general description the SP1410 is a monolithic step down switch mode regulator with a built in internal power mosfet. it achieves 2a continuous output current over a wide input supply range with excellent load and line regulation. current mode operation provides fast transient response and ea ses loop stabilization. fault condition protection includes cycle-by-cycle current limiting and thermal shutdown. in shutdown mode the regulator draws 25 a of supply current. the SP1410 requires a minimum number of readily available standard external components. features ? 2a output current. ? 0.22 ? internal power mosfet switch, ? stable with low esr output ceramic capacitors ? up to 95% efficiency. ? 25 a shutdown mode ? thermal shutdown ? cycle-by-cycle over current protection ? wide 4.75 to 25v operating input range ? output adjustable from 1.22 to 21v ? programmable under voltage lockout ? standard sop8 packages applications ? pc monitors. ? distributed power systems ? battery charger ? pre-regulator for linear regulators functional diagram 1 of 9 SP1410 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
ordering information part number package operating ambient temperature SP1410es sop8 -20 to +85 c pin description symbol pin no. description bs 1 high-side gate drive boost input. bs supplies the drive for the high-side n-channel mosfet switch.connect a 10nf or greater capacitor from sw to bs to power the high-side switch. in 2 power input. in supplies the power to the ic, as well as the step-down converter switches. drive in with a 4.75v to 25v power source. bypass in to gnd with a suitably large capacitor to eliminate noise on the input to the ic. see input capacitor. sw 3 power switching output. sw is the switching node that supplies power 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 power the high-side switch. gnd 4 ground. fb 5 feedback input. fb senses the output voltage to regulate that voltage. drive fb with a resistive voltage divider from the output voltage. the feedback threshold is 1.22v. see setting the output voltage. comp 6 compensation node. comp is used to compensate the regulation control loop. connect a series rc network from comp to gnd to compensate the regulation control loop. see compensation. en 7 enable input. en is a digital input that turn s the regulator on or off. drive en high to turn on the regulator, drive it low to turn it off. for automatic startup, leave en unconnected. n/c 8 no connect 2 of 9 SP1410 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
figure 1: typical application circuit absolute maximum ratings parameter value unit in supply voltage -0.3v to 25 v sw voltage -1 to v in +1 v bs voltage v sw -0.3 tov sw +6 v all other pins -0.3 to 6 v junction to ambient thermal resistance ( ja) 105 c/w operating ambient temperature -20 to 85 c operating junction temperature -40 to 125 c storage temperature -65 to 150 c lead temperature (soldering, 10 sec) 260 c 3 of 9 SP1410 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
electrical characteristics (unless otherwise specified refer to circ uit of figure 1, ven=5v, vin=12v, ta=25 c ) parameters condition min. typ. max. units feedback voltage 4.75v v in 25v 1.184 1.222 1.258 v upper switch on resistance 0.22 ? lower switch on resistance 10 ? upper switch leakage v en =0v; v sw =0v 10 a current limit 2.60 a oscillator frequency 370 420 470 khz short circuit frequency v fb = 0v 41 khz maximum duty cycle v fb = 1.1v 90 % minimum duty cycle v fb = 1.4v 0 % enable threshold 0.7 1.0 1.3 v under voltage lockout threshold high going 2.0 2.5 3.0 v under voltage lockout threshold hysteresis 200 mv shutdown supply current v en =0v 25 50 a operating supply current v en =3v; v fb =1.4v 2.2 ma thermal shutdown 160 c note 1 . exceeding these ratings may damage the device. note 2 . the device is not guaranteed to f unction outside its operating rating. note 3 . measured on 1? square of 1 oz. copper fr4 board. 4 of 9 SP1410 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
functional description the SP1410 is a current-mode step-down switch-mode regulator. it regulates input voltages from 4.75v to 25v down to an output voltage as low as 1.22v, and is able to supply up to 2a of load current. the SP1410 uses current-mode control to regulate the output voltage. the output voltage is measured at fb through a resistive voltage divider and amplified through the internal error amplifier. the output current of the transconductance error amp lifier is presented at comp where a network compensates the regulation control system. the voltage at comp is compared to the switch current measured internally to control the output voltage. the converter uses an internal n-channel mosfet switch to step down the input voltage to the regulated output volt age. since the mosfet requires a gate voltage greater than the input voltage, a boost capaci tor connected between sw and bs drives the gate. the capacitor is internally charged while the switch is off. an internal 10 ? switch from sw to gnd is used to insure that sw is pulled to gnd when the switch is off to fully charge the bs capacitor. application information the output voltage is set using a resistive voltage divider from the output voltage to fb(see figure 3). the voltage divider divides the output voltage down by the ratio: vfb = vout * r3 / (r2 + r3) thus the output voltage is: vout = 1.222 * (r2 + r3) / r3 r3 can be as high as 100k , but a typical value is 10k ? . using that value, r2 is determined by: r2 ~= 8.18 * (vout ? 1.222) (k ? ) for example, for a 3.3v output voltage, r3 is 10k ? , and r2 is 17k ? . inductor the inductor is require d to supply constant current to the output load while being driven by the switched input voltage. a larger value inductor results in less ripple current that in turn results in lower output ripple voltage. however, the larger value inductor has a larger physical size, higher series resistance, and/or lower saturation current. choose an inductor that does not saturate under the worst-case load conditions. a good rule for determining the induct ance is to allow the peak-to-peak ripple current in the inductor to be approximately 30% of the maximum load current. also, make sure that the peak inductor current (the load current plus half the peak-to-peak inductor ripple current ) is below the 2.4a minimum current limit. the inductance value can be calcul ated by the equation: l = (vout) * (vin-vout) / vin * f * i where vout is the output voltage, vin is the input voltage, f is the switching frequency, and i is the peak-to-peak induc tor ripple current. table 2 lists a number of suitable inductors from various manufacturers. table 2: inductor selection guide 5 of 9 SP1410 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
input capacitor the input current to the step-down converter is discontinuous, and therefore an input capacitor c1 is required to supply the ac current to the step-down converter while maintaining the dc input voltage. a low esr capacitor is required to keep the noise at the ic to a minimum. ceramic capacitors are preferred, but tantalum or low-esr electrolytic capacitors may also suffice. the input capacitor value should be greater than 10 f. the capacitor can be electrolytic, tantalum or ceramic. however since it absorbs the input switching current it requires an adequate ripple current rating. its rms current rating should be greater than approximately 1/2 of the dc load current. for insuring stable operation c2 should be placed as close to the ic as possible. alternately a smaller high quality ceramic 0.1 f capacitor may be placed closer to the ic and a larger capacitor placed further away. if using this technique, it is recommended that the larger capacitor be a tantalum or electrolytic type. all ceramic capacitors should be placed close to the SP1410. output capacitor the output capacitor is required to maintain the dc output voltage. low esr capacitors are preferred to keep the out put voltage ripple low. the characteristics of the output capacitor also affect the stability of the regulation control system. ceramic, tantalum, or low esr electrolytic capacitors are recommended. in the case of ceramic capacitors, the impedance at the switching frequency is dominated by the capacitance, and so the output voltage ripple is mostly independent of the esr. the output voltage ripple is estimated to be: vripple ~= 1.4 * vin * (flc/fsw)^2 where vripple is the output ripple voltage, vin is the input voltage, flc is the resonant frequency of the lc filter , fsw is the switching frequency. in the case of tanatalum or low-esr electrolytic capacitors , the esr dominates the impedance at the switching frequency, and so the output ripple is calculated as: vripple ~= i * resr where vripple is the output voltage ripple, i is the inductor ripple current, and resr is the equivalent series resi stance of the output capacitors. output rectifier diode the output rectifier diode supplies the current to the inductor when the high- side switch is off. to reduce losses due to the diode forward voltage and recovery times, use a schottky rectifier. tables 3 provides the schottky rectifier part numbers based on the maximum input voltage and current rating. table 4 lists some rectifier manufacturers. table 3: schottky re ctifier selection guide vendor/model core type core material package dimensions (mm) w l h sumida cr25 open ferrite 7.0 7.8 5.5 cdh74 open ferrite 7.3 8.0 5.2 cdrh5d28 shielded ferrite 5.5 5.7 5.5 cdrh5d28 shielded ferrite 5.5 5.7 5.5 cdrh6d28 shielded ferrite 6.7 6.7 3.0 cdrh104r shielded ferrite 10.1 10.0 3.0 toko d53lc type a shielded ferrite 5.0 5.0 3.0 d75c shielded ferrite 7.6 7.6 5.1 d104c shielded ferrite 10.0 10.0 4.3 d10fl open ferrite 9.7 11.5 4.0 coilcraft do3308 open ferrite 9.4 13.0 3.0 do3316 open ferrite 9.4 13.0 5.1 6 of 9 SP1410 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
v in (max) 2a load current part numbe r v endo r 15v 30bq15 4 25v b230 1 sk23 6 sr23 6 table 4: schottky diode manufacturers # vendor web site 1 diodes, inc. www.diodes.com 2 fairchild semiconductor www.fairchildsemi.com 3 general semiconductor www.gensemi.com 4 nternational rectifier www.irf.com 5 on semiconductor www.onsemi.com 6 pan jit international www.panjit.com.tw choose a rectifier who?s maximum reverse voltage rating is greater than the maximum input voltage, and who?s current rating is greater than the maximum load current. compensation the system stability is controlled through the comp pin. comp is the output of the internal transconductance error amplifier. a series capacitor-resistor combination sets a pole-zero combination to control t he characteristics of the control system. the dc loop gain is: avdc = (vfb / vout) * avea * gcs * rload where: vfb is the feedback threshold voltage, 1.222v vout is the desired output regulation voltage avea is the transconduc tance error amplifier voltage gain, 400 v/v gcs is the current sense gain, (roughly the output current divided by the voltage at comp), 1.95 a/v rload is the load resistance (vout / iout where iout is the output load current) the system has 2 poles of importance, one is due to the compensation capacitor (c5), and the other is due to the output capacitor (c7). these are: fp1 = gmea / (2 *avea*c5) where p1 is the first pole, and gmea is the error amplifier transconductance (770 s). and fp2 = 1 / (2 *rload *c7) the system has one zero of importance, due to the compensation capacitor (c5) and the compensation resistor (r1). the zero is: fz1 = 1 / (2 *r1*c5) if a large value capacitor (c7) with relatively high equivalent?series- resistance (esr) is used, the zero due to the capacitance and esr of the output capacitor c an be compensated by a third pole set by r1 and c4. the pole is: fp3 = 1 / (2 *r1*c4) the system crossover frequency (the frequency where the loop gain drops to 1, or 0db) is important. a good rule of thumb is to set the crossover frequency to approximately 1/10 of the switching frequency. in this case, the switching frequency is 420khz, so use a crossover frequency(fc) of 40khz. lower crossover frequencies result in slower response and worse transient load recovery. higher crossover frequencies can result in instability. table 5: compensation values for typical output voltage/capacitor combinations vout c7 r1 c5 c4 2.5v 22 f ceramic 7.5k ? .2.2nf none 3.3v 22 f ceramic 10k ? 2nf none 5v 22 f ceramic 15k ? 1.2nf none 12v 22 f ceramic 33k ? 1nf none 2.5v 560 f/6.3v (30m ? esr) 150k ? 1nf 100pf 3.3v 560 f/6.3v (30m ? esr) 200k ? 1nf 82pf 5v 470 f/10v (30m ? esr) 250k ? 1nf 56pf 12v 220 f/25v (30m ? esr) 250k ? 1nf 27pf choosing the compensation components 7 of 9 SP1410 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
the values of the compensation components given in table 5 yield a stable control loop for the output voltage and capacitor given. to optimize the compensation components for conditions not listed in t able 5, use the following procedure: choose the compensation resistor to set the desired crossover frequency. determine the value by the following equation: r1 = 2 *c7*vout*fc / (gea*gcs*vfb) putting in the know constants and setting the crossover frequency to the desired 40khz: r1 1.37*108 c7*vout the value of r1 is limited to 10k ? to prevent output overshoot at star tup, so if the value calculated for r1 is greater than 10k ? , use 10k ? . in this case, the actual crossover frequency is less than the desired 40khz, and is calculated by: fc = r1*gea*gcs*vfb / (2 *c7*vout) or fc 2.92 / (c7 * vout) choose the compensation capacitor to set the zero to ? of the crossover frequency. determine the value by the following equation: c5 = 2 / *r1*fc 1.59*10-5 / r1 if r1 is less than 10k ? , or if r1 = 10k ? use the following equation: c5 = 4c7*vout / (r12*gea*gcs*vfb) c5 2.2*10-5* c7 * vout determine if the second compensation capacitor, c4 is required. it is required if the esr zero of the output capacitor happens at less than four times the crossover frequency. or: 8 *c7*resr*fc 1 where resr is the equivalent series resistance of the output capacitor. if this is the case, then add the second compensation resistor. determine the value by the equation: c4 = c7*resr(max) / r1 where resr(max) is the maximum esr of the output capacitor. example: vout =3.3v c7 = 22 f ceramic (esr = 10m ? ) r1 (1.37 x 108) (22 x 10-6)(3.3v) = 9.9k ? use the nearest standard value of 10k ? . c5 >(0.22x22x10-6x3.3/(10x103) = 1.6nf. use the nearest standard value of 1.5nf. 2 x c2x resrx fc = 0.014 which is less than 1, therefore no second compensation capacitor is required. figure 3. SP1410 step down from 25v to 3.3v @ 2a 8 of 9 SP1410 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
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