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TMP35 Datasheet(PDF) 7 Page - Analog Devices |
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TMP35 Datasheet(HTML) 7 Page - Analog Devices |
7 / 16 page REV. C TMP35/TMP36/TMP37 –7– APPLICATIONS SECTION Shutdown Operation All TMP3x devices include a shutdown capability that reduces the power supply drain to less than 0.5 µA maximum. This feature, available only in the SOIC-8 and the SOT-23 packages, is TTL/ CMOS level compatible, provided that the temperature sensor supply voltage is equal in magnitude to the logic supply voltage. Internal to the TMP3x at the SHUTDOWN pin, a pull-up current source to VIN is connected. This permits the SHUTDOWN pin to be driven from an open-collector/drain driver. A logic LOW, or zero-volt condition on the SHUTDOWN pin, is required to turn the output stage OFF. During shutdown, the output of the temperature sensors becomes a high impedance state where the potential of the output pin would then be determined by external circuitry. If the shutdown feature is not used, it is recommended that the SHUTDOWN pin be connected to V IN (Pin 8 on the SOIC-8, Pin 2 on the SOT-23). The shutdown response time of these temperature sensors is illustrated in TPCs 9, 10, and 11. Mounting Considerations If the TMP3x temperature sensors are thermally attached and protected, they can be used in any temperature measurement application where the maximum temperature range of the medium is between –40 °C to +125°C. Properly cemented or glued to the surface of the medium, these sensors will be within 0.01 °C of the surface temperature. Caution should be exercised, especially with TO-92 packages, because the leads and any wiring to the device can act as heat pipes, introducing errors if the surrounding air-surface interface is not isothermal. Avoiding this condition is easily achieved by dabbing the leads of the temperature sensor and the hookup wires with a bead of thermally conductive epoxy. This will ensure that the TMP3x die temperature is not affected by the surrounding air temperature. Because plastic IC packaging technology is used, excessive mechanical stress should be avoided when fastening the device with a clamp or a screw-on heat tab. Thermally conductive epoxy or glue, which must be electrically nonconductive, is recommended under typical mounting conditions. These temperature sensors, as well as any associated circuitry, should be kept insulated and dry to avoid leakage and corrosion. In wet or corrosive environments, any electrically isolated metal 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 many printed circuit board coatings and varnishes. Thermal Environment Effects The thermal environment in which the TMP3x sensors are used determines two important characteristics: self-heating effects and thermal response time. Illustrated in Figure 3 is a thermal model of the TMP3x temperature sensors that is useful in understanding these characteristics. TJ JC TC CA CCH CC PD TA Figure 3. Thermal Circuit Model In the TO-92 package, the thermal resistance junction-to-case, θJC, is 120°C/W. The thermal resistance case-to-ambient, θCA, is the difference between θJA and θJC, and is determined by the characteristics of the thermal connection. The temperature sensor’s power dissipation, represented by PD, is the product of the total voltage across the device and its total supply current (including any current delivered to the load). The rise in die temperature above the medium’s ambient temperature is given by: TP T DC CA A JJ =× + ()+ θθ Thus, the die temperature rise of a TMP35 “RT” package mounted into a socket in still air at 25 °C and driven from a 5 V supply is less than 0.04 °C. The transient response of the TMP3x sensors to a step change in the temperature is determined by the thermal resistances and the thermal capacities of the die, CCH, and the case, CC. The thermal capacity of the case, CC, varies with the measurement medium since it includes anything in direct contact with the package. In all practical cases, the thermal capacity of the case is the limiting factor in the thermal response time of the sensor and can be represented by a single-pole RC time constant response. TPCs 12 and 14 illustrate the thermal response time of the TMP3x sensors under various conditions. The thermal time constant of a temperature sensor is defined as the time required for the sensor to reach 63.2% of the final value for a step change in the temperature. For example, the thermal time constant of a TMP35 “S” package sensor mounted onto a 0.5" by 0.3" PCB is less than 50 sec in air, whereas in a stirred oil bath, the time constant is less than 3 seconds. Basic Temperature Sensor Connections Figure 4 illustrates the basic circuit configuration for the TMP3x family of temperature sensors. The table shown in the figure illustrates the pin assignments of the temperature sensors for the three package types. For the SOT-23, Pin 3 is labeled as “NC” as are Pins 2, 3, 6, and 7 on the SOIC-8 package. It is recommended that no electrical connections be made to these pins. If the shutdown feature is not needed on the SOT-23 or the SOIC-8 package, the SHUTDOWN pin should be connected to VS. 2.7V < Vs < 5.5V VOUT TMP3x 0.1 F Vs GND PACKAGE VS GND VOUT SHDN SOIC-8 8 4 1 5 SOT-23-5 2 5 1 4 TO-92 1 3 2 NA PIN ASSIGNMENTS SHDN Figure 4. Basic Temperature Sensor Circuit Configuration |
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