temperature differentials, and to account for thermal drift
errors, such as resistor matching tempcos, amplifier error drift,
and the like, where appropriate. Circuit design with the TMP01
requires a slightly different perspective regarding the thermal
behavior of electronic components.
THERMAL RESPONSE TIME
The time required for a temperature sensor to settle to a speci-
fied accuracy is a function of the thermal mass of the sensor,
and the thermal conductivity between the sensor and the object
being sensed. Thermal mass is often considered equivalent to
capacitance.
Thermal conductivity is commonly specified using the symbol
Q, and can be thought of as the reciprocal of thermal resistance.
It is commonly specified in units of degrees per watt of power
transferred across the thermal joint. Thus, the time required
for the TMP01 to settle to the desired accuracy is dependent
on the package selected, the thermal contact established in that
particular application, and the equivalent power of the heat
source. In most applications, the settling time is probably best
determined empirically.
Rev. E | Page 10 of 20
 

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