description the SP1484 is a monolit hic synchronous buck regulator. the device integrates top and bottom 85m ? mosfets that provide 3a of continuous load current over a wide operating input voltage of 4.75v to 23v. current mode control provides fast transient response and cycle-by-cycle current limit. an adjustable soft-start prevents inrush current at turn-on and in shutdown mode, the supply current drops below 1a. the SP1484 is pin compatible to the mp1482 2a/23v/synchronous step-down converter. features ? 3a continuous output current ? wide 4.75v to 23v operating input range ? integrated 85m ? power mosfet switches ? output adjustable from 0.925v to 20v ? up to 95% efficiency ? programmable soft-start ? stable with low esr ceramic output capacitors ? fixed 340khz frequency ? cycle-by-cycle over current protection ? input under voltage lockout ? thermally enhanced 8-pin soic package applications ? fpga,asic, dsp power supplies ? lcd tv ? green electronics/appliances ? notebook computers typical application 1 of 10 SP1484 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
package reference absolute maximum rati(1) supply voltage vin??????-0.3v to +24v switch voltage vsw???? -1v to vin +0.3v boost voltage vbs??vsw -0.3v to vsw +6v all other pins?????????-0.3v to +6v junction temperature 150 lead temperature 260 storage temperature -65 to +150 recommended operating conditions(2) input voltage v in ????????4.75v to 23v output voltage v out ??????0.925v to 20v ambient operating temp .-20 to +85 thermal resistance (3) ? ja ? jc soic8n(exposed pad) 50 10 /w notes: 1) exceeding these ratings may damage the device. 2) the device is not guaranteed to function outside of its operating conditions 3) measured on approximately 1 ? square of 1 oz copper electrical characteristics (vin=12v, ta=+25 , unless otherwise noted.) parameter symbol condition min typ max units shutdown supply current v en =0v 0.3 3.0 a supply current ven=2.0v, vfb=1.0v 1.3 1.5 ma feedback voltage v fb 4.75v vin 23v 0.900 0.925 0.950 v feedback overvoltage threshold 1.1 v error amplifier voltage gain(4) a ea 400 v/v error amplifier trans conductance g ea ? ic=10a 820 a/v high-side/low-side switch on-resistance (4) 85 m ? high-side switch leakage current ven=0v, vsw=0v 0 10 a upper switch current limit minimum duty cycle 3.8 5.3 a lower switch current limit from drain to source 0.9 a comp to current sense transconductance g cs 5.2 a/v 2 of 10 SP1484 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
oscillation frequency f osc1 300 340 380 k hz maximum duty cycle d max vfb=1.0v 110 k hz minimum on time (4) t on 220 ns en shutdown threshold voltage ven rising 1.1 1.5 2.0 v en shutdown threshold voltage hysterisis 220 mv en lockout threshold voltage 2.2 2.5 2.7 v en lockout hysterisis 210 mv input under voltage lockout threshold 3.80 4.05 4.40 v input under voltage lockout threshold hysterisis 210 mv soft-start current vss=0v 6 a soft-start period css=0.1f 15 ms thermal shutdown (4) 160 note: 4) guaranteed by design, not tested. pin functions pin# name description 1 bs high-side gate drive boost input. bs supplies the drive for the high-side n-channel mosfet switch. connect a 0.01f or greater capacitor from sw to bs to power the high side switch. 2 in power input. in supplies the power to the ic, as well as the step-down converter switches. drive in with a 4.75v to 23v pow er source. see input capacitor. 3 sw 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. 4 gnd ground (connect the exposed pad to pin 4). 5 fb feedback input. fb senses the output voltage and regulates it. drive fb with a resistive voltage divider connected to it 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 network from comp to gnd. in some cases, and additional capacitor form comp to gnd is required. see compensation components. 7 en enable input. en is a digital input that turns the regulator on or off. drive en high to turn on the regulator; low to turn it off. attach to in with a 100k ? pull up 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-sta rt feature, leave ss unconnected. 3 of 10 SP1484 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
typical performanc e characteristics (c1=4.7uf,c2=2x10uf, l=10uh, css=0.1uf, ta=+25 ,unless otherwise noted) 4 of 10 SP1484 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
openration functi onal description the SP1484 regulates input voltages from 4.75v to 23v down to an output voltage as low as 0.925v, and supplies up to 3a of load current. the SP1484 uses current-mode control to regulate the output voltage. the out put voltage is measured at fb through a resistive voltage divider and amp lified through the internal transconductance error amplifier. the voltage at the comp pin is compared to the switch current (measured internally) to control the output voltage. the converter uses internal n-channel mosfet switches to step-down the input voltage to the regulated output voltage. since the high side mosfet requires a gat e voltage greater than the input voltage, a boost capacitor connec ted between sw and bs is needed to drive the high side gate. the boost capacitor is charged from the in ternal 5v rail when sw is low. when the fb pin voltage exceeds 20 % of the nominal regulation valu e of 0.925v, the over voltage comparator is tipped and the comp pin and the ss pi n are discharged to gnd, forcing the high-side switch off. applications information component selection setting the output voltage the output voltage is set using a resistive voltage divider connected from the output voltage to fb. the voltage divider divides the output voltage down to the feedback voltage by the ratio: vfb = vout *[r2/(r1+r2)] thus the output voltage is: vout=0.925 x [(r1+r2)/r2] r2 can be as high as 100k ? , but a typical value is 10k ? . using the typical value for r2, r1 is determined by: r1=10.81 x (vout -0.925) (k ? ) 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 5 of 10 SP1484 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
table 1-recommended resistance values vout r1 r2 1.8v 9.52k ? 10 k ? 2.5v 16.9 k ? 10 k ? 3.3v 26.1 k ? 10 k ? 5v 44.2 k ? 10 k ? 12v 121 k ? 10 k ? inductor the inductor is require d to supply constant current to the load while being driven by the switched input voltage. a larger value inductor will result in less ripple current that will in turn result in lower output ripple voltage. however, the larger value inductor will have a larger physical size, higher series resistance, and/or lower saturation current. a good rule for determining inductance is to allow the peak-to-peak ripple current to be approximately 30% or the maximum switch current limit. also, make sure that the peak inductor current is below the maximum switch current limit. the inductance value can be calculated by: l=vout/(fs x ? il)x (1-vout/vin) where vout is the output voltage, vin is the input voltage, fs is the switching frequency, and ? il is the peak-to-peak inductor ripple current. choose an inductor that w ill not saturate under the maximum inductor peak current, calculated by: ilp=iload + vout/(2xfs x l)x(1-vout/vin) where i load is the load current. the choice of which styl e inductor to use mainly depends on the price vs. size requirements and any emi constraints. optional schottky diode during the transition between the high-side switch and low-side switch, the body diode of the low-side power mosfet conducts the inductor current. the forward voltage of this body diode is high. an optional schottky diode may be paralleled between the sw pin and gnd pin to improve overall efficiency. table 2 lists example schottky diodes and t heir manufcturers. table 2---diode selection guide 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, therefore a capacitor is required to supply the ac current while maintaining the dc input voltage. use low esr capacitors for the best performance. ceramic capacitors are preferred, but tantalum or low-esr electrolytic capacitors will also suffice. choose x5r or x7r dielectrics when using ceramic capacitors. since the input capacitor (c 1) absorbs the input switching current, it requires and adequate ripple current rating. the rms current in the input capacitor can be estimated by: the worst-case condition occurs at vin=2vout, where ic1 = iload/2. fo r simplification, use an 6 of 10 SP1484 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
input capacitor with a rms current rating greater than half of the maximum load current. the input capacitor can be electrolytic, tantalum or ceramic. when using electrolytic or tantalum capacitor, i.e. 0.1f, should be placed as close to the ic as possible. when using ceramic capacitors, make sure that they have enough capacitance to provide sufficient charge to prevent excessive voltage ripple at input. the input voltage ripple for low esr capacitors can be estimated by: where c1 is the input capacitance value. output capacitor the output capacitor (c2) is required to maintain the dc output voltage. ceramic, tantalum, or low esr electrolytic capacitors are recommended. under typical applicati on conditions, a minimum ceramic capacitor value of 20f is recommended on the output. low esr capacitors are preferred to keep the output voltag e ripple low. the output voltage ripple can be estimated by: ? vout=vout /(fs x l )x (1- vout/vin)x (resr +1/(8xfsxc2)) where c2 is the output capacitance value and resr is the equivalent series resistance (esr) value of the output capaci tor. when using ceramic capacitors, the impedance at the switching frequency is dominated by the capacitance which is the main cause for the output voltage ripple. for simplification, the output voltage ripple can be estimated by: when using tantalum or el ectrolytic capacitors. the esr dominates t he impedance at the switching frequency. for si mplification, the output ripple can be approximated to: the characteristics of the output capacitor also affect the stability of t he regulation system. the SP1484 can be optimized for a wide range of capacitance and esr values. compensation components SP1484 employs current mode control for easy compensation and fast tr ansient response. the system stability and transient re sponse are 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 govern the characteristics of the control system. the dc gain of the voltage feedback loop is given by: where vfb is the feedback voltage (0.925v), avea is the error amplifier voltage gain, gcs is the current sense tr ansconductance and rload is the load resistor val ue. the system has two poles of importance. one is due to the compensation capacitor (c3) and the output resistor of the error amplifier, and the other is due to the output capacitor and the load resistor. these poles are located at: where gea is the error amplifier transconductance. the sys tem has one zero of 7 of 10 SP1484 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
importance, due to the compensation capacitor (c3) and the compensation resistor (r3). this zero is located at: the system may have another zero of importance, if the output capacitor has a large capacitance and/or a high esr value. the zero, due to the esr and capacitance of the output capacitor, is located at: in this case, a third pole set by the compensation capacitor (c6) and the compensation resistor (r3) is used to compensate 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 transfer func tion to get a desired loop gain. the system crossover frequency where the feedback loop has the unity gain is important. lower crossover frequencies result in slower line and load transient responses, while higher crossover frequencies could cause system instability. a good standard is to set the crossover frequency below one-tenth of the switching frequency. to optimize the compens ation components, the following procedure can be used. 1. choose the compensation resistor (r3) to set the desired crossover frequency. determine r3 by the following equation: where fc is the desired crossover frequency which is typically below o ne tenth of the switching frequency. 2. choose the compensation capacitor (c3) to achieve the desired phase margin. for applications with typica l inductor values, setting the compensation zero (fz1) below one-forth of the crossover frequency provides sufficient phase margin. determine c3 by the following equation: where r3 is the compensation resistor 3. determine if t he second compensation capacitor (c6) is required. it is required if the esr zero of the output capa citor is located at less than half of the switching frequency, or the following relationship is valid: if this is the case, then add the second compensation capacitor (c6) to set pole fp3 at the location of the esr zero. determine c6 by the equation: external bootstrap diode an external bootstrap diode may enhance the efficiency of the regulator, the applicable conditions of external bs diode are: vout is 5v or 3.3v; and in these cases, and external bs diode is recommended from the output of the voltage regulator to bs pin, as shown in fig.2 8 of 10 SP1484 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
figure 2-add optional external bootstrap diode to enhance efficiency the recommended external bs diode is in4148, and the bs cap is 0.1~1f. typic alapplications circuit 9 of 10 SP1484 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
package description esop8 package out line dimensions 10 of 10 SP1484 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
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