november 2005 1 mic2777 mic2777 micrel, inc. mic2777 dual micro-power low voltage supervisor general description the mic2777 is a dual power supply supervisor that provides under-voltage monitoring, manual reset capability, and power- on reset generation in a compact 5-pin sot package. fea- tures include two under-voltage detectors, one ?xed and one adjustable, and both active-high and active-low res et outputs. one under-voltage detector compares v dd against a ?xed threshold. ten factory-programmed thresholds are available. the other under-voltage detector is user-adjustable. the reset outputs are asserted for no less than 140ms at power-on and any time v dd or the input voltage drops below the corresponding reference voltage. they remain asserted for the timeout period after the input voltage and v dd sub- sequently rise back above the threshold boundaries. a reset can be generated at any time by pulling down on the adjust- able input. hysteresis is included to prevent chattering due to noise. typical supply current is a low 3.5a. typical application in r1r2 /rst vdd rst otherlogic mic2777 microprocessor manualreset /reset v core v core 1.0v v i/o 1.8v v i/o gnd gnd typical application features ? monitors two independent power supplies for under-volt- age conditions ? one ?xed and one user adjustable input ? 1.5% theshold accuracy ? choice of factory-programmed thresholds ? user-adjustable input can monitor supplies as low as 0.3v ? generates 140ms (minimum) power-on reset pulse ? manual reset capability ? both active-high and active-low reset outputs ? input may be pulled above v dd (abs. max.) ? /rst output valid down to 1.2v ? ultra-low supply current, 3.5a typical ? rejects brief input transients ? ittybitty? 5-pin sot-23 package applications ? monitoring processor, asic, or fpga core and i/o volt- ages ? computer systems ? pdas, hand-held pcs ? embedded controllers ? telecommunications systems ? power supplies ? wireless / cellular systems ? networking hardware micrel, inc. ? 2180 fortune drive ? san jose, ca 95131 ? usa ? tel + 1 (408) 944-0800 ? fax + 1 (408 ) 474-1000 ? http://www.micrel.com ittybitty? is a trademark of micrel, inc. ordering information part number junction tem. range package standard marking pb-free marking mic2777-xxbm5 unxx mic2777-xxym5 unxx C40oc to +85oc sot-23-5 downloaded from: http:///
mic2777 micrel, inc. mic2777 2 november 2005 pin con?guration /rst vdd in rst 1 3 4 5 2 gnd sot-23-5 (m5) standard voltage options* voltage typical nominal threshold code application (v dd ) voltage (v th ) 46 5.0v 5% 4.68 44 5.0v 10% 4.43 31 3.3v 5% 3.09 29 3.3v 10% 2.93 28 3.0v 5% 2.81 26 2.85v 5% 2.67 25 2.70v 5% 2.53 23 2.5v 5% 2.34 22 2.4v 5% 2.25 17 1.8v 5% 1.69 *there are ten standard versions available with an order increment of 3000 pieces. samples of stand ard versions are normally available from stock. contact factory for information on non-standard versions. available i n tape-and-reel only. pin description pin number pin name pin function 1 /rst digital (output): asserted low whenever v in or v dd falls below the reference voltage. it will remain asserted for no less than 140ms after v in and v dd return above the threshold limits. 2 gnd ground 3 rst digital (output): asserted high whenever v in or v dd falls below the refer- ence voltage. it will remain asserted for no less than 140ms after v in and v dd return within above the threshold limit. 4 in analog (input): the voltage on this pin is compared to the internal 300mv reference. an undervoltage condition will trigger a reset sequence. manual reset capability can be achieved by adding a switch between this pin and ground. 5 vdd analog (input): power supply input for internal circuitry and input to the ?xed voltage monitor: the voltage on this pin is compared against the internal voltage reference. an undervoltage condition will trigger a reset sequence. downloaded from: http:///
november 2005 3 mic2777 mic2777 micrel, inc. absolute maximum ratings (note 1) supply voltage (v dd ) .......................................C0.3v to +7v input voltage (v in ) ...........................................C0.3v to +7v rst, (/rst) current ................................................... 20ma storage temperature (t s ) ........................ C65c to +150c esd rating, note 3 .................................................... 1.5kv operating ratings (note 2) supply voltage (v dd ) .................................. +1.5v to +5.5v input voltage (v in ) .............................................................C0.3v to +6.0v ambient temperature range (t a ) .............. C40c to +85c package thermal resistance ( ja ) ....................... 256c/w electrical characteristicsnote 5 ; t a = +25c, bold values indicate C40c t a +85c; unless noted symbol parameter condition min typ max units i dd supply current v dd = v in 3.5 a note 5 ; /mr, rst, /rst open vdd voltage threshold under-voltage threshold on v dd v th -1.5% v th v th +1.5% v v hyst hysteresis voltage 1 % in, under-voltage dectector input v ref under-voltage threshold 295 300 305 mv v hyst hysteresis voltage 3 mv i in input current 5 pa t min t a t max 10 na rst, /rst outputs t prop propagation delay v in = (v ref(max) + 100mv) to 20 s v in = (v ref(min) C 100mv) or v dd = (v th + 2.5 % ) to (v th C 2.5 % ), v in =v dd t rst reset pulse width 140 280 ms v ol rst or /rst output voltage low i sink = 1.6ma; 0.3 v v dd 1.6v i sink = 100a; 0.3 v v dd 1.2v, note 4 v oh rst or /rst output voltage high i source = 500a; 0.8v dd v v dd 1.5v rst asserted, i source = 10a; 0.8v dd v v dd 1.2v, note 4 note 1. exceeding the absolute maximum rating may damage the device. note 2. the device is not guaranteed to function outside its operating rating. note 3. devices are esd sensitive. handling precautions recommended. human body model, 1.5k in series with 1 00pf. note 4. v dd operating range is 1.5v to 5.5v. output is guaranteed to be asserted down to v dd = 1.2v. note 5. v dd equals the nominal typical application (v dd ) as shown in standard voltage options table. downloaded from: http:///
mic2777 micrel, inc. mic2777 4 november 2005 timing diagram v th v in 0v v hyst v dd 0v v /rst (active low) v oh v ol v rst (active high) v oh v ol t rst t rst a a a v ref v hyst propagation delays not shown for clarity. note a. the mic2777 ignores very brief transients. see applications information for details. downloaded from: http:///
november 2005 5 mic2777 mic2777 micrel, inc. functional diagram gnd v dd in mic2777 v ref v ref rst r s q /q /rst delay line one shot functional description in, under-voltage detector input the voltage present at the in pin is compared to the internal 300mv reference voltage. a reset is triggered if and when v in falls below v ref . typically, a resistor divider is used to scale the input voltage to be monitored such that v in will fall below v ref as the voltage being monitored falls below the desired trip-point. hysteresis is employed to prevent chattering due to noise. the comparator on the in input is relatively immune to very brief negative-going transients. v dd input the v dd pin is both the power supply terminal and a monito red input voltage. the voltage at this pin is continually compared against the internal reference. the trip-point at which a reset occurs is factory programmed. a reset is triggered if and when v dd falls below the trip-point. hysteresis is employed to prevent chattering due to noise. the comparator on the v dd input is relatively immune to very brief negative-going transients. rst, /rst reset output typically, the mic2777 is used to monitor the power supplies of intelligent circuits such as microcontrollers and micropro- cessors. by connecting the appropriate reset output of a mic2777 to the reset input of a c or p, the processor will be properly reset at power-on and during power-down and brown-out conditions. the reset outputs are asserted any time v dd or v in drops below the corresponding threshold voltage. the reset outputs remain asserted for t rst (min) after v in and/or v dd subsequent return above the threshold boundaries and/or /mr is released. a reset pulse is also generated at power-on. manual reset the ability to initiate a reset via external logic or a manual switch is provided in addition to the mic2777s automatic supervisory functions. typically, a momentary push-button switch is connected such that in is shorted to ground when the switch contacts close. assuming v dd and v in are within tolerance when the switch is released, the reset outputs will be de-asserted no less than 140ms later. in can als o be driven by an open-drain or open-collector logic signal. downloaded from: http:///
mic2777 micrel, inc. mic2777 6 november 2005 application information programming the voltage threshold referring to the typical application circuit, the voltage threshold on the in pin is calculated as follows: v v r1 r2 r2 th ref = + ( ) where v ref = 0.300v in order to provide the additional criteria needed to solve for the resistor values, the resistors can be selected such that the two resistors have a given total value, that is, r1 + r2 = r total . imposing this condition on the resistor val- ues provides two equations that can be solved for the two unknown resistor values. a value such as 1m for r total is a reasonable choice since it keeps quiescent current to a generally acceptable level while not causing any measurable errors due to input bias currents. the larger the resistors, the larger the potential errors due to input bias current (i in ). the maximum recommended value of r total is 3m. applying this criteria and rearranging the v th expression to solve for the resistor values gives: r2 r v v total ref th = ( ) ( ) r1 = r total C r2 application example figure 1 illustrates a hypothetical mic2777-23 application in which the mic2777-23 is used to monitor the core and i/o supplies of a high-performance cpu or dsp. the core supply, v core , in this example is 1.0v 5%. the main power rail and i/o voltage, v i/o , is 2.5v 5%. as shown in figure 1, the mic2777 is powered by v i/o . the minimum value of v i/o is 2.5v C5% = 2.375v; the maximum is 2.5v +5% = 2.625v. this is well within the mic2777s power supply range of 1.5v to 5.5v. resistors r1 and r2 must be selected to correspond to the v core supply of 1.0v. the goal is to insure that the core supply voltage is adequate to insure proper operation, i.e., v core (1.0v C5%) = 0.950v. because there is always a small degree of uncertainty due to the accuracy of the resis- tors, variations in the devices voltage reference, etc., the threshold will be set slightly below this value. the potential variation in the mic2777s input voltage reference (v ref ) is speci?ed as 1.5%. the resistors chosen will have their own tolerance speci?cation. this example will assume the use of 1% accurate resistors. the potential worst-case error contri- bution due to input bias current can be calculated once the resistor values are chosen. if the guidelines above regarding the maximum total value of r1+r2 are followed, this error contribution will be very small thanks to the mic2777s very low input bias current. to summarize, the various potential error sources are: ? variation in v ref : speci?ed at 1.5% ? resistor tolerance: chosen by designer (typically 1%) ? input bias current, i in : calculated once resistor values are known, typically very small taking the various potential error sources into account, the threshold voltage will be set slightly below the mi nimum v core speci?cation of 0.950v so that when the actual threshold voltage is at its maximum, it will not intrude into the normal operating range of v core . the target threshold voltage will be set as follows: given that the total tolerance on v th for the in pin is [v ref tolerance] + [resistor tolerance] = 1.5% + 1% = 2.5%, and v th(max) = v core(min) , then v core(min) = v th + 2.5% v th = 1.025 v th , therefore, solving for v th results in v = v 1.025 = 0.950 1.025 = 0.9268v th core(min) solving for r1 and r2 using this value for v th and the equa- tions above yields: r1 = 676.3k 673k r2 = 323.7k 324k the resulting circuit is shown in figure 1.input bias current effects now that the resistor values are known, it is possible to cal- culate the maximum potential error due to input bias current, i in . as shown in the electrical characteristics table, the maximum value of i in is 10na. (note that the typical value is a much smaller 5pa) the magnitude of the offset caused by i in is given by: v error = i in(max) (r1||r2) = v error = 1 10 -8 a 2.189 10 5 ? = v error = 2.189 10 -3 v = v error = 2.189mv the typical error is about three orders of magnitude lower than thisC close to one microvolt! generally, the error due to input bias can be discounted. if it is to be taken into account, simply adjust the target threshold voltage downward by this amount and recalculate r1 and r2. the resulting value will be very close to optimum. if accuracy is more important than the quiescent current in the resistors, simply reduce the value of r total to minimize offset errors. downloaded from: http:///
november 2005 7 mic2777 mic2777 micrel, inc. in /rst rst vdd gnd mic2777-23 r1676k 1% r2324k 1% /reset vcore gnd v core 1.0v 5% microprocessor vi/o v i/o 2.5v 5% figure 1. mic2777 example design transient response the mic2777 is inherently immune to very short negative- going glitches. very brief transients may exceed the voltage thresholds without tripping the output. as shown in figure 2a and 2b, in general the narrower the transient, the deeper the threshold overdrive that will be ignored by the mic2777. the graph represents the typical allowable transient duration for a given amount of threshold overdrive that will not generate a reset. 0 5 10 15 20 25 30 35 40 0 100 200 300 reset comp. overdrive, v ref Cv in(mv) typical input transient response figure 2a. typical input transient response 0 20 40 60 80 100 0 500 1000 1500 2000 reset comp. overdrive, v ref Cv dd (mv) typical v dd transient response figure 2b. typical v dd transient response ensuring proper operation at low supply at levels of v dd below 1.2v, the mic2777s /rst output cannot turn on suf?ciently to produce a valid logic-low on /rst. in this situation, other circuits driven by /rst could be allowed to ?oat, causing undesired operation. (in most cases, however, it is expected that the circuits driven by the mic2777 will be similarly inoperative at v dd 1.2v.) if a given application requires that /rst be valid below v dd = 1.2v, this can be accomplished by adding a pull-down re- sistor to the /rst output. a value of 100k is recommended as this is usually an acceptable compromise of quiescent current and pull-down current. the resistors value is not critical, however. see figure 3. the statements above also apply to the mic2777s rst out- put. that is, to ensure valid rst signal levels at v dd <1.2v, a pull-up resistor (as opposed to a pull-down) should be added to the rst output. a value of 100k is typical for this application as well. see figure 4. in /rst rst vdd gnd mic2777-xx r1r2 /reset vcc gnd v cc microprocessor 100krpull-down figure 3. mic2777 valid /rst below 1.2v in rst /rst vdd gnd mic2777-xx r1r2 reset vcc gnd v cc microprocessor 100krpull-up figure 4. mic2777 valid rst below 1.2v downloaded from: http:///
mic2777 micrel, inc. mic2777 8 november 2005 micrel inc. 2180 fortune drive san jose, ca 95131 usa tel + 1 (408) 944-0800 fax + 1 (408) 474-1000 web http://www.micrel.com this information furnished by micrel in this data sheet is believed to be accurate and reliable. however no responsibility is assumed by micrel for its use. micrel reserves the right to change circuitry and speci?cations at any time without noti?cation to t he customer. micrel products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product can reasonably be expected to result in personal injury. life support devices or systems are devices or systems that (a) are intended for surgical implant into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a signi?cant injury to the user. a purchaser's use or sale of micrel products for use in life support appliances, devices or systems is a purchaser's own risk and purchaser agrees to fully indemnify micrel for any damages resulting from such use or sale. ? 2000 micrel, inc. package information sot-23-5 (m5) downloaded from: http:///
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