2-2.5a, 23v, 450 khz synchronous rectified step-down converter description the mxt2482sx is a mon o lithic synchronous bu ck regulator. the device integrates 150m ? mosfets that provide 2a continuous load curr ent over a wide operati ng input voltage of 4.5 to 23v. current mode cont rol provides fast transient re sponse and cycle-by-cycle current limit. an adjustable soft-start p r eve n ts inrush curr ent at turn-on. in s hutdown mo de, the suppl y current drops below 1a. this device, availabl e in an 8-pin soic pack age, provid es a very compact system solutio n with minimal reliance on external components. package reference features . ? 2-2.5a output current . ? wide 4.5-23v operating input range . ? integrated 150m ? power mosfet switches . ? output adjustable from from 0.925 to 20v . ? up to 95% efficiency . ? programmable soft-start . ? stable with low esr ceramic output capacitors . ? fixed 450khz frequency . ? cycle-by-cycle over current protection . ? input under voltage lockout . ? thermally enhanced 8-pin soic/soic-e package applications . ? distributed power systems . ? networking systems . ? fpga, dsp, asic power supplies . ? green electronics/ appliances . ? notebook computers 1 / 1 mxt2482sx/a free datasheet http:///
absolute maximum ratings (1 ) supply voltage (v in ).......................-0.3v to 26v switch voltage (v sw )............. ?1v to v in + 0.3v bootstrap voltage (v bs ) ...vsw-0.3v to v sw + 6v enable/uvlo voltage (v en )...........?0.3v to +6v comp voltage (v com p ) ...................?0.3v to +6v feedback voltage (v fb ) ................?0.3v to +6v junction temperat ure ........................... + 150c lead temperatur e ................................. + 260c storage temperature .............. ?55c to +150c recommended operating conditions (2 ) input voltage (v in ) ......................... 4.5v to 23v output voltage (v sw ) ....................... 0.925 to 20v operating temperatur e...............?20c to +85c thermal resistance (3 ) ja jc soic8n (4) .............................. 50...... 10... c/w n o tes: 1) e x ceeding these ratings may damage the dev ice. 2) t he dev ice is not guaranteed to function outside of its operating conditions. 3) measured on approx imately 1? square of 1 oz copper. 4) measured only w i th solder ex posed pad to board 2 / 2 mxt2482sx mxt2482sx mxt2482sx mxt2482sx mxt2482sx mxt2482sx mxt2482sx mxt2482sx MXT2482SXA 2-2.5a, 23v, 450 khz synchronous rectified step-down converter mxt2482sx/a free datasheet http:///
electrical characteristics v in = 12v, t a = +25 c, unless otherw ise noted. paramet e r condition min ty p max units shutdow n supply current v en 0.3v 0.3 3.0 a supply current v en 2.6v, v fb = 1.0v 1.3 1.5 ma feedback voltage 4.5v v in 23v 900 925 950 mv feedback overvoltage threshold 1.1 v error amplifier voltage 480 v/v error amplifier transconductance ? i c = ? 10a 800 a/v high-side sw itch-on resistance 150 (4 ) m ? low -side sw itch-on resistance 150 (4 ) m ? high-side sw itch leakage v en = 0v, v sw = 0v 0.1 10 a upper sw itch current limit 2.4 3.7 a low er sw itch current limit 0.9 a comp to current sense transconductance 4.0 /v oscillator frequency 400 450 500 khz short circuit frequency v fb = 0v 160 khz max i mum duty cycle v fb = 0.8v 90 % minimum on time 120 ns en shutdow n threshold voltage v en rising 1.1 1.5 2.0 v en shutdow n threshold voltage hysterisis 140 (4 ) mv en lockout threshold voltage 2.2 2.5 2.7 v en lockout hysterisis 140 (4 ) mv input uvlo threshold rising v in rising 3.80 4.10 4.40 v input uvlo threshold hysteresis 210 mv soft-start current v ss = 0v 6.0 a soft-start period c ss = 0.1f 15 ms thermal shutdow n 4 160 c note: 4) guaranteed by design 3 / 3 2-2.5a, 23v, 450 khz synchronous rectified step-down converter mxt2482sx/a free datasheet http:///
typical application pin functions pin # name description 1 bs high-side gate drive boost input. bs supplies the drive for the high-side n-channel mosfet sw itch. connect a 0.01f or greater capacitor from sw to bs to pow er the high side sw itch. 2 in pow e r input. in supplies the pow er to the ic, as w e ll as the step-dow n converter sw itches. drive in w i th a 4.5v to 23v pow er source. bypass in to gnd w i th a suitably large capacitor to eliminate noise on the input to the ic. see input capacitor . 3 sw pow e r sw itching output. sw is the sw itching node that supplies pow er to the output. connect the output lc filter from sw to the output load. note that a capacitor is required from sw to bs to pow er the high-side sw itch. 4 gnd ground (connect ex posed pad to pin 4). 5 fb feedback input. fb senses the output voltage to regulate that voltage. drive fb w i th a resistive voltage divider from the output voltage. the feedback threshold is 0.925v. see setting the output voltage . 6 comp compensation node. comp is used to compensate the regulation control loop. connect a series rc netw o rk from comp to gnd to compensate the regulation control loop. in some cases, an additional capacitor from comp to gnd is required. see com p ensation com ponents. 7 en enable input. en is a digital input that turns the regulator on or off. drive en high to turn on the regulator, drive it low to turn it off. pull up w i th 100k ? resistor for automatic startup. 8 ss soft-start control input. ss controls the soft-start period. connect a capacitor from ss to gnd to set the soft-start period. a 0.1f capacitor sets the soft-start period to 15ms. to disable the soft-start feature, leave ss unconnected. 4 / 4 mxt2482sx 2-2.5a, 23v, 450 khz synchronous rectified step-down converter mxt2482sx/a free datasheet http:///
operation functional description the mxt2482sx is a synchronous re ctified, current-mode, step- down regulator. it regulates input voltages from 4.5v to 23v down to an output voltage as low as 0.925v, and supplies up to 2a of load current. the mxt2482sx u s es curr ent-mode control to regul ate the output vol t age. the output voltage is measured at fb through a resistive voltage divider and amplified through the internal transconductance error am plifier. the voltage at the comp pin is compared to the switch current measured in ternally to control the output voltage. the converter uses internal n-channel mosfet switches to step-down the input voltage to the regulated output voltage. si nce the high side mosfet r equires a gate voltage greater than the input voltage, a boos t capacitor connect ed between sw and bs is needed to drive the high side gate. the boos t capacitor is charged from the in ternal 5v rail when sw is low. when the mxt2482sx fb pin exceeds 20% of the nominal regul ation voltage of 0.925v, the over voltage comparator is tripped and the co mp p i n and the ss pin are discharged to g nd, forcing the high-side switch off. figure 1 ? functional block diagram 5 / 5 2-2.5a, 23v, 450 khz synchronous rectified step-down converter mxt2482sx/a free datasheet http:///
application information component selection setting the output voltage the output voltage is set using a resistive voltage divider from the output voltage to fb (see typical application circuit on page 1). the voltage divider divides the output voltage down by the rat io: where v fb is the feedback voltage and v out is the output voltage. thus the output voltage is: r2 can be as high as 100k ? , but a typical value is 10k ? . using the typical value for r2, r1 is determined by: for example, for a 3.3v output voltage, r2 is 10k ? , and r1 is 26.1k ? . table 1 lists recommended resistance values of r1 and r2 for standard output voltages. calculated by: where v out is the output voltage, v in is the input voltage, f s is the switching frequency, and ? i l is the peak-to-peak inductor ri pple current. choose an inductor that will not saturate under the maximum inductor peak current. the peak inductor current can be calculated by: where i load is the load current. the choice of whic h style inductor to use mainly depends on the price vs. size requirement s and any emi requirements. optional schottky diode during the transition between high-side switch and low-side switch, the body diode of the lowside power mosfet conducts the inductor current. the forward voltage of this body diode is high. an optional schottky diode ma y be paralleled between the sw pin and gnd pin to improve overall efficiency. tabl e 2 lists example sc hottky diodes and their manufacturers. table 2 ? diode selection guide 6 / 6 2-2.5a, 23v, 450 khz synchronous rectified step-down converter mxt2482sx/a free datasheet http:///
inductor the inductor is required to supply con s tant cu rrent to the output load while being dri v en by the switched input voltage. a larger value inductor will result in less ripple current that will result in lower output ripple vo ltage. however, the larger va lue inductor will have a lar ger physical size, higher series resistance, and/ or lower saturation current. a good rule for determining the induct ance to use is to allow the peak-to- pe ak ripple current in the inductor to be app ro ximately 30% of the maximum switch cu rrent limit. also, make sur e that the pe ak inductor current is below the maximum switch current limit. the i nductance value can be part number voltage/current rating vendor b130 30v, 1a diodes, inc. sk13 30v, 1a diodes, inc. mbrs130 30v, 1a international rectifier input capacitor the input current to the step-down converter is discontinuous, theref ore a capacitor is requir ed to supply the ac current to the step- down converter w h ile maintaining the dc in put voltage. use low esr capacitors for th e best per for m ance. cera mic capac itors are prefe r r ed, but tantalum or low-esr elec trolytic capacitors may also suffice. choose x5r or x7r dielectrics w hen using c e ramic capacitors. si nce the input capacitor abs orbs the inp u t switching current it requir e s an adeq u a te ripple cu rrent rating. the rms current in the i nput capacitor can be estimated by: the worst-case condition occurs at v in = 2v out , where i cin = i load /2. for simplification, choose the input capacit or wh ose r m s current rating gre a ter than half of the maximu m load current. the input capacitor can be electrolytic, ta ntalum or ce ramic. when using electr olytic or tantalum capacitors, a sma ll, high quality cera mic capacito r , i.e. 0.1f, should be p l aced as close to the ic as po ssible. when using ce ra mic capacitors, make sure that they have enoug h capa citance to provide su fficient charge to prevent exce ssive voltage ripple at in p u t. the input voltage ripple for low esr capacitors can be estimated by: where c in is the input c apacitance value. 7 / 7 2-2.5a, 23v, 450 khz synchronous rectified step-down converter mxt2482sx/a free datasheet http:///
output capacitor the output capacitor is re quired to maintain the dc output vo ltage. ceramic, tantalum, or low esr electrolytic capacitor s ar e recommended. lo w esr capacitors are pre f erred to kee p the output voltage ripple low. the output voltageripple can be estimated by: where c o is the output capac itance value and r esr is the equivalent series resistance (esr) value of the output capacitor. in the case of ceramic c apacitors, the impedance at the switching frequency is dominated by the capacitance. the output voltage ripple is mainly caused by the capac itance. for simplificati on, the output voltage rippl e can be estimated by: in the case of tantalum or electrolyti c c apacitors, the esr do minat es the impedance a t the switching frequency. for simplification, the output ripple can be approximated to: the characteristics of th e output capacitor also affect the sta b ility of the regulation system. themxt2482sx can be optimiz ed for a wide range of c apacitance and esr values. compensation components mxt2482sx employs current mode c ontrol for easy compensation and fast transient response. the system stability and transient response are controlled thr ough the comp pin. comp pin is the output of the internal transconductance error amplifier. a series capacitor-resistor combination sets a pole-zero combination to cont rol the characteristics of the control system. the dc gain of the voltage f eedback loop is given by: where v fb is the feedback voltage, 0.925v; a vea is the error amplifier voltage gain; g cs is the current sense tr ansconductance and r load is the load resistor value. the system has two poles of importance. one is due to the compensati on capacitor (c3) and the output resistor of the error amplifier, and t he other is due to the output c apacitor and the load resistor. these poles are located at: where g ea is the error amplifie r transconductance. the system has one zero of im portance, due to the compensat ion capacitor (c3) and the 8 / 8 2-2.5a, 23v, 450 khz synchronous rectified step-down converter mxt2482sx/a free datasheet http:///
compensation resistor (r3). th is zero is located at: the system may ha ve another ze r o of importa nce, if the output c apa citor has a large capacitance and/or a hig h esr valu e. the zero, due to t he esr and capacitance of the output capacitor, is located at: in this case ( a s shown in figure 2), a third pole set by the compensation capacitor (c6) and the compensation resistor (r3) is used to com pensate the effect of the esr zero on the loop gain. this pole is located at: the goal of compensation design is to shape the converter tra n sf er function to get a de sired loop gain. t he system crossover fre quency w her e the feedb a ck loop has the unity gai n is important. lower crossover freq uenc ies result in slower line a nd load tra n s ient responses, while high er crossover fre quencies could cause sy stem instability. a good ru le of thumb is to set the crossover frequency below one-t enth of the switching frequency. to optimize the compensat ion components, the followi ng procedure can be used. 1. choose the compensation resistor (r3) to set the desired crossover frequency. determine the r3 value by the following equation: where f c is the desired crossover frequency which is typically below one tenth of the switching frequency. 2. 2. choose the compensati on capacitor (c3) to achi eve the desired phase margin. for applications with typical inductor values, setting the compensation zero, f z1 , below one-forth of the crossover frequency provides sufficient phase margin. determine the c3 value by the following equation: r3 is the compensation resistor. 3. determine if the second com pensation capacitor (c6) is requi red. it is required if the esr zero of the output capacitor is located at less than half of the switching frequency, or the following relationship is valid: if this is the case, then a dd the se cond co mpensation capacitor (c 6) to set the pole f p3 at the location of the esr zero. determ ine the c6 value by the equation: 9 / 9 2-2.5a, 23v, 450 khz synchronous rectified step-down converter mxt2482sx/a free datasheet http:///
external bootstrap diode an external bootstrap diode may en hance the e fficiency of the regul ator, the applica b le conditions of extern al bst diode a r e v out =5v or 3.3v. this diode is also recommende d for high duty cycle operation (when output voltage (v out >12v) applications. figure 2?external bootstrap diode the recommended external bst diode is in4148, and the bst cap is 0.1~1uf typical application circuits figure 3 ? mxt2482sx w i th 3.3v output, 22f/6.3v ceramic output capacitor 1 0 / 1 0 mxt2482sx mxt2482sx 2-2.5a, 23v, 450 khz synchronous rectified step-down converter mxt2482sx/a free datasheet http:///
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