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rev. 0 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 which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of analog devices. a airflow and temperature sensor TMP12* ? analog devices, inc., 1995 one technology way, p.o. box 9106, norwood. ma 02062-9106, u.s.a. tel: 617/329-4700 fax: 617/326-8703 features temperature sensor includes 100 w heater heater provides power ic emulation accuracy 6 3 c typ. from 2 40 c to 1 100 c operation to 1 150 c 5 mv/ c internal scale-factor resistor programmable temperature setpoints 20 ma open-collector setpoint outputs programmable thermal hysteresis internal 2.5 v reference single 5 v operation 400 m a quiescent current (heater off) minimal external components applications system airflow sensor equipment over-temperature sensor over-temperature protection power supply thermal sensor low-cost fan controller general description the TMP12 is a silicon-based airflow and temperature sensor designed to be placed in the same airstream as heat generating components that require cooling. fan cooling may be required continuously, or during peak power demands, e.g. for a power supply, and if the cooling systems fails, system reliability and/or safety may be impaired. by monitoring temperature while emu- lating a power ic, the TMP12 can provide a warning of cooling system failure. the TMP12 generates an internal voltage that is linearly pro- portional to celsius (centigrade) temperature, nominally 1 5 mv/ c. the linearized output is compared with voltages from an external resistive divider connected to the TMP12s 2.5 v precision reference. the divider sets up one or two refer- ence voltages, as required by the user, providing one or two temperature setpoints. comparator outputs are open-collector transistors able to sink over 20 ma. there is an on-board hys- teresis generator provided to speed up the temperature-setpoint output transitions, this also reduces erratic output transitions in noisy environments. hysteresis is programmed by the external resistor chain and is determined by the total current drawn from the 2.5 v reference. the TMP12 airflow sensor also incorpo- rates a precision, low temperature coefficient 100 w heater resistor that may be connected directly to an external 5 v sup- ply. when the heater is activated it raises the die temperature in the dip package approximately 20 c above ambient (in still air). the purpose of the heater in the TMP12 is to emulate a power ic, such as a regulator or pentium cpu which has a high internal dissipation. when subjected to a fast airflow, the package and die tempera- tures of the power device and the TMP12 (if located in the same airstream) will be reduced by an amount proportional to the rate of airflow. the internal temperature rise of the TMP12 may be reduced by placing a resistor in series with the heater, or reducing the heater voltage. the TMP12 is intended for single 5 v supply operation, but will operate on a 12 v supply. the heater is designed to operate from 5 v only. specified temperature range is from 2 40 c to 1 125 c, operation extends to 1 150 c at 5 v with reduced accuracy. the TMP12 is available in 8-pin plastic dip and so packages. functional block diagram vref over 1k w + - v+ gnd set high heater hysteresis voltage window comparator current mirror hysteresis current voltage reference and sensor i hys + - + - under set low 100 w pinouts dip and so top view (not to scale) 8 v+ 1 vref 7 over 2 set high 6 under set low 3 5 heater gnd 4 *protected by u.s. patent no. 5,195,827.
parameter symbol conditions min typ max units accuracy accuracy (high, low setpoints) t a = 1 25 c 6 2 6 3 c accuracy (high, low setpoints) t a = 2 40 c to 1 100 c 6 3 6 5 c internal scale factor t a = 2 40 c to 1 100 c 1 4.9 1 5 1 5.1 mv/ c power supply rejection ratio psrr 4.5 v 1 v s 5.5 v 0.1 0.5 c/v linearity t a = 2 40 c to 1 125 c 0.5 c repeatability t a = 2 40 c to 1 125 c 0.3 c long term stability t a = 1 125 c for 1 k hrs 0.3 c setpoint inputs offset voltage v os 0.25 mv output voltage drift tcv os 3 m v/ c input bias current i b 25 100 na vref output output voltage vref t a = 1 25 c, no load 2.49 2.50 2.51 v output voltage vref t a = 2 40 c to 1 100 c, 2.5 6 0.015 v no load output drift tc vref 2 10 ppm/ c output current, zero hysteresis i vref 7 m a hysteresis current scale factor sf hys 5 m a/ c open-collector outputs output low voltage v ol i sink = 1.6 ma 0.25 0.4 v output low voltage v ol i sink = 20 ma 0.6 v output leakage current i oh v s = 12 v 1 100 m a fall time t hl see test load 40 ns heater resistance r h t a = 1 25 c 97 100 103 w temperature coefficient t a = 2 40 c to 1 125 c 100 ppm/ c maximum continuous current i h see note 1 60 ma power supply supply range 1 v s 4.5 5.5 v supply current i sy unloaded at 1 5 v 400 600 m a i sy unloaded at 1 12 v 2 450 m a notes 1 guaranteed but not tested. 2 TMP12 is specified for operation from a 5 v supply. however, operation is allowed up to a 12 v supply, but not tested at 12 v. maximum heater supply is 6 v. specifications subject to change without notice. TMP12Cspecifications rev. 0 C2C (v s = 1 5 v, 2 40 c t a 1 125 c unless otherwise noted.) 20pf 1k w test load
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 TMP12 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. TMP12 rev. 0 C3C parameter symbol conditions min typ max units accuracy accuracy (high, low setpoints) t a = 1 25 c 6 3 c internal scale factor t a = 1 25 c 1 4.9 1 5 1 5.1 mv/ c setpoint inputs input bias current i b 100 na vref output output voltage vref t a = 1 25 c, no load 2.49 2.51 v open-collector outputs output low voltage v ol i sink = 1.6 ma 0.4 v output leakage current i oh v s = 12 v 100 m a heater resistance r h t a = 1 25 c 97 100 103 w power supply supply range 1 v s 4.5 5.5 v supply current i sy unloaded at 1 5 v 600 m a note electrical tests are performed at wafer probe to the limits shown. due to variations in assembly methods and normal yield loss, yield after packaging is not guaranteed for standard product dice. consult factory to negotiate specifications based on dice lot qualification through sample lot assembly and testing. dice characteristics die size 0.078 3 0.071 inch, 5,538 sq. mils (1.98 3 1.80 mm, 3.57 sq. mm) transistor count: 105 wafer test limits (v s = 1 5 v, gnd = o v, t a = 1 25 c, unless otherwise noted.) 1. vref 2. set high input 3. set low input 4. gnd 5. heater 6. under output 7. over output 8. v 1 for additional dice ordering information, refer to databook. 8 3 7 5 4 1 2 warning! esd sensitive device 6
TMP12 rev. 0 C4C absolute maximum ratings* supply voltage . . . . . . . . . . . . . . . . . . . . . . . . 2 0.3 v to 1 15 v heater voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 6 v setpoint input voltage . . . . . . . . . . . 2 0.3 v to [(v 1 ) 1 0.3 v] reference output current . . . . . . . . . . . . . . . . . . . . . . . . 2 ma open-collector output current . . . . . . . . . . . . . . . . . . 50 ma open-collector output voltage . . . . . . . . . . . . . . . . . . . 1 15 v operating temperature range . . . . . . . . . . 2 55 c to 1 150 c dice junction temperature . . . . . . . . . . . . . . . . . . . . . 1 175 c storage temperature range . . . . . . . . . . . . 2 65 c to 1 160 c lead temperature(soldering, 60 sec) . . . . . . . . . . . . . 1 300 c package type q ja q jc units 8-pin plastic dip (p) 103 1 43 c/w 8-lead soic (s) 158 2 43 c/w notes 1 q ja is specified for device in socket (worst case conditions). 2 q ja is specified for device mounted on pcb. caution 1. stresses above those listed under absolute maximum ratings may cause permanent damage to the device. this is a stress rating only and functional operation at or above this specification is not implied. exposure to the above maximum rating conditions for extended periods may affect device reli- ability. 2. digital inputs and outputs 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. ordering guide temperature package package model/grade range 1 description option TMP12fp xind plastic dip n-8 TMP12fs xind soic so-8 TMP12gbc 1 25 c die note 1 xind = 2 40 c to 1 125 c functional description the TMP12 incorporates a heating element, temperature sen- sor, and two user-selectable setpoint comparators on a single substrate. by generating a known amount of heat, and using the setpoint comparators to monitor the resulting temperature rise, the TMP12 can indirectly monitor the performance of a systems cooling fan. the TMP12 temperature sensor section consists of a bandgap voltage reference which provides both a constant 2.5 v output and a voltage which is proportional to absolute temperature (vptat). the vptat has a precise temperature coefficient of 5 mv/k and is 1.49 v (nominal) at 1 25 c. the comparators compare vptat with the externally set temperature trip points and generate an open-collector output signal when one of their respective thresholds has been exceeded. the heat source for the TMP12 is an on-chip 100 w low tempco thin-film resistor. when connected to a 5 v source, this resistor dissipates: p d = v 2 r 5 2 v 100 w = 0.25 w , = which generates a temperature rise of about 32 c in still air for the so packaged device. with an airflow of 450 feet per minute (fpm), the temperature rise is about 22 c. by selecting a temp- erature setpoint between these two values, the TMP12 can provide a logic-level indication of problems in the cooling sys tem. a proprietary, low tempco thin-film resistor process, in conjunc- tion with production laser trimming, enables the TMP12 to provide a temperature accuracy of 6 3 c (typ) over the rated temperature range. the open-collector outputs are capable of sinking 20 ma, allowing the TMP12 to drive small control re- lays directly. operating from a single 1 5 v supply, the quiescent current is only 600 m a (max), without the heater resistor current.
TMP12 rev. 0 C5C heater resistor power dissipation ?mw 35 30 0 0 50 100 150 200 20 15 10 5 25 air flow rates v = 5v so? soldered to .5 " .3" cu pcb junction temperature rise above ambient ? c 250 a. 0 fpm c. 450 fpm b. 250 fpm d. 600 fpm a b c d figure 1. soic junction temperature rise vs. heater dissipation heater resistor power dissipation ?mw junction temperature rise above ambient ? c 25 20 0 0 250 50 100 150 200 15 10 5 air flow rates v = 5v pdip soldered to 2" 1.31" cu pcb a. 0 fpm c. 450 fpm b. 250 fpm d. 600 fpm a b c d figure 2. pdip junction temperature rise vs. heater dissipation time ?sec 25 20 0 15 10 5 30 35 40 45 50 55 60 65 70 0 10 20 30 40 50 60 70 80 90 100 110 120 130 v = 5v rheater to external supply turned on @ t = 5 sec so? soldered to .5" .3" copper pcb pdip soldered to 2" 1.31 copper pcb a. so?, htr @ 5v b. pdip, htr @ 5v c. so?, htr @ 3v d. pdip, htr @ 3v a b c d junction temperature ? c figure 3. junction temperature rise in still air transition from 100 c stirred bath to forced 25 c air v = 5v, no load, heater off so? soldered to .5" .3" cu pcb pdip soldered to 2" 1.31" cu pcb air velocity ?fpm 140 0 0 700 100 time constant ?sec 200 300 400 500 600 120 80 60 40 20 100 a. pdip pcb b. soic pcb a b figure 4. package thermal time constant in forced air time ?sec 80 30 0 02 junction temperature ? c 46810121416 18 20 70 40 20 10 60 50 90 100 110 120 v = 5v, no load, heater off so? soldered to .5" .3" cu pcb pdip soldered to 2" 1.31" cu pcb a. so? pcb b. pdip pcb a b transition from still 25 c air to stirred 100 c bath figure 5. thermal response time in stirred oil bath temperature ? c 102 99.5 98 -75 heater resistance ? w -25 25 75 125 175 101.5 100 99 98.5 101 100.5 v+ = +5v figure 6. heater resistance vs. temperature
TMP12 rev. 0 C6C temperature ? c 2.52 2.505 2.49 -75 175 reference voltage ?v -25 25 75 125 2.515 2.51 2.5 2.495 v = 5v, no load, heater off figure 7. reference voltage vs. temperature temperature ? c 5 4 3 -75 175 start-up supply voltage ?v -25 25 75 125 4.5 3.5 start-up voltage defined as output reading being within 5 c of output at 5v no load, heater off figure 8. start-up voltage vs. temperature temperature ? c 500 400 300 475 450 350 325 425 375 -75 175 supply current ?? -25 25 75 125 v = 5v, no load, heater off figure 9. supply current vs. temperature temperature ? c -50 -25 25 75 125 accuracy error ? c 0 50 100 2 -3 1 -2 -4 -5 0 -1 3 4 5 6 -6 a. maximum limit c. minimum limit b. accuracy error a b c figure 10. accuracy error vs. temperature supply voltage ?v 400 0 350 200 150 100 50 300 250 450 500 08 1 supply current ?? 23 4 5 67 ta = 25 c, no load, heater off figure 11. supply current vs. supply voltage temperature ? c 0.4 0 -75 power supply rejection ? c/v -25 25 75 125 175 0.2 0.1 0.3 0.5 v = 4.5 to 5.5v no load, heater off figure 12. vptat power supply rejection vs. temperature
TMP12 rev. 0 C7C temperature ? c 36 20 -75 open collector sink current ?ma -25 25 75 125 175 28 24 32 40 v ol = 1v, v = 5v 22 38 34 30 26 figure 13. open-collector output sink current vs. temperature temperature ? c 700 0 -75 175 open?ollector output voltage ?mv -25 25 75 125 600 500 400 300 200 100 a. load = 10ma c. load = 1ma b. load = 5ma a b c v = 5v figure 14. open-collector voltage vs. temperature applications information a typical application for the TMP12 is shown in figure 15. the TMP12 package is placed in the same cooling airflow as a high-power dissipation ic. the TMP12s internal resistor pro- duces a temperature rise which is proportional to air flow, as shown in figure 16. any interruption in the airflow will produce an additional temperature rise. when the TMP12 chip tempera- ture exceeds a user-defined setpoint limit, the system controller can take corrective action, such as: reducing clock frequency, shutting down unneeded peripherals, turning on additional fan cooling, or shutting down the system. power i.c. pga package pga socket pc board air flow TMP12 figure 15. typical application 35 40 45 50 55 60 65 50 100 150 200 250 0 die temperature ( c) TMP12 p d (mw) a. TMP12 die temp no air flow c. low set point b. high set point d. TMP12 die temp max air flow e. system ambient temperature a b c d e figure 16. choosing temperature setpoints temperature hysteresis the temperature hysteresis at each setpoint is the number of degrees beyond the original setpoint temperature that must be sensed by the TMP12 before the setpoint compar- ator will be reset and the output disabled. hysteresis prevents chatter and motorboating in feedback control systems. for monitoring temperature in computer systems, hysteresis prevents multiple interrupts to the cpu which can reduce system performance. figure 17 shows the TMP12s hysteresis profile. the hyster- esis is programmed, by the user, by setting a specific load current on the reference voltage output vref. this output current, i ref , is also called the hysteresis current. i ref is mir- rored internally by the TMP12, as shown in the functional block diagram, and fed to a buffer with an analog switch. lo hi output voltage over, under temperature hysteresis low hysteresis high = hysteresis low t setlow t sethigh hysteresis high figure 17. TMP12 hysteresis profile after a temperature setpoint has been exceeded and a com- parator tripped, the hysteresis buffer output is enabled. the result is a current of the appropriate polarity which gener- ates a hysteresis offset voltage across an internal 1 k w resistor at the comparator input. the comparator output remains on until the voltage at the comparator input, now equal to the temperature sensor voltage vptat summed with the hysteresis effect, has returned to the pro- grammed setpoint voltage. the comparator then returns
TMP12 rev. 0 C8C low, deactivating the open-collector output and disabling the hysteresis current buffer output. the scale factor for the pro- grammed hysteresis current is: i = i vref = 5 m a/ c 1 7 m a thus, since vref = 2.5 v, a reference load resistance of 357 k w or greater (output current of 7 m a or less) will produce a tem- perature setpoint hysteresis of zero degrees. for more details, see the temperature programming discussion below. larger values of load resistance will only decrease the output current below 7 m a, but will have no effect on the operation of the device. the amount of hysteresis is determined by selecting an appropriate value of load resistance for vref, as shown below. programming the TMP12 the basic thermal monitoring application only requires a simple three-resistor ladder voltage divider to set the high and low setpoints and the hysteresis. these resistors are programmed in the following sequence: 1. select the desired hysteresis temperature. 2. calculate the hysteresis current, i vref 3. select the desired setpoint temperatures. 4. calculate the individual resistor divider ladder values needed to develop the desired comparator setpoint voltages at the set high and set low inputs. the hysteresis current is readily calculated, as shown above. for example, to produce 2 degrees of hysteresis i vref should be set to 17 m a. next, the setpoint voltages v sethigh and v setlow are determined using the vptat scale factor of 5 mv/k = 5 mv/ ( c 1 273.15), which is 1.49 v for 1 25 c. finally, the divider resistors are calculated, based on the setpoint voltages. the setpoint voltages are calculated from the equation: v set = ( t set 1 273.15)(5 mv/ c ) this equation is used to calculate both the v sethigh and the v setlow values. a simple 3-resistor network, as shown in figure 18, determines the setpoints and hysteresis value. the equations used to calculate the resistors are: r1 ( k w ) = ( v ref 2 v sethigh )/ i vref = (2.5 v 2 v sethigh )/ i vref r2 ( k w ) = ( v sethigh 2 v setlow )/ i vref r3 ( k w ) = v setlow / i vref 1 2 3 4 8 7 6 5 ( vref ?v sethigh ) / i vref = r1 TMP12 (v sethigh ?v setlow ) / i vref = r2 v setlow / i vref = r3 v sethigh v setlow vref = 2.5 v i vref gnd v+ heater under over figure 18. TMP12 setpoint programming for example, setting the high setpoint for 1 80 c, the low setpoint for 1 55 c, and hysteresis for 3 c produces the following values: i hys = i vref = (3 c 3 5 m a/ c ) 1 7 m a = 15 m a 1 7 m a = 22 m a v sethigh = ( t sethigh 1 273.15)(5 mv/ c ) = (80 c 1 273.15)(5 mv/ c ) = 1.766 v v setlow = ( t setlow 1 273.15)(5 mv/ c ) = (55 c 1 273.15) (5 mv/ c ) = 1.641 v r1 ( k w ) = ( vref 2 v sethigh )/ i vref = (2.5 v 2 1.766 v )/ 22 m a = 33.36 k w r2 ( k w ) = ( v sethigh 2 v setlow )/ i vref = (1.766 v 2 1.641 v )/ 22 m a = 5.682 k w r3 ( k w ) = v setlow /i vref = (1.641 v )/22 m a = 74.59 k w the total of r1 1 r2 1 r3 is equal to the load resistance needed to draw the desired hysteresis current from the reference, or i vref . the nomograph of figure 19 provides an easy method of determining the correct vptat voltage for any temperature. simply locate the desired temperature on the appropriate scale (k, c or f) and read the corresponding vptat value from the bottom scale. 218 248 273 298 323 348 373 398 ?5 ?5 ?8 0 25 50 75 100 125 ?7 ?5 0 32 50 77 100 150 200 212 257 vptat k f 1.09 1.24 1.365 1.49 1.615 1.74 1.865 1.99 ? figure 19. temperature 2 vptat scale the formulas shown above are also helpful in understanding the calculations of temperature setpoint voltages in circuits other than the standard two-temperature thermal/airflow monitor. if a setpoint function is not needed, the appropriate comparator in- put should be disabled. sethigh can be disabled by tying it to v 1 or vref, setlow by tying it to gnd. either output can be left disconnected. selecting setpoints choosing the temperature setpoints for a given system is an em- pirical process, because of the wide variety of thermal issues in any practical design. the specific setpoints are dependent on such factors as airflow velocity in the system, adjacent compo- nent location and size, pcb thickness, location of copper ground planes, and thermal limits of the system. the TMP12s temperature rise above ambient is proportional to airflow (figures 1, 2 and 16). as a starting point, the low setpoint temperature could be set at the system ambient temp- erature (inside the enclosure) plus one half of the temperature rise above ambient (at the actual airflow in the system). with this setting, the low limit will provide a warning either if the fan output is reduced or if the ambient temperature rises (for ex- ample, if the fans cool air intake is blocked). the high setpoint could then be set for the maximum system temperature to pro- vide a final system shutdown control.
TMP12 rev. 0 C9C measuring the TMP12 internal temperature as previously mentioned, the TMP12s vptat generator repre- sents the chip temperature with a slope of 5 mv/k. in some cases, selecting the setpoints is made easier if the TMP12s internal vptat voltage (and therefore the chip temperature) is known. for example, the case temperature of a high power microprocessor can be monitored with a thermistor, thermocouple, or other mea- surement method. the case temperature can then be correlated with the TMP12s temperature to select the setpoints. the TMP12s vptat voltage is not available externally, so indi- rect methods must be used. since the vptat voltage is applied to the internal comparators, measuring the voltage at which the digital output changes state will reflect the vptat voltage. a simple method of measuring the TMP12 vptat is shown in figure 20. to measure vptat, adjust potentiometer r1 until the led turns on. the voltage at pin 2 of the TMP12 will then match the TMP12s internal vptat. vptat TMP12 vref set high gnd v+ heater 1 2 3 4 5 6 7 8 200k 200k +5v nc r1 +5v r1 330 +5v led over under set low figure 20. measuring vptat with a potentiometer the method described in figure 20 can be automated by replac- ing the discrete resistors with a digital potentiometer. the improved circuit, shown in figure 21, permits the vptat volt- age to be monitored with a microprocessor or other digital controller. the ad8402-100 provides two 100 k w potentiom- eters which are adjusted to 8-bit resolution via a 3-wire se- rial interface. the controller simply sweeps the wiper of potentiometer 1 from the a1 terminal to the b1 terminal (digital value = 0), while monitoring the comparator output at pin 7 of the TMP12. when pin 7 goes low, the voltage at pin 2 equals the vptat voltage. this circuit sweeps pin 2's voltage from maximum to minimum, so that the TMP12's setpoint hystersis will not affect the reading. the circuit of figure 21 provides approximately 1 c of resolution. the two potentiometers divide vref by two, and the 8-bit potentiometer further divides vref by 256, so the resolution is: resolution = = 4.9 mv vref 2 2 n 2.5 v 2 28 = where vref is the voltage reference output (pin 1 of the TMP12) and n is the resolution of the ad8402. since the vptat has a slope of 5 mv/k, the ad8402 provides 1 c of resolution. the adjustment range of this circuit extends from vref/2 (i.e. 1.25 v, or 2 23 c) to vref 2 1 lsb (i.e. 2.5 v 2 4.9 mv, or 226 c). the vptat is therefore: vptat = 1.25 v + ( digital count 4.9 mv ) where digital count is the value sent to the ad8402 which caused the setpoint 1 output to go low. a third way to measure the vptat voltage is to close a feedback loop around one of the TMP12s comparators. this causes the comparator to oscillate, and in turn forces the voltage at the comparator input to equal the vptat voltage. figure 22 is a typical circuit for this measurement. an op193 operational amplifier, operating as an integrator, provides additional loop-gain to ensure that the TMP12 comparator will oscillate. temperature sensor & voltage reference 1 2 3 4 hysteresis generator window comparator vptat vref 7 8 5 6 TMP12 serial data interface 15 6 7 8 9 10 11 b2 2 a2 3 w2 4 w1 12 a1 13 b1 14 100 dgnd agnd sdi clk cs ad8402?00 ? interface v dd rs shdn over +5v +5v nc nc figure 21. measuring vptat with a digital potentiometer
TMP12 rev. 0 C10C understanding error sources the accuracy of the vptat sensor output is well characterized and specified, however preserving this accuracy in a thermal monitoring control system requires some attention to minimiz- ing the various potential error sources. the internal sources of setpoint programming error include the initial tolerances and temperature drifts of the reference voltage vref, the setpoint comparator input offset voltage and bias current, and the hyster- esis current scale factor. when evaluating setpoint programming errors, remember that any vref error contribution at the com- parator inputs is reduced by the resistor divider ratios. each comparators input bias current drops to less than 1 na (typ) when the comparator is tripped. this change accounts for some setpoint voltage error, equal to the change in bias current multi- plied by the effective setpoint divider ladder resistance to ground. the thermal mass of the TMP12 package and the degree of thermal coupling to the surrounding circuitry are the largest fac- tors in determining the rate of thermal settling, which ultimately determines the rate at which the desired temperature measure- ment accuracy may be reached (see graph in figure 3). thus, one must allow sufficient time for the device to reach the final temperature. the typical thermal time constant for the soic plastic package is approximately 70 seconds in still air. there- fore, to reach the final temperature accuracy within 1%, for a temperature change of 60 degrees, a settling time of 5 time con- stants, or 6 minutes, is necessary. refer to figure 4. external error sources to consider are the accuracy of the external programming resistors, grounding error voltages, and thermal gra- dients. the accuracy of the external programming resistors directly impacts the resulting setpoint accuracy. thus, in fixed-temperature applications the user should select resistor tolerances appropriate to the desired programming accuracy. since setpoint resistors are typically located in the same air flow as the TMP12, resistor tem- perature drift must be taken into account also. this effect can be minimized by selecting good quality components, and by keep- ing all components in close thermal proximity. careful circuit board layout and component placement are necessary to mini- mize common thermal error sources. also, the user should take care to keep the bottom of the setpoint programming divider ladder as close to gnd (pin 4) as possible to minimize errors due to ir voltage drops and coupling of external noise sources. in any case, a 0.1 m f capacitor for power supply bypassing is always recommended at the chip . safety considerations in heating and cooling system design designers should anticipate potential system fault conditions that may result in significant safety hazards which are outside the control of and cannot be corrected by the TMP12-based cir- cuit. governmental and industrial regulations regarding safety requirements and standards for such designs should be observed where applicable. 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 TMP12 open-collector output device is dissipating: p diss = 0.6 v 0.020 a = 12 mw which in a surface-mount so package accounts for a tempera- ture increase due to self-heating of: d t = p diss ja = 0.012 w 158 c/w = 1.9 c this increase is for still air, of course, and will be reduced at high airflow levels. however, the user should still be aware that self-heating effects can directly affect the accuracy of the TMP12. for setpoint 2, self-heating will add to the setpoint temperature (that is, in the above example the TMP12 will switch the setpoint 2 output off 1.9 degrees early). self-heating will not affect the temperature at which setpoint 1 turns on, but will add to the hysteresis. several circuits for adding external driver transistors and other buffers are presented in following sections of this data sheet. these buffers will reduce self-heating and improve accuracy. buffering the voltage reference the reference output vref is used to generate the temperature setpoint programming voltages for the TMP12. since the hyster- esis is set by the reference current, external circuits which draw current from the reference will increase the hysteresis value. 5 6 7 8 op193 vref set high set low gnd v+ heater 1 2 3 4 TMP12 130k +5v ~1.5v 200k 5k nc nc +5v +5v 1uf 300k 10k 0.1uf vptat over under figure 22. an analog measurement circuit for vptat
TMP12 rev. 0 C11C the on-board vref output buffer is typically capable of 500 m a output drive into as much as 50 pf load (max). exceeding this load will affect the accuracy of the reference voltage, could cause thermal sensing errors due to excess heat build-up, and may induce oscillations. external buffering of vref with a low-drift voltage follower will ensure optimal reference accuracy. amplifiers which offer low drift, low power consumption, and low cost appropriate to this application include the op284, and members of the op113 and op193 families. with excellent drift and noise characteristics, vref offers a good voltage reference for data acquisition and transducer excitation ap- plications as well. output drift is typically better than 2 10 ppm/ c, with 315 nv/hz (typ) noise spectral density at 1 khz. preserving accuracy over wide temperature range operation the TMP12 is unique in offering both a wide-range temperature sensor and the associated detection circuitry needed to implement a complete thermostatic control function in one monolithic device. the voltage reference, setpoint comparators, and output buffer amplifiers have been carefully compensated to maintain accuracy over the specified temperature ranges in this application. since the TMP12 is both sensor and control circuit, in many applications the external components used to program and interface the device are subjected to the same temperature extremes. thus, it is necessary to place components in close thermal proximity minimizing large temperate differentials, and to account for thermal drift errors where appropriate, such as resistor matching temperature coeffi- cients, amplifier error drift, and the like. circuit design with the TMP12 requires a slightly different perspective regarding the ther- mal behavior of electronic components. pc board layout considerations the TMP12 also requires a different perspective on pc board lay- out. in many applications, wide traces and generous ground planes are used to extract heat from components. the TMP12 is slightly different, in that ideal path for heat is via the cooling system air flow. thus, heat paths through the pc traces should be minimized. this constraint implies that minimum pad sizes and trace widths should be specified in order to reduce heat conduction. at the same time, analog performance should not be compromised. in particular, the bottom of the setpoint resistor ladder should be located as close to gnd as possible, as discussed in the under- standing error sources section of this data sheet. thermal response time the time required for a temperature sensor to settle to a specified 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 is the inverse of thermal resistance. it is commonly specified in units of degrees per watt of power transferred across the thermal joint. figures 3 and 5 illustrate the typical rc time constant response to a step change in ambient temperature. thus, the time required for the TMP12 to settle to the desired accuracy is dependent on the package selected, the thermal contact established in the particular application, and the equivalent thermal con- ductivity of the heat source. for most applications, the settling-time is probably best determined empirically. switching loads with the open-collector outputs in many temperature sensing and control applications some type of switching is required. whether it be to turn on a heater when the temperature goes below a minimum value or to turn off a motor that is overheating, the open-collector outputs can be used. for the majority of applications, the switches used need to handle large currents on the order of 1 amp and above. because the TMP12 is accurately mea- suring temperature, the open-collector outputs should handle less than 20 ma of current to minimize self-heating. clearly, the trip point outputs should not drive the equip- ment directly. instead, an external switching device is required to handle the large currents. some examples of these are relays, power mosfets, thyristors, igbts, and darlington transistors. this section shows a variety of circuits where the TMP12 controls a switch. the main consideration in these circuits, such as the relay in figure 23, is the current required to ac- tivate the switch. hysteresis generator window comparator vptat vref 100 motor shutdown 2604-12-311 coto in4001 or equiv +12v r1 r2 r3 1 2 3 4 8 5 7 6 TMP12 nc +12 v 140 w temperature sensor & voltage reference figure 23. reed relay drive it is important to check the particular relay you choose to ensure that the current needed to activate the coil does not exceed the TMP12s recommended output current of 20 ma. this is easily determined by dividing the relay coil voltage by the specified coil resistance. keep in mind that the inductance of the relay will create large voltage spikes that can damage the TMP12 output unless protected by a commutation diode across the coil, as shown. the relay shown has contact rating of 10 watts maximum. if a relay capable of handling more power is desired, the larger con- tacts will probably require a commensurably larger coil, with lower coil resistance and thus higher trigger current. as the contact power handling capability increases, so does the current needed for the coil, in some cases an external driving transistor should be used to remove the current load on the TMP12 as explained in the next section.
TMP12 rev. 0 C12C power fets are popular for handling a variety of high current dc loads. figure 24 shows the TMP12 driving a p-channel mosfet transistor for a simple heater circuit. when the out- put transistor turns on, the gate of the mosfet is pulled down to approximately 0.6 v, turning it on. for most mosfets a gate-to-source voltage or vgs on the order of -2 v to -5 v is suf- ficient to turn the device on. figure 25 shows a similar circuit for turning on an n-channel mosfet, except that now the gate to source voltage is positive. for this reason an external transistor must be used as an inverter so that the mosfet will turn on when the trip point pulls down. nc irfr9024 or equiv heating element 2.4k w (12v) 1.2k w (6v) 5% v+ temperature sensor & voltage reference hysteresis generator window comparator vptat vref 100 1 2 3 4 8 5 7 6 +5v TMP12 nc = no connect figure 24. driving a p-channel mosfet irf130 nc 2n1711 heating element v+ 4.7k w 4.7k w temperature sensor & voltage reference hysteresis generator window comparator vptat vref 100 1 2 3 4 8 5 7 6 +5v TMP12 figure 25. driving an n-channel mosfet isolated gate bipolar transistors (igbts) combine many of the benefits of power mosfets with bipolar transistors and are used for a variety of high power applications. because igbts have a gate similar to mosfets, turning on and off the devices is relatively simple as shown in figure 26. the turn on voltage for the igbt shown (irgb40s) is between 3.0 and 5.5 volts. this part has a continuous collector current rating of 50 a and a maximum collector to emitter voltage of 600 v, enabling it to work in very demanding applications. irgbc40s nc 2n1711 v+ 4.7k w 4.7k w motor control +5v temperature sensor & voltage reference hysteresis generator window comparator vptat vref 100 1 2 3 4 8 5 7 6 TMP12 nc = no connect figure 26. driving an igbt the last class of high power devices discussed here are thyristors, which include scrs and triacs. triacs are a useful alternative to relays for switching ac line voltages. the 2n6073a shown in fig- ure 27 is rated to handle 4 a (rms). the opto-isolated moc3021 triac shown features excellent electrical isolation from the noisy ac line and complete control over the high power triac with only a few additional components. nc v+ = 5v 300 w 150 w moc3011 1 2 34 5 6 load ac 2n6073a nc = no connect temperature sensor & voltage reference hysteresis generator window comparator vptat vref 100 1 2 3 4 8 5 7 6 +5v TMP12 figure 27. controlling the 2n6073a triac
TMP12 rev. 0 C13C high current switching as mentioned earlier, internal dissipation due to large loads on the TMP12 outputs will cause some temperature error due to self-heating. external transistors buffer the load from the TMP12 so that virtually no power is dissipated in the internal transistors and minimal self-heating occurs. this section shows several examples using external transistors. the simplest case uses a single transistor on the output to invert the output signal is shown in figure 28. when the open-collector of the TMP12 turns on and pulls the output down, the external transistor q1s base will be pulled low, turning off the transistor. another transistor can be added to re-invert the signal as shown in figure 29. now, when the output of the TMP12 is pulled down, the first transistor, q1, turns off and its collector goes high, which turns q2 on, pulling its collector low. thus, the output taken from the collector of q2 is identical to the output of the TMP12. by picking a transistor that can accommodate large amounts of current, many high power devices can be switched. 2n1711 v+ 4.7k w i c q1 temperature sensor & voltage reference hysteresis generator window comparator vptat vref 100 1 2 3 4 8 5 7 6 TMP12 figure 28. an external transistor minimizes self-heating motor switch relay +12v 2n1711 v+ 4.7k w i c 4.7k w tip-110 temperature sensor & voltage reference hysteresis generator window comparator vptat vref 100 1 2 3 4 8 5 7 6 +5v TMP12 figure 30. darlington transistor can handle large currents q1 q2 2n1711 v+ 4.7k w 4.7k w 2n1711 temperature sensor & voltage reference hysteresis generator window comparator vptat vref 100 1 2 3 4 8 5 7 6 TMP12 i c figure 29. second transistor maintains polarity of TMP12 output an example of a higher power transistor is a standard darlington configuration as shown in figure 30. the part cho- sen, tip-110, can handle 2 a continuous which is more than enough to control many high power relays. in fact the darlington itself can be used as the switch, similar to mosfets and igbts.
TMP12 rev. 0 C14C outline dimensions dimensions shown in inches and (mm). 8-pin epoxy dip 0.015 (0.381) 0.008 (0.204) 0.160 (4.06) 0.115 (2.93) 0.130 (3.30) min 0.210 (5.33) max 0.015 (0.381) typ 0.430 (10.92) 0.348 (8.84) 0.280 (7.11) 0.240 (6.10) 4 5 8 1 0.070 (1.77) 0.045 (1.15) 0.022 (0.558) 0.014 (0.356) 0.325 (8.25) 0.300 (7.62) 0? 15? 0.100 (2.54) bsc seating plane 0.195 (4.95) 0.115 (2.93) 8-pin soic seating plane 0.0500 (1.27) bsc 4 5 8 1 0.2440 (6.20) 0.2284 (5.80) 0.1574 (4.00) 0.1497 (3.80) 0.1968 (5.00) 0.1890 (4.80) 0.0500 (1.27) 0.0160 (0.41) 0? 8 45? 0.0196 (0.50) 0.0099 (0.25) 0.0098 (0.25) 0.0075 (0.19) 0.0192 (0.49) 0.0138 (0.35) 0.0098 (0.25) 0.0040 (0.10) 0.102 (2.59) 0.094 (2.39) c2074-10-10/95 printed in u.s.a.

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