november 2005 1 mic2776 mic2776 micrel, inc. mic2776 micro-power low voltage supervisor general description the mic2776 is a power supply supervisor which provides under-voltage monitoring and power-on reset generation in a compact 5-pin sot package. features include an adjustable under-voltage detector, a delay-generator, a manual reset input, and a choice of active-high, active-low, or open-drain active-low reset output. the user-adjustable monitoring input is compared against a 300mv reference. this low reference voltage allows monitoring voltages lower than those supported by previous supervisor ics. the reset output is asserted for no less than 140ms at power- on and any time the input voltage drops below the reference voltage. it remains asserted for the timeout period after the input voltage subsequently rises back above the threshold boundary. a reset can be generated at any time by asserting the manual reset input, /mr. the reset output will remain ac- tive at least 140ms after the release of /mr. the /mr input can also be used to daisy-chain the mic2776 onto existing power monitoring circuitry or other supervisors. hysteresis is included to prevent chattering due to noise. typical supply current is a low 3.0a. typical application in /rst vdd /mr gnd mic2776l r1r2 /reset vcore gnd power_good v core 1.0v manualreset microprocessor vi/o v i/o 2.5v features ? user-adjustable input can monitor supplies as low as 0.3v ? 1.5% threshold accuracy ? separate v dd input ? generates power-on reset pulse (140ms min.) ? manual reset input ? choice of active-high, active-low or open-drain active- low reset output ? inputs can be pulled above v dd (7v abs. max.) ? open-drain output can be pulled above v dd (7v abs. max.) ? ultra-low supply current, 3.0a typical ? rejects brief input transients ? ittybitty? sot-23-5 package applications ? monitoring processor, asic, or fpga core voltage ? 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. downloaded from: http:///
mic2776 micrel, inc mic2776 2 november 2005 pin con?guration rst vdd in /mr 1 3 4 5 2 gnd sot-23-5 (m5) h version /rst vdd in /mr 1 3 4 5 2 gnd sot-23-5 (m5) l and n version pin description pin number pin number pin name pin function mic2776h mic2776l mic2776n 1 rst digital (output): asserted high whenever v in falls below the reference volt- age. it will remain asserted for no less than 140ms after v in returns above the threshold limit. 1 /rst digital (output): asserted low whenever v in falls below the reference volt- age. it will remain asserted for no less than 140ms after v in returns above the threshold limit. (open-drain for n version) 2 2 gnd ground 3 3 /mr digital (input): driving this pin low initiates an immediate and unconditional reset. assuming in is above the threshold when /mr is released (returns high), the reset output will be de-asserted no less than 140ms later. /mr may be driven by a logic signal or a mechanical switch. /mr has an internal pull-up to v dd and may be left open if unused. 4 4 in analog (input): the voltage on this pin is compared to the internal 300mv reference. an under-voltage condition will trigger a reset sequence. 5 5 vdd analog (input): independent supply input for internal circuitry. ordering information part number reset output temperature range package standard marking pb-free marking mic2776n-bm5 ukaa mic2776n-ym5 ukaa open-drain, active-low / rst C40oc to +85oc sot-23-5 mic2776h-bm5 ulaa mic2776h-ym5 ulaa active-high, complementary rst C40oc to +85oc sot-23-5 mic2776l-bm5 umaa mic2776l-ym5 umaa active-low, complementary /rst C40oc to +85oc sot-23-5 downloaded from: http:///
november 2005 3 mic2776 mic2776 micrel, inc. absolute maximum ratings (note 1) supply voltage (v dd ) .......................................C0.3v to +7v input voltages (v in , v /mr ) ...............................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 voltages (v in , v /mr ) ............................C0.3v to +6.0v output voltages v /rst (n version) ......................................C0.3v to +6.0v v /rst , v rst (h and l versions) ........ C0.3v to v dd + 0.3v ambient temperature range (t a ) .............. C40c to +85c package thermal resistance ( ja ) ....................... 256c/w electrical characteristicsv dd = 3.3v; 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 = 3.3v; /mr, rst, /rst open 3.0 a in, under-voltage detector input v ref under-voltage threshold t a = 25c 295 300 305 mv v hyst hysteresis voltage 3 mv i in input current 5 pa t min t a t max 10 na reset outputs (/rst, rst) t prop propagation delay v in = (v ref(max) + 100mv) to 20 s v in = (v ref(min) C 100mv) 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 (h and l version only) i source = 10a; 0.8v dd v v dd 1.2v, note 4 manual reset inputs (/mr) v ih input high voltage 1.5v v dd 5.5v 0.7v dd v v il input low voltage 1.5v v dd 5.5v 0.3v dd v t prop propagation delay v /mr < v il 5 s t min minimum input pulse width reset occurs, v /mr < v il 33 ns i pu internal pull-up current 100 na i in input current, /mr v /mr < v il 100 na 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. downloaded from: http:///
mic2776 micrel, inc mic2776 4 november 2005 timing diagram v oh v ol v /mr v in v dd 0v 0v v oh v ol t rst t rst v /rst t rst t rst v oh v ol v rst >t min a a v hyst v ref propagation delays not shown for clarity. note a. the mic2776 ignores very brief transients. see applications information for details. downloaded from: http:///
november 2005 5 mic2776 mic2776 micrel, inc. 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 chat- tering due to noise. rst, /rst reset output typically, the mic2776 is used to monitor the power supply of intelligent circuits such as microcontrollers and micropro- cessors. by connecting the reset output of a mic2776 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 condi- tions. in addition, asserting /mr, the manual reset input, will activate the reset function. functional diagram rs q /q /mr in mic2776 * pinout and polarity vary by device type.see ordering information table. v dd /rst*rst* v ref gnd delay one shot i pu the reset outputs are asserted any time /mr is asserted or if v in drops below the threshold voltage. the reset outputs remain asserted for t rst (min) after v in subsequently returns above the threshold boundary and /mr is released. a reset pulse is also generated at power-on. /mr, manual reset input the ability to initiate a reset via external logic or a manual switch is provided in addition to the mic2776s automatic supervisory functions. driving the /mr input to a logic low causes an immediate and unconditional reset to occur. as- suming v in is within tolerance when /mr is released (returns high), the reset output will be de-asserted no less than t rst later. /mr may be driven by a logic signal, or mechanical switch. typically, a momentary push-button switch is con- nected such that /mr is shorted to ground when the switch contacts close. the switch may be connected directl y between /mr and gnd. /mr has an internal 100na pull-up current to v dd and may be left open if unused. downloaded from: http:///
mic2776 micrel, inc mic2776 6 november 2005 application information programming the voltage threshold referring to the typical application circuit, the voltage threshold 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 below illustrates a hypothetical mic2776 application in which the mic2776 is used to monitor the core supply 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 volt- age, v i/o , is 2.5v 5%. as shown in figure 1, the mic2776 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 mic2776s 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 cor e 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 resistors, variations in the devices voltage reference, etc., the threshold will be set slightly below this value. the potential variation in the mic2776s voltage reference 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 contribution 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 mic2776s 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 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 this - 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 mic2776 mic2776 micrel, inc. in /rst vdd /mr gnd mic2776 r1676k 1% r2324k 1% /reset vcore gnd v core 1.0v 5% manualreset microprocessor vi/o v i/o 2.5v 5% figure 1. mic2776 example design interfacing to processors with bidirectional reset pins some microprocessors have reset signal pins that are bi- directional, rather than input only. the motorola 68hc11 family is one example. because the mic2776ns output is open-drain, it can be connected directly to the processors reset pin using only the pull-up resistor normally required. see figure 2. in /rst vdd /mr gnd mic2776n r1r2 /reset vcc gnd v cc microprocessor 100k figure 2. interfacing to bidirectional reset pin transient response the mic2776 is inherently immune to very short negative- going glitches. very brief transients may exceed the voltage threshold without tripping the output. as shown in figure 3, the narrower the transient, t he deeper the threshold overdrive that will be ignored by the mic2776. the graph represents the typical allowable transient duration for a given amount of threshold overdrive that will not g enerate 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 3. typical input transient response ensuring proper operation at low supply at levels of v dd below 1.2v, the mic2776ls /rst output driver cannot turn on suf?ciently to produce a valid logic-low on the /rst output. in this situation, other circuits driven by /rst could be allowed to ?oat, causing undesired opera- tion. (in most cases, however, it is expected that the circuits driven by the mic2776l 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 resis- tor to the /rst output. a value of 100k is recommended as this is usually an acceptable compromise of leakage current and pull-down current. the resistors value is not critical, however. see figure 4. the statements above also apply to the mic2776hs rst output. 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 5. in /rst vdd /mr gnd mic2776l r1r2 /reset vcc gnd v cc manualreset microprocessor 100krpull-down figure 4. mic2776l valid /reset below 1.2v in rst vdd /mr gnd mic2776h r1r2 reset vcc gnd v cc manualreset microprocessor 100krpull-up figure 5. mic2776h valid reset below 1.2v downloaded from: http:///
mic2776 micrel, inc mic2776 8 november 2005 package information sot-23-5 (m5) 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. downloaded from: http:///
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