application note tsm101 used in a voltage supervisor AN895/0299 by s. laffont ? this technical note shows how to use the tsm101 integrated circuit to realize a voltage supervisor. an example of realization is given. 1 - tsm101 presentation the tsm101 integrated circuit incorporates a high stability series band gap voltage reference, two ored operational amplifiers and a current source (figure 1). 1 2 3 4 8 5 6 7 vref figure 1 : tsm101 schematic diagram 2 - principle of operation and schematic diagram the two operational amplifiers are used as compa- rators. the first operator has its + input wired to the internal 1.24v reference and the second one its - input wired to 1.24v. as the two operators are or-ed internally, the output (pin 6) is low if : the negative input (v7) of the first operator (pin 7) is higher than 1.24v the positive input (v5) of the second operator (pin 5) is lower than 1.24v. a resistor network (r1, r2, r3) is used to bias the input of the two operators, when the input voltage is out of the limits, the output (pin 6) is close to zero. this signal can be used directly to power an opto- coupler. in our application, we have added a transistor (q1). this transistor is driven via a zener diode (d1). this circuit has two advantages : 1 - this zener diode avoids problems encoun- tered with many voltage supervisors : when the supply voltage is lower than the operating volt- age of those circuits, the behavior is erratic. the minimum operating voltage of the tsm101 is 5v. with a 4.7v zener diode, the transistor q1 will never be saturated if the supply voltage is lower than 5v whatever the output of the tsm101. 2 - the optocoupler is on only if the input voltage is inside the specified r ange. 3 - calculation of the elements let assume : v min. = minimum threshold voltage v max. = maximum threshold voltage v cc = nominal voltage we have the following equations : v5 = ( v min. x ( r1 + r2 )) s r = v ref v7 = ( v max. x r1 ) s r = v ref r1 = v re x s r f ) v max. r2 = v ref x s r v min. = - r1 1/2
example : supervision of a 12v +-5% power supply : v min. = 11.4v v max. = 12.6v we take r s = 24k w (500 m a in the resistor net- work) d1 = 4.7v zener diode ib (q1) = 500 m a when on and a current of 10ma in the opto-coupler when on then : r1 = 2.36k w r2 = 200 w r3 = 21.43k w r4 = 12k w r5 = 10k w r6 = 1k w the complete schematic of the voltage supervisor is represented on figure 2. figure 2 : voltage supervisor with tsm101 information furnished is believed to be accurate and reliable. however, stmi croelectronics 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 f rom 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 publication supersedes and replaces all information previously supplied. stmicroelectronics products are not authorized for use as critical components in life support devices or s ystems without express written approval of stmicroelectronics. ? the st logo is a trademark of stmi croelectronics ? 1999 stmi croelectronics C printed in italy C all rights reserved stmi croelectronics 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 application note 2/2
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