Datasheet TMP35, TMP36, TMP37 (Analog Devices) - 9

Data Sheet TMP35/TMP36/TMP37 APPLICATIONS INFORMATION SHUTDOWN OPERATION THERMAL ENVIRONMENT EFFECTS
All TMP35/TMP36/TMP37 devices include a shutdown The thermal environment in which the TMP35/TMP36/TMP37 capability, which reduces the power supply drain to less than sensors are used determines two important characteristics: self- 0.5 µA maximum. This feature, available only in the SOIC_N heating effects and thermal response time. Figure 23 il ustrates a and the SOT-23 packages, is TTL/CMOS level-compatible, thermal model of the TMP35/TMP36/TMP37 temperature provided that the temperature sensor supply voltage is equal in sensors, which is useful in under-standing these characteristics. magnitude to the logic supply voltage. Internal to the TMP35/
T
θ θ
J JC TC CA
TMP36/TMP37 at the SHUTDOWN pin, a pul -up current source to +VS is connected. This allows the SHUTDOWN pin to
C
be driven from an open-col ector/drain driver. A logic low, or
P CH CC T D A
zero-volt condition, on the SHUTDOWN pin is required to turn off the output stage. During shutdown, the output of the 00337-021 Figure 23. Thermal Circuit Model temperature sensors becomes high impedance where the potential of the output pin is then determined by external In the T-3 package, the thermal resistance junction-to-case, θJC, circuitry. If the shutdown feature is not used, it is recommended is 120°C/W. The thermal resistance case-to-ambient, CA, is the that the SHUTDOWN pin be connected to +V difference between θ S (Pin 8 on the JA and θJC, and is determined by the char- SOIC_N; Pin 2 on the SOT-23). acteristics of the thermal connection. The power dissipation of the temperature sensor, PD, is the product of the total voltage The shutdown response time of these temperature sensors is across the device and its total supply current, including any shown in Figure 14, Figure 15, and Figure 16. current delivered to the load. The rise in die temperature above
MOUNTING CONSIDERATIONS
the ambient temperature of the medium is given by If the TMP35/TMP36/TMP37 temperature sensors are thermally TJ = PD × (θJC + θCA) + TA attached and protected, they can be used in any temperature Thus, the die temperature rise of a TMP35 SOT-23 package measurement application where the maximum temperature mounted into a socket in stil air at 25°C and driven from a 5 V range of the medium is between −40°C and +125°C. Properly supply is less than 0.04°C. cemented or glued to the surface of the medium, these sensors are within 0.01°C of the surface temperature. Caution should be The transient response of the TMP35/TMP36/TMP37 sensors exercised, especial y with T-3 packages, because the leads and to a step change in the temperature is determined by the any wiring to the device can act as heat pipes, introducing thermal resistances and the thermal capacities of the die, CCH, errors if the surrounding air-surface interface is not isothermal. and the case, CC. The thermal capacity of CC varies with the Avoiding this condition is easily achieved by dabbing the leads measurement medium because it includes anything in direct of the temper-ature sensor and the hookup wires with a bead of contact with the package. In all practical cases, the thermal thermally conductive epoxy. This ensures that the TMP35/TMP36/ capacity of CC is the limiting factor in the thermal response time TMP37 die temperature is not affected by the surrounding air of the sensor and can be represented by a single-pole RC time temperature. Because plastic IC packaging technology is used, constant response. Figure 17 and Figure 19 show the thermal excessive mechanical stress should be avoided when fastening the response time of the TMP35/TMP36/TMP37 sensors under device with a clamp or a screw-on heat tab. Thermal y conductive various conditions. The thermal time constant of a temperature epoxy or glue, which must be electrically nonconductive, is sensor is defined as the time required for the sensor to reach recommended under typical mounting conditions. 63.2% of the final value for a step change in the temperature. For example, the thermal time constant of a TMP35 SOIC These temperature sensors, as well as any associated circuitry, package sensor mounted onto a 0.5" × 0.3" PCB is less than should be kept insulated and dry to avoid leakage and corrosion. 50 sec in air, whereas in a stirred oil bath, the time constant is In wet or corrosive environments, any electrically isolated metal less than 3 sec. or ceramic well can be used to shield the temperature sensors. Condensation at very cold temperatures can cause errors and should be avoided by sealing the device, using electrically non- conductive epoxy paints or dip or any one of the many printed circuit board coatings and varnishes. Rev. H | Page 9 of 19