TMP35FT9

a Low Voltage Temperature Sensors TMP35/TMP36/TMP37 FEATURES Low Voltage Operation (2.7 V to 5.5 V) Calibrated Directly in ⴗC 10 mV/ⴗC Scale Factor (20 mV/ⴗC on TMP37) ⴞ2ⴗC Accuracy over Temperature (Typ) ⴞ0.5ⴗC Linearity (Typ) Stable with Large Capacitive Loads Specified –40ⴗC to +125ⴗC, Operation to +150ⴗC Less than 50 ␮A Quiescent Current Shutdown Current 0.5 ␮A Max Low Self-Heating APPLICATIONS Environmental Control Systems Thermal Protection Industrial Process Control Fire Alarms Power System Monitors CPU Thermal Management FUNCTIONAL BLOCK DIAGRAM +Vs (2.7V to 5.5V) TMP35/ TMP36/ TMP37 SHUTDOWN VOUT PACKAGE TYPES AVAILABLE RT-5 (SOT-23) 5 GND VOUT 1 +VS 2 TOP VIEW (Not to Scale) 4 SHUTDOWN NC 3 NC = NO CONNECT PRODUCT DESCRIPTION The TMP35, TMP36, and TMP37 are low voltage, precision centigrade temperature sensors. They provide a voltage output that is linearly proportional to the Celsius (Centigrade) temperature. The TMP35/TMP36/TMP37 do not require any external calibration to provide typical accuracies of ± 1°C at +25°C and ± 2°C over the –40°C to +125°C temperature range. The low output impedance of the TMP35/TMP36/TMP37 and its linear output and precise calibration simplify interfacing to temperature control circuitry and A/D converters. All three devices are intended for single-supply operation from 2.7 V to 5.5 V maximum. Supply current runs well below 50 µA, providing very low self-heating—less than 0.1°C in still air. In addition, a shutdown function is provided to cut supply current to less than 0.5 µA. The TMP35 is functionally compatible with the LM35/LM45 and provides a 250 mV output at 25°C. The TMP35 reads temperatures from 10°C to 125°C. The TMP36 is specified from –40°C to +125°C, provides a 750 mV output at 25°C, and operates to +125°C from a single 2.7 V supply. The TMP36 is functionally compatible with the LM50. Both the TMP35 and TMP36 have an output scale factor of 10 mV/°C. The TMP37 is intended for applications over the range 5°C to 100°C and provides an output scale factor of 20 mV/°C. The TMP37 provides a 500 mV output at 25°C. Operation extends to 150°C with reduced accuracy for all devices when operating from a 5 V supply. RN-8 (SOIC) 8 +VS VOUT 1 7 NC TOP VIEW (Not to Scale) 6 NC NC 3 NC 2 5 SHUTDOWN GND 4 NC = NO CONNECT TO-92 1 2 3 BOTTOM VIEW (Not to Scale) PIN 1, +Vs; PIN 2, VOUT; PIN 3, GND The TMP35/TMP36/TMP37 are all available in low cost 3-lead TO-92, SOIC-8, and 5-lead SOT-23 surface-mount packages. REV. C Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781/329-4700 www.analog.com Fax: 781/326-8703 © Analog Devices, Inc., 2002 (VS = 2.7 V to 5.5 V, –40
C ≤ TA ≤ +125
C, unless TMP35/TMP36/TMP37–SPECIFICATIONS1 otherwise noted.) Parameter Symbol Conditions ACCURACY TMP35/TMP36/TMP37F TMP35/TMP36/TMP37G TMP35/TMP36/TMP37F TMP35/TMP36/TMP37G Scale Factor, TMP35 Scale Factor, TMP36 Scale Factor, TMP37 TA = 25°C TA = 25°C Over Rated Temperature Over Rated Temperature 10°C ≤ TA ≤ 125°C –40°C ≤ TA ≤ +125°C 5°C ≤ TA ≤ 85°C 5°C ≤ TA ≤ 100°C 3.0 V ≤ +VS ≤ 5.5 V 0 µA ≤ IL ≤ 50 µA –40°C ≤ TA ≤ +105°C –105°C ≤ TA ≤ +125°C TA = 25°C 3.0 V ≤ +VS ≤ 5.5 V Load Regulation Power Supply Rejection Ratio Min PSRR Linearity Long-Term Stability TA = 150°C for 1 kHrs SHUTDOWN Logic High Input Voltage Logic Low Input Voltage VIH VIL OUTPUT TMP35 Output Voltage TMP36 Output Voltage TMP37 Output Voltage Output Voltage Range Output Load Current Short-Circuit Current Capacitive Load Driving Device Turn-On Time VS = 2.7 V VS = 5.5 V Max Unit ±1 ±1 ±2 ±2 10 10 20 20 ±2 ±3 ±3 ±4 9.8/10.2 9.8/10.2 19.6/20.4 19.6/20.4 °C °C °C °C mV/°C mV/°C mV/°C mV/°C 6 25 30 50 0.5 0.4 20 60 100 m°C/µA m°C/µA m°C/V m°C/V °C °C 1.8 400 TA = 25°C TA = 25°C TA = 25°C 250 750 500 Note 2 No Oscillations2 Output within ± 1°C 100 kΩ
100 pF Load2 1000 10000 0.5 1 0.01 5.5 50 0.5 V µA µA 2000 50 250 2.7 +VS ISY (ON) ISY (OFF) Unloaded Unloaded V mV mV mV mV mV µA µA pF ms 100 0 IL ISC CL POWER SUPPLY Supply Range Supply Current Supply Current (Shutdown) Typ NOTES 1 Does not consider errors caused by self-heating. 2 Guaranteed but not tested. Specifications subject to change without notice. 50 LOAD REG – m
C/A 40 30 20 10 0 –50 0 50 TEMPERATURE –
C 100 150 Figure 1. Load Reg vs. Temperature (m°C/µ A) –2– REV. C TMP35/TMP36/TMP37 ABSOLUTE MAXIMUM RATINGS 1, 2, 3 FUNCTIONAL DESCRIPTION Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 V Shutdown Pin . . . . . . . . . . . . . . GND ≤ SHUTDOWN ≤ +VS Output Pin . . . . . . . . . . . . . . . . . . . . . . GND ⱕ VOUT ⱕ +VS Operating Temperature Range . . . . . . . . . . –55°C to +150°C Dice Junction Temperature . . . . . . . . . . . . . . . . . . . . . . 175°C Storage Temperature Range . . . . . . . . . . . . –65°C to +160°C Lead Temperature (Soldering, 60 sec) . . . . . . . . . . . . . 300°C An equivalent circuit for the TMP3x family of micropower, centigrade temperature sensors is shown in Figure 2. At the heart of the temperature sensor is a band gap core, which is comprised of transistors Q1 and Q2, biased by Q3 to approximately 8 µA. The band gap core operates both Q1 and Q2 at the same collector current level; however, since the emitter area of Q1 is 10 times that of Q2, Q1’s VBE and Q2’s VBE are not equal by the following relationship: NOTES 1 Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation at or above this specification is not implied. Exposure to maximum rating conditions for extended periods may affect device reliability. 2 Digital inputs are protected; however, permanent damage may occur on unprotected units from high energy electrostatic fields. Keep units in conductive foam or packaging at all times until ready to use. Use proper antistatic handling procedures. 3 Remove power before inserting or removing units from their sockets. A  ∆VBE =VT × ln  E ,Q1  A  E ,Q2  +VS SHDN 25A Package Type
JA
JC Unit TO-92 (T9 Suffix) SOIC-8 (S Suffix) SOT-23 (RT Suffix) 162 158 300 120 43 180 °C/W °C/W °C/W 3X 2X θJA is specified for device in socket (worst-case conditions). Q2 1X Q4 R1 ORDERING GUIDE Q1 10X Model Accuracy at 25C (C max) Linear Operating Temperature Range Package Options1 TMP35FT9 TMP35GT9 TMP35FS TMP35GS TMP35GRT2 ± 2.0 ± 3.0 ± 2.0 ± 3.0 ± 3.0 10°C to 125°C 10°C to 125°C 10°C to 125°C 10°C to 125°C 10°C to 125°C TO-92 TO-92 RN-8 RN-8 RT-5 TMP36FT9 TMP36GT9 TMP36FS TMP36GS TMP36GRT2 ± 2.0 ± 3.0 ± 2.0 ± 3.0 ± 3.0 –40°C to +125°C –40°C to +125°C –40°C to +125°C –40°C to +125°C –40°C to +125°C TO-92 TO-92 RN-8 RN-8 RT-5 TMP37FT9 TMP37GT9 TMP37FS TMP37GS TMP37GRT2 ± 2.0 ± 3.0 ± 2.0 ± 3.0 ± 3.0 5°C to 100°C 5°C to 100°C 5°C to 100°C 5°C to 100°C 5°C to 100°C TO-92 TO-92 RN-8 RN-8 RT-5 R3 NOTES 1 SOIC = Small Outline Integrated Circuit; RT = Plastic Surface Mount; TO = Plastic. 2 Consult factory for availability. R2 +VOUT 7.5A Q3 2X 6X GND Figure 2. Temperature Sensor Simplified Equivalent Circuit Resistors R1 and R2 are used to scale this result to produce the output voltage transfer characteristic of each temperature sensor and, simultaneously, R2 and R3 are used to scale Q1’s VBE as an offset term in VOUT. Table I summarizes the differences between the three temperature sensors’ output characteristics. Table I. TMP3x Output Characteristics Sensor Offset Voltage (V) Output Voltage Scaling (mV/C) Output Voltage @ 25C (mV) TMP35 TMP36 TMP37 0 0.5 0 10 10 20 250 750 500 The output voltage of the temperature sensor is available at the emitter of Q4, which buffers the band gap core and provides load current drive. Q4’s current gain, working with the available base current drive from the previous stage, sets the short-circuit current limit of these devices to 250 µA. CAUTION ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although the TMP35/TMP36/TMP37 features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality. REV. C –3– WARNING! ESD SENSITIVE DEVICE TMP35/TMP36/TMP37 – Typical Performance Characteristics 2.0 1.8 OUTPUT VOLTAGE – V 1.6 POWER SUPPLY REJECTION –
C/V 100 a. TMP35 b. TMP36 c. TMP37 VS = 3V c 1.4 b 1.2 1.0 0.8 a 0.6 0.4 31.6 10 3.16 1 0.32 0.1 0.032 0.2 0 50 25 0 25 50 75 TEMPERATURE –
C 100 0.01 20 125 TPC 1. Output Voltage vs. Temperature 100 1k FREQUENCY – Hz 10k 100k TPC 4. Power Supply Rejection vs. Frequency 5 5 MINIMUM SUPPLY VOLTAGE – V 4 ACCURACY ERROR –
C 3 a. MAXIMUM LIMIT (G GRADE) b. TYPICAL ACCURACY ERROR c. MINIMUM LIMIT (G GRADE) a 2 1 0 1 b 2 3 4 5 4 NO LOAD 3 b 2 a 1 a. TMP35/TMP36 b. TMP37 c 0 20 40 60 80 100 TEMPERATURE –
C 120 0 50 140 TPC 2. Accuracy Error vs. Temperature 25 0 25 50 75 TEMPERATURE –
C 100 125 TPC 5. Minimum Supply Voltage vs. Temperature 60 0.4 a. V+ = 5V b. V+ = 3V V+ = 3V to 5.5V, NO LOAD 50 0.3 SUPPLY CURRENT – A POWER SUPPLY REJECTION –
C/V MINIMUM SUPPLY VOLTAGE REQUIRED TO MEET DATA SHEET SPECIFICATION 0.2 0.1 NO LOAD 40 a 30 b 20 0 50 25 0 25 50 75 TEMPERATURE –
C 100 10 50 125 TPC 3. Power Supply Rejection vs. Temperature 25 0 25 50 75 TEMPERATURE –
C 100 125 TPC 6. Supply Current vs. Temperature –4– REV. C TMP35/TMP36/TMP37 400 50 = SHUTDOWN PIN HIGH TO LOW (3V TO 0V) TA = 25°C, NO LOAD 300 RESPONSE TIME – s SUPPLY CURRENT – A 40 30 20 200 100 = SHUTDOWN PIN LOW TO HIGH (0V TO 3V) VOUT SETTLES WITHIN ±1°C 10 0 0 1 2 3 4 5 SUPPLY VOLTAGE – V 6 0 50 8 7 TPC 7. Supply Current vs. Supply Voltage 100 125 0.8 a. V+ = 5V b. V+ = 3V 30 20 a 10 0 25 50 75 TEMPERATURE –
C 100 0.4 0.2 0 1.0 0.8 0.6 TA = 25 C 0.4 V+ AND SHUTDOWN = SIGNAL 0.2 b 25 TA = 25 C V+ = 3V SHUTDOWN = SIGNAL 0.6 OUTPUT VOLTAGE – V SUPPLY CURRENT – nA 25 50 75 TEMPERATURE –
C 1.0 NO LOAD 0 50 0 TPC 10. VOUT Response Time for Shutdown Pin vs. Temperature 50 40 25 0 50 125 TPC 8. Supply Current vs. Temperature (Shutdown = 0 V) 0 50 100 150 200 250 TIME – µs 300 350 400 450 TPC 11. VOUT Response Time to Shutdown and V+ Pins vs. Time 400 110 a 100 90 PERCENT OF CHANGE – % RESPONSE TIME – s 300 = V+ AND SHUTDOWN PINS HIGH TO LOW (3V TO 0V) 200 = V+ AND SHUTDOWN PINS LOW TO HIGH (0V TO 3V) VOUT SETTLES WITHIN ±1°C 100 c b 80 VIN = 3V, 5V 70 60 50 40 a. TMP35 SOIC SOLDERED TO 0.5" x 0.3" Cu PCB b. TMP36 SOIC SOLDERED TO 0.6" x 0.4" Cu PCB c. TMP35 TO-92 IN SOCKET SOLDERED TO 1" x 0.4" Cu PCB 30 20 10 0 50 25 0 25 50 75 TEMPERATURE –
C 100 0 125 0 TPC 9. VOUT Response Time for V+ Power-Up/PowerDown vs. Temperature REV. C 100 200 300 TIME – sec 400 500 600 TPC 12. Thermal Response Time in Still Air –5– TMP35/TMP36/TMP37 140 a. TMP35 SOIC SOLDERED TO 0.5" x 0.3" Cu PCB b. TMP36 SOIC SOLDERED TO 0.6" x 0.4" Cu PC c. TMP35 TO-92 IN SOCKET SOLDERED TO 1" x 0.4" Cu PCB 10mV 1ms 100 90 100 VOLT/DIVISION TIME CONSTANT – sec 120 80 VIN = 3V, 5V 60 b 40 10 c 0% 20 a 0 0 100 200 300 400 500 AIR VELOCITY – FPM 600 TIME/DIVISION 700 TPC 15. Temperature Sensor Wideband Output Noise Voltage. Gain = 100, BW = 157 kHz TPC 13. Thermal Response Time Constant in Forced Air 110 2400 a 2200 VOLTAGE NOISE DENSITY – nV/ Hz 100 90 c CHANGE – % 80 70 VIN = 3V, 5V b 60 50 40 30 20 a. TMP35 SOIC SOLDERED TO 0.5" x 0.3" Cu PCB b. TMP36 SOIC SOLDERED TO 0.6" x 0.4" Cu PCB c. TMP35 TO-92 IN SOCKET SOLDERED TO 1" x 0.4" Cu PCB 10 20 30 TIME – sec 40 50 1600 1400 1200 1000 800 600 a 400 0 10 60 b 1800 200 10 0 0 2000 a. TMP35/36 b. TMP37 100 1k FREQUENCY – Hz 10k TPC 16. Voltage Noise Spectral Density vs. Frequency TPC 14. Thermal Response Time in Stirred Oil Bath –6– REV. C TMP35/TMP36/TMP37 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 VIN (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 nonconductive epoxy paints or dip or any one of many printed circuit board coatings and varnishes. 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: TJ = PD × (θJC + θ CA ) + TA 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 0.1F Vs TMP3x SHDN GND 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 PD CCH JC TC PIN ASSIGNMENTS CA CC TA PACKAGE VS SOIC-8 SOT-23-5 TO-92 8 2 1 GND 4 5 3 VOUT SHDN 1 1 2 5 4 NA Figure 4. Basic Temperature Sensor Circuit Configuration Figure 3. Thermal Circuit Model REV. C VOUT –7– TMP35/TMP36/TMP37 Note the 0.1 µF bypass capacitor on the input. This capacitor should be a ceramic type, have very short leads (surface mount would be preferable), and be located as close a physical proximity to the temperature sensor supply pin as practical. Since these temperature sensors operate on very little supply current and could be exposed to very hostile electrical environments, it is important to minimize the effects of RFI (radio frequency interference) on these devices. The effect of RFI on these temperature sensors in specific and analog ICs in general is manifested as abnormal dc shifts in the output voltage due to the rectification of the high frequency ambient noise by the IC. In those cases where the devices are operated in the presence of high frequency radiated or conducted noise, a large value tantalum capacitor (⬎2.2 µF) placed across the 0.1 µF ceramic may offer additional noise immunity. The same circuit principles can be applied to the TMP36, but because of the TMP36’s inherent offset, the circuit uses two less resistors as shown in Figure 5b. In this circuit, the output voltage transfer characteristic is 1 mV/°F but is referenced to the circuit’s common; however, there is a 58 mV (58°F) offset in the output voltage. For example, the output voltage of the circuit would read 18 mV were the TMP36 placed in –40°F ambient environment and 315 mV at 257°F. VS VS 0.1F TMP36 VOUT Fahrenheit Thermometers Although the TMP3x temperature sensors are centigrade temperature sensors, a few components can be used to convert the output voltage and transfer characteristics to directly read Fahrenheit temperatures. Shown in Figure 5a is an example of a simple Fahrenheit thermometer using either the TMP35 or the TMP37. This circuit can be used to sense temperatures from 41°F to 257°F, with an output transfer characteristic of 1 mV/°F using the TMP35 and from 41°F to 212°F using the TMP37 with an output characteristic of 2 mV/°F. This particular approach does not lend itself well to the TMP36 because of its inherent 0.5 V output offset. The circuit is constructed with an AD589, a 1.23 V voltage reference, and four resistors whose values for each sensor are shown in the figure table. The scaling of the output resistance levels was to ensure minimum output loading on the temperature sensors. A generalized expression for the circuit’s transfer equation is given by:  R1   R3  VOUT =   TMP 35 +  R3 + R4 AD589 R1+ R2     ( ) ( R1 45.3k GND R2 10k VOUT @ 1mV/
F – 58
F VOUT @ –40
F = 18mV VOUT @ +257
F = 315mV Figure 5b. TMP36 Fahrenheit Thermometer Version 1 At the expense of additional circuitry, the offset produced by the circuit in Figure 5b can be avoided by using the circuit in Figure 5c. In this circuit, the output of the TMP36 is conditioned by a singlesupply, micropower op amp, the OP193. Although the entire circuit operates from a single 3 V supply, the output voltage of the circuit reads the temperature directly, with a transfer characteristic of 1 mV/°F, without offset. This is accomplished through the use of an ADM660, a supply voltage inverter. The 3 V supply is inverted and applied to the P193’s V– terminal. Thus, for a temperature range between –40°F and +257°F, the output of the circuit reads –40 mV to +257 mV. A general expression for the circuit’s transfer equation is given by: ) where: TMP35 = Output voltage of the TMP35, or the TMP37, at the measurement temperature, TM, and AD589 = Output voltage of the reference = 1.23 V. Note that the output voltage of this circuit is not referenced to the circuit’s common. If this output voltage were to be applied directly to the input of an ADC, the ADC’s common should be adjusted accordingly.  R6   R4  R4 VS  VOUT =   1+ R3 TMP 36 −  R3  2  R5 + R6       ( ) Average and Differential Temperature Measurement VS In many commercial and industrial environments, temperature sensors are often used to measure the average temperature in a building, or the difference in temperature between two locations on a factory floor or in an industrial process. The circuits in Figures 6a and 6b demonstrate an inexpensive approach to average and differential temperature measurement. In Figure 6a, an OP193 is used to sum the outputs of three temperature sensors to produce an output voltage scaled by 10 mV/°C that represents the average temperature at three locations. The circuit can be extended to as many temperature sensors as required as long as the circuit’s transfer equation is maintained. In this application, it is recommended that one temperature sensor type be used throughout the circuit; otherwise, the output voltage of the circuit will not produce an accurate reading of the various ambient conditions. 0.1F VS R1 TMP35/37 VOUT R2 GND VOUT AD589 1.23V R3 R4 PIN ASSIGNMENTS SENSOR TCVOUT R1 (k) R2 (k) R3 (k) R4 (k) TMP35 TMP37 1mV/
F 2mV/
F 45.3 45.3 10 10 10 10 374 182 Figure 5a. TMP35/TMP37 Fahrenheit Thermometers –8– REV. C TMP35/TMP36/TMP37 +3V R1 50k R3 R4 0.1F C1 10F R2 50k 8 2 VS VOUT TMP36 10F/0.1F R5 3 –40
F ⱕ TA ⱕ +257
F 6 4 R6 GND VOUT @ 1mV/
F OP193 8 5 1 –3V NC ELEMENT TMP36 R2 R4 R5 R6 2 258.6k 10k 47.7k 10k 10F 10F ADM660 6 4 NC 3 7 Figure 5c. TMP36 Fahrenheit Thermometer Version 2 The circuit in Figure 6b illustrates how a pair of TMP3x sensors can be used with an OP193 configured as a difference amplifier to read the difference in temperature between two locations. In these applications, it is always possible that one temperature sensor would be reading a temperature below that of the other sensor. To accommodate this condition, the output of the OP193 is offset to a voltage at one-half the supply via R5 and R6. Thus, the output voltage of the circuit is measured relative to this point, as shown in the figure. Using the TMP36, the output voltage of the circuit is scaled by 10 mV/°C. To minimize error in the difference between the two measured temperatures, a common, readily available thin-film resistor network is used for R1–R4. 2.7V < VS < 5.5V 2.7V < +VS < 5.5V 0.1F 2 0.1F VTEMP( AVG) @ 10mV/
C FOR TMP35/36 @ 20mV/
C FOR TMP35/36 7 TMP36 @ T1 1 3 R1* R2* R8 25k OP193 4 0.1F R5 100k 7 R1 300k 2 TMP3x 0.1F R6 7.5k R2 300k TMP36 @ T2 R3* OP193 6 3 4 R7 100k R9 25k TMP3x VOUT CENTERED AT FOR R1 = R2 = R3 = R; R3 300k TMP3x R4 7.5k R4* VTEMP( AVG) = 1 (TMP3x1 + TMP3x2 + TMP3x3) 3 R5 = R1 3 0 ⱕ TA ⱕ 125 C R4 = R6 R5 100k 1F R6 100k VOUT = T2 – T1 @ 10mV/
C V CENTERED AT S 2 *R1–R4, CADDOCK T914–100k–100, OR EQUIVALENT Figure 6a. Configuring Multiple Sensors for Average Temperature Measurements REV. C Figure 6b. Configuring Multiple Sensors for Differential Temperature Measurements –9– TMP35/TMP36/TMP37 Microprocessor Interrupt Generator Thermocouple Signal Conditioning with Cold-Junction Compensation 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 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. 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 kΩ and R2 is set to 1 MΩ. An expression for this circuit’s hysteresis is given by:  R1 VHYS =   VLOGIC SWING, CMP402  R2 ( 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 kΩ sets R5 equal to 1.22 MΩ. Since the closest 1% value for R5 is 1.21 MΩ, a 50 kΩ potentiometer 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 operate 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 temperature sensor can be susceptible to contamination from these noise sources. ) 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 R2 1M VS 0.1F R1A 10k VOUT 0.1F 3 R1B 10k 6 TMP35 R5 100k 4 2 C1 CL 1000pF INTERRUPT 14 5 GND 0.1F 13 2 3 REF191 4 6 R3 16k VREF R4 10k 1F >80
C