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february 2007 rev 1 1/14 AN2504 application note using stmicroelectronics voltage detectors 1 introduction voltage detectors are designed to monitor th e system supply voltage and assert the output signal, out , every time it goes below a defined voltage threshold. the major advantages of these circuits are low current consumption and a precise temperature-compensated voltage reference, v ref (see figure 1 ). stmicroelectronics voltage detectors are also laser programmed to the desired voltage threshold over the range of 1.6v to 6.0v in 100mv steps and they have a good transient immunity (see figure 5 ). figure 1. n-channel open drain output block diagram note that voltage detector output state simply indicates if the supply voltage is above or below a specific threshold, which can be used for early power fail warning. however microprocessors' inputs require some minimum input pulse width to register the change of signal on the logical input. in this case we recommend using reset circuits, which guarantee the minimum pulse width known as reset time-out period. reset circuits have some other advantages compared to voltage detectors (e.g. specification across temperature, better voltage threshold accuracy, and lower temperature coefficient). on the other hand voltage detectors are usually able to operate at higher voltages, have lower current consumption, greater hysteresis and usually a lower price. the qualities mentioned above cause voltage detectors to be preferentially used in the following applications: ? battery monitoring power-supply monitoring portable medical devices power fail detection pdas notebook computers back-up supply switching portable/battery- powered electronics cell phones. ai10482b v cc r1 r2 r3 v ss + ? v ref v out www.st.com
contents AN2504 - application note 2/14 contents 1 introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3 operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4 transient immunity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5 open drain output advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 6 modifying the voltage threshold, v th? . . . . . . . . . . . . . . . . . . . . . . . . . . 8 7 simulation of the reset time-out period . . . . . . . . . . . . . . . . . . . . . . . . 9 8 output mosfet protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 8.1 external n-channel transistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 8.2 external p-channel transistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 9 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
AN2504 - application note list of figures 3/14 list of figures figure 1. n-channel open drain output block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 figure 2. stm1061n active-low, open drain typical hardware hookup . . . . . . . . . . . . . . . . . . . . . . 4 figure 3. voltage timing waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 figure 4. maximum transient duration vs. reset threshold overdrive . . . . . . . . . . . . . . . . . . . . . . . 6 figure 5. additional transient immunity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 figure 6. voltage detectors with wire or connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 figure 7. voltage detector monitors different supply voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 figure 8. modifying the voltage threshold, v th? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 figure 9. hardware hookup with rc element on the voltage detector output . . . . . . . . . . . . . . . . . . 9 figure 10. voltage waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 figure 11. circuit with external n-channel transistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 figure 12. circuit with external p-channel transistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
description AN2504 - application note 4/14 2 description stmicroelectronics offers e.g. the stm1061 voltage detector. one of its typical uses is shown in the figure 2 . the stm1061n open drain output voltage detector is monitoring the 3.6v battery. this allows the voltage detector to detect a voltage drop early, assert the output signal (out ) so the mcu can start safeguard routines even before the regulated supply voltage for the mcu starts to fall. the voltage detector open drain output sinks current when the output is asserted. it is necessary to connect a pull-up resistor from out to any supply voltage (see figure 2 ). the resistor value must be large enough to register a logic low and small enough to register a logic high while all of the input current and leakage paths connected to the reset output line are being supplied. a10k ? pull-up is sufficient in most applications. figure 2. stm1061n active-low, open drain typical hardware hookup ai10484a v cc v in + v ss v cc mcu stm1061n (open drain) dc/dc converter out 3.6v v out v ss v ss suspend regulated 3v r v out
AN2504 - application note operation 5/14 3 operation the voltage detector monitors v cc voltage input continuously (see figure 1 ) and compares it with the precision voltage reference, v ref . when v cc falls below a specified trip point threshold, the output (out ) is forced low and remains asserted as long as the v cc input remains below v th+ , where v th+ = v th? + v hyst (hysteresis) see figure 3 . remember that a pull-up resistor on the voltage detector open-drain output is required for proper functionality (see figure 2 ). figure 3. voltage timing waveform ai10483 v cc v cc (min) v ss v th+ v th? t pr release delay t pd detect delay release voltage (v th+ = v th? + v hyst ) detect voltage v ss v out
transient immunity AN2504 - application note 6/14 4 transient immunity the stm1061 device is relatively immune to negative-going v cc transients (glitches). the graph (see figure 4 ) indicates the maximum pulse width a negative v cc transient can have without asserting output signal, out . as the magnitude of the transient increases (further below the threshold), the maximum allowable pulse width decreases. any combination of duration and overdrive which lies under the curve will not assert the output signal, out . a 0.1f bypass capacitor, cb, mounted as close as possible to the v cc pin provides additional transient immunity (see figure 5 ). figure 4. maximum transient duration vs. reset threshold overdrive figure 5. additional transient immunity threshold overdrive (mv) transient duration (s) ai11123b 0 20 40 60 80 100 120 140 160 1 10 100 1000 10000 v th? = 3.4v v th? = 1.6v ai12905a v cc v cc c b stm1061n out v ss power good r
AN2504 - application note open drain output advantages 7/14 5 open drain output advantages the advantages of open drain output are the ability to connect more open drain outputs in parallel (wired or connections, see figure 6 ) as well as connect the output to a power supply voltage different from v cc (see figure 7 ). the hook up on figure 6 monitors 2 independent supply voltages (3v and 5v). every time either the first or the second supply voltage goes below defined threshold, the input signal of the mcu, suspend , goes low. figure 6. voltage detectors with wire or connection a voltage detector with an open drain output can also monitor supply voltages different from the mcu supply voltage (see figure 7 ). the logic high on the mcu suspend input is adequate to supply voltage of the mcu. figure 7. voltage detector monitors different supply voltage ai12906 v cc v ss v cc mcu stm1061n out 5v 3v r v ss suspend v cc stm1061n out v ss ai12907 v ss v cc mcu 5v 3v r suspend v cc stm1061n out v ss
modifying the voltage threshold, v th? AN2504 - application note 8/14 6 modifying the voltage threshold, v th? although the stm1061 voltage thresholds, v th? , are adjusted in fine steps (100mv), it is sometimes necessary to make adjustments during prototyping. this can be achieved by connecting of external divider (see figure 8 ). this hook up can be used if the required threshold of voltage detector is lower than the desired monitored voltage. to maintain detector accuracy, the current flow through the divider should be significantly higher than the 0.9a operating current required by the stm1061. a 90a bleeder current is sufficient in most applications. figure 8. modifying the voltage threshold, v th? example: the voltage on the divider is: that is why: the value of r2 resistor is then: we will choose the nearest values of the resistors (e.g. from e24): r1 = 18k ? , r2 = 2.2k ? . ai12910 v supply r2 v cc stm1061n out v ss r1 threshold of stm1061 detector: v th? = 1.6v desired threshold: v supply = 1.8v recommended bleeder current: i = 90a r1+r2 is then: (r1+r2) = v supply /i = 1.8v/90a = 20k ? v cc r 1 r 1 r 2 + -------------------- v supply v th ? == r 1 v th ? v supply ------------------------ -r 1 r 2 + () 1.6v 1.8v ------------ 2 0 k ? 17.7k ? === r 2 20k ? 17.7k ? 2.2k ? = ? =
AN2504 - application note simulation of the reset time-out period 9/14 7 simulation of the reset time-out period it is possible to simulate reset time-out period with an rc element on the output of voltage detector (see figure 9 ). figure 9. hardware hookup with rc element on the voltage detector output if the supply voltage, v cc , drops below the defined voltage threshold, v rst , the capacitor, c, is discharged through the output transistor of the voltage detector (see figure 10 ). when the supply voltage, v cc , rises above the voltage threshold plus hysteresis, v rst +v hyst , the output transistor disconnects the output from the ground and the capacitor, c, is charged through the resistor, r. figure 10. voltage waveforms ai12911a v ss v cc mcu v cc r c suspend v cc stm1061n out v ss v cap ai12912 t t 3 t 2 t 1 v ih v rst v cap v cc v rst ? v hyst v capacitor is charging capacitor is discharging
simulation of the reset time-out period AN2504 - application note 10/14 the voltage on the stm1061n output resp. capacitor, v cap , rises as capacitor is charged. when it reaches input high voltage trip point of the mcu (time t 3 ), the high state is detected. if there is no rc element on the output, the out pin goes high at the time t 2 . that is why simulated reset time-out period is t 3 -t 2 . use following formula for determining the correct capacitor value: where, c = capacitor of rc element in farads, r = resistor of rc element in ohms, t = desired time-out period (t 3 -t 2 ) in seconds, v cc = supply voltage, v ih = input voltage trip point for the mcu ln = natural logarithm. example: the stm1061n27wx6f is used to monitor a 3v supply voltage. the mcu detects input high state at v ih =0.7 v cc = 0.7 3v = 2.1v. desired time-out period is 140ms. an r = 100 ? resistor is chosen. the value of the capacitor is following: the closest higher capacitance value from the most common e12 series (i.e. c = 1.2f) is chosen. the time-out delay will be slightly greater by choosing a higher value of capacitor. remember that this circuitry should be used only for temporary purposes. in the case of real need of time-out delay the usage of reset device is highly recommended as mentioned in the introduction . c t r i n v cc v cc v ih ? -------------------------- ? ------------------------------------------- - = c t r i n v cc v cc v ih ? -------------------------- ? ------------------------------------------- 140 10 3 ? ? 100 10 3 i n 3 32.1 ? ----------------- ?? --------------------------------------------------------- 1.16 10 6 ? ? 1.16 f == ==
AN2504 - application note output mosfet protection 11/14 8 output mosfet protection for applications requiring higher current drive capabilities an external mosfet might be used, which protect the output of a voltage detector from overloading and destroying. it is possible to use either n channel or p channel transistor (see below). 8.1 external n-channel transistor if the load should be connected to the supply voltage during the regular operation (output, out , is not asserted), an external n-channel transistor will be needed (see figure 11 ). when the supply voltage v cc drops below the threshold of the voltage detector, the output signal, out , is asserted, the external n-channel mosfet is switched off and disconnects the load from the ground. figure 11. circuit with external n-channel transistor ai12913 r load v cc v cc r v cc stm1061n out v ss
output mosfet protection AN2504 - application note 12/14 8.2 external p-channel transistor if the load should be connected to the supply voltage during the output, out , assertion, an external p-channel trans istor will be needed (see figure 12 ). when the supply voltage v cc drops below the threshold of the voltage detector, the output signal, out , is asserted, the external p-channel mosfet is switched on and connects the load to the supply voltage, v cc . figure 12. circuit with external p-channel transistor ai12913a r load v cc v cc r v cc stm1061n out v ss
AN2504 - application note revision history 13/14 9 revision history table 1. document revision history date revision changes 06-feb-2007 1 initial release.
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