application note tsm101 used in a thermostat AN921/0299 by r. liou ? this technical note shows how to use the tsm101 integrated circuit to realize a simple thermostat controlling a fan in cooling applications. an example of realization is given with the corre- sponding calculations. tsm101 presentation the tsm101 integrated circuit incorporates a high stability series band gap voltage reference (1.24v 2% precision or 1% with the a version), two ored operational amplifiers and a current source (1.4ma) as shown on figure 1. application context and principle of operation a temperature controlled fan is a common applica- tion in all electronic systems confined in a reduced and enclosed volume. as an example, the tsm101 can be used in such applicationsfor mother boards cooling, or in smps (switch mode power sup- plies). this thermostat is to be used in association with a temperature sensor (ex lm335). one operational amplifier of the tsm101 compares the voltage given by the temperature sensor to an internal precision voltage reference, and as soon as the temperature is higher than the preset limit, the output of the operational amplifier is set active (active low) and can switch a fan motor on. figure 2 shows how tsm101 is to be used in this application. resistor bridge r1/r2 sets the tem- perature limit. c1 stabilizes the output voltage of the temperature sensor. r3 ensures the hysteresis of the system. an improvement is shown on figure 3 where the current source is used to supply the temperature sensor (with 1.4macathode current). this requires that pin 2 and pin 5 be connected to the system ground. this saves the polarization resistor of the temperature sensor. figure 1 : tsm101 schematic diagram figure 2 : basic thermostat function 1/3
figure 4 : an overcurrent and overtemperature fan control figure 3 : the internal current source can supply the temperature sensor an other improvement can be achieved for switch mode power supplies (smps) where it is useful to start the cooling device as soon as the temperature is too high or when the overall current is above a preset limit (the cooling device can therefore antici- pate on the temperature elevation). this is shown on figure 4 where the drop voltage across the sense resistor r8 is compared to a set limit given by the resistor bridge r6/r7. in this configuration, the fan motor is started either by an overtemperature, or by an overcurrent. calculation of the elements the following calculations apply to an overcurrent and overtemperature fan controller (figure 4). temperature control : the temperature upper limit is determined by the resistor bridge r1/r2. ? vref = vsensor (t ) x r2 / (r1 + r2) if the sensor is an lm335, then the voltage function of the temperature is a direct translation of the temperature in kelvin degrees following : ? v(t )=t ( k) / 100. as an example, at 25 c, the output voltage of the lm335 is (273 + 25) / 100=2.98v. let us assume that an acceptable upper tempera- ture limit is 50 c, therefore : ? 1.24 = 3.23 x r2 / (r1+ r2) with r1+ r2 = 30k w as a good compromise precision/consumption. ? r2 = 12k w , r1 = 18k w starts the fan at 50 c. ? c1 = 0.1 m f stabilizes the lm335 output. ? r = 10k w supplies the lm335. current control : the current limit is determined by the resistor bridge r6/r7 and the sense resistor r8. in many applications where it is necessary to re- duce temperature with air flow, a common current scale is in between 1 and 10 amps. application note 2/3
information furnished is believed to be accurate and reliable. however, stmicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement 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 stmicroelectronics. specifications mentioned in this publication are subject to change without notice. this publ ication supersedes and replaces all information previously supplied. stmicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of stmicroelectronics. ? the st logo is a trademark of stmicroelectronics ? 1999 stmicroelectronics printed in italy all rights reserved stmicroelectronics group of companies australia - brazil - canada - china - france - germany - italy - japan - korea - malaysia - malta - mexico - morocco the netherlands - singapore - spain - sweden - switzerland - taiwan - thailand - united kingdom - u.s.a. ? http://www.st.com let us assume that tsm101 is used as a thermo- stat for an application which has a maximum con- sumption of 10a and requires air flow starting at a current consumption of 2a. the voltage drop through the sense resistor r8 is given by : ? vdrop = vref x r7 / (r6 + r7) at 10a, a tolerable voltage drop can be chosen as 50mv, therefore, the voltage drop corresponding to 2a is 10mv. ? 0.01 = 1.24 x r7 / (r6 + r7) where r6 + r7 ~ 1.2k w to ensure proper charge for the voltage reference. ? r7 = 10 w , r6 = 1.2k w the sense resistor r8 determines the upper cur- rent limit following : ? vdrop = r8 x imax ? r8 = 5m w motor control : the power transistor q1 is controlled via its base resistor r4. to limit the base current to 10ma, r4 should be chosen in the range of 1k w . the pull up resistor r5 can be chosen in the range of 10k w . ? r4 = 1k w , r5 = 10k w the hysteresis on the temperature is determined by r3 : when q1 is on, the negative input voltage of the operational amplifier is pulled up ; when it is off, the negative input is pulled down. to make a unidirectional hysteresis, a diode is inserted in series with r3 in order to achieve hysteresis when the fan is on. r3 must be greater than r2 // r1 not to offset the measurement of the temperature sen- sor. ? r2 // r1 = r1 x r2 / (r1 + r2) = 7.2k w = req let usassume that the precision of the temperature control is in the range of +/- 1 c, and that we expect a5 c hysteresis. on the input (pin 7) of tsm101, 1 c corresponds to 10mv x r2 / (r1 + r2) = 4mv, therefore,5 c will correspond to vhyst = 20mv. the resistor r3 should have an influence of 20mv on pin 7, following the equation : ? vcc x req / (r3 + req) = vhyst ? r3 = 4.3m w the capacitors c2 and c3 stabilize respectively the command of the power transistor and filter the noise due to the motor. application note 3/3
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