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eup3472 ds3472 ver 1.0 jul. 2010 1 2a, 28v, 500khz synchronous step-down converter description the eup3472 is a 500khz fixed frequency synchronous current mode buck regulator. the device integrates both 150m ? high-side switch and 100m ? low-side switch that provide 2a of continuous load current over a wide operating input voltage of 4.5v to 28v.the internal synchronous power switch increases efficiency and eliminates the need for an external schottky diode. current mode control provides fast transient response and cycle- by-cycle current limit. the eup3472 features short circuit and thermal protection circuits to increase system reliability. externally programmable soft -start allows for proper power on sequencing with respect to other power supllies and avoids input inrush current during startup. in shutdown mode, the supply current drops below 1 a. the eup3472 is available in sop-8 package. features z 2a continuous output current z 110ns minimum on time z integrated 150m ? high side switch z integrated 100m ? low side switch z wide 4.5v to 28v operating input range z output adjustable from 0.8v to 24v z up to 95% efficiency z programmable soft-start z <1 a shutdown current z available in 500khz fixed switching frequency z thermal shutdown and over current protection z input under voltage lockout z available in sop-8 package z rohs compliant and 100% lead (pb)-free halogen-free applications z distributed power systems z networking systems z fpga, dsp, asic power supplies typical application circuit figure 1.
eup3472 ds3472 ver 1.0 jul. 2010 2 typical application circuit (continued) figure 2. eup3472 typical applicatio n circuit with electrolytic capacitors block diagram figure 3. eup3472 functional block diagram
eup3472 ds3472 ver 1.0 jul. 2010 3 pin configurations package type pin configurations sop-8 pin description number pin name description 1 bs high-side gate drive boost input. bs supp lies the drive for the high-side n-channel dmos switch. connect a 0.01f or greater cap acitor 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 4.5v to 28v power source. bypass in to gnd with a suitably large capacitor to eliminate noise on the input to the ic. 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 0.8v. 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, an additional capacitor from 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 tu rn it off. connect to in with a 100k pull up resistor for automatic startup. 8 ss soft-start control input. ss co ntrols the soft-start period . connect a capacitor from ss to gnd to set the external soft-start period , or leave ss floating to set the internal soft-start period. a 0.1 f capacitor sets the soft-start period to about 15ms. leave ss pin floating, the internal soft -start period is about 300 s.
eup3472 ds3472 ver 1.0 jul. 2010 4 ordering information order number package type marking operating temperature range EUP3472DIR1 sop-8 xxxxx p3472 -40 c to +85c eup3472 lead free code 1: lead free, halogen free 0: lead packing r: tape & reel operating temperature range i: industry standard package type d: sop
eup3472 ds3472 ver 1.0 jul. 2010 5 absolute maximum ratings (1) ? supply voltage (v in ) -------------------------------------------------------- -0.3v to +30v ? en voltage (v en ) -------------------------------------------------------- -0.3v to +6v ? switch voltages (v sw ) ------------------------------------------------------ -1v to v in +0.3v ? bootstrap voltage (v bs ) -------------------------------------------- v sw -0.3v to v sw +6v ? all other pins ---------------------------------------------------------------------- -0.3v to +6v ? junction temperature -------------------------------------------------------------------- 150c ? lead temperature ------------------------------------------------------------------------ 260c ? storage temperature -------------------------------------------------------- -65c to 150c ? output voltage v out ----------------------------------------------------------- 0.9v to 26v ? thermal resistance ja (sop-8) ----------------- ---------------------- --------------------- ---------------- 125c /w ? esd ratings human body mode --------------------------------------------------------------------------- 2kv recommend operating conditions (2) ? input voltage (v in ) --------------------------------------------------------------- 4.5v to 28v ? operating temperature range ------------------- ---------------------------- -40c to +85c note (1): stress beyond those listed under ?absolute maximum ratings? may damage the device. note (2): the device is not guaranteed to function outside the recommended operating conditions. electrical characteristics unless otherwise specified, v in =12v ,t a =+25c. eup3472 symbol parameter conditions min. typ. max. unit i shut shutdown supply current v en =0v 0.1 3 a i q supply current v en =2v, v comp =0.35v 1.1 1.5 ma v fb feedback voltage 4.5v Q v in Q 28v 0.784 0.800 0.816 v a ea error amplifier voltage gain 400 v/v g ea error amplifier transconductance i c = 10a 400 a/v r ds(on) 1 high-side switch on-resistance i sw =300ma 150 r ds(on) 2 low-side switch on-resistance i sw =300ma 100 m ? i leakage high-side switch leakage current v en =0v, v sw =0v 0 10 a i limit upper switch current limit minimum duty cycle 3.6 4.8 i neg low-side switch reverse current limit from drain to source -1 a g cs comp to current sense transconductance 5.6 a/v f osc1 oscillation frequency v fb =0.76v 400 500 600 khz f osc2 short circuit oscillation frequency v fb =0v 100 khz d max maximum duty cycle v fb =0.76v 90 % t on minimum on time 110 ns v en en shutdown threshold voltage v en rising 1.1 1.5 2 v ehhys en shutdown threshold voltage hysterisis 0.2 v uvlo input under voltage lockout threshold v in rising 3.8 4.0 4.2 v uvlohys input under voltage lockout threshold hysteresis 0.2 v i ss soft-start current v ss =0v 6 a t ss soft-start period c ss =0.1f 15 ms t sd thermal shutdown 160 t sdhys thermal shutdown hysteresis 20 c
eup3472 ds3472 ver 1.0 jul. 2010 6 typical operating characteristics (c in =10f, c out =22f, l=6.8h, c ss =0.1f ,t a =+25c, unless otherwise noted.)
eup3472 ds3472 ver 1.0 jul. 2010 7 typical operating characteristics (continued) (c in =10f, c out =22f, l=6.8h, c ss =0.1f ,t a =+25c, unless otherwise noted.) output ripple output ripple
eup3472 ds3472 ver 1.0 jul. 2010 8 typical operating characteristics (continued) (c in =10f, c out =22f, l=6.8h, c ss =0.1f ,t a =+25c, unless otherwise noted.) external soft-start external soft-start internal soft-start (without c ss capacitor) internal soft-start (without c ss capacitor) shut down shut down
eup3472 ds3472 ver 1.0 jul. 2010 9 typical operating characteristics (continued) (c in =10f, c out =22f, l=6.8h, c ss =0.1f ,t a =+25c, unless otherwise noted.) load transient response load transient response short circuit short circuit recovery
eup3472 ds3472 ver 1.0 jul. 2010 10 application information setting the output voltage the output voltage is set using a resistive voltage divider connected from the output voltage to v fb . the voltage divider divides the output voltage down to the feedback voltage by the ratio: 2 1 2 out fb r r r v v + = thus the output voltage is: 2 2 1 out r r r v 8 . 0 v + = r 2 can be as high as 100k ? , but a typical value is 10k ? . using the typical value for r 2 , r 1 is determined by: k 5 . 12 ) v 8 . 0 v ( r out 1 ? = for example, for a 3.3v output voltage, r 2 is 10k ? and r 1 is 31.25k ? . inductor the inductor is required 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 results 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% of 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: ) v v 1 ( i f v l in out l s out ? = 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 ripple current. choose an inductor that will not saturate under the maximum inductor peak current, calculated by: ) v v 1 ( l f 2 v i i in out s out load lp ? + = where i load is the load current. the choice of which style 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. 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 in ) absorbs the input switching current, it requires an adequate ripple current rating. the rms current in the input capacitor can be estimated by: ) v v 1 ( v v i i in out in out load cin ? = the worst-case condition occurs at v in = 2v out , where ic in = i load /2. for simplification, use an 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 capacitors, a small high quality ceramic 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: ) v v 1 ( v v f c i v in ut o in ut o s in oad l in ? = where c in is the input capacitor value. for simplification, choose the input capacitor whose rms current rating greater than half of the maximum load current. output capacitor the output capacitor (c out ) is required to maintain the dc output voltage. ceramic, tantalum, or low esr electrolytic capacitors are recommended. low esr capacitors are preferred to keep the output voltage ripple low. the output voltage ripple can be estimated by: where c out is the output capacitance value and r esr is the equivalent series resistance (esr) value of the output capacitor. 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: ) v v 1 ( c l f 8 v v in out out 2 s out out ? = ) c f 8 1 r ( ) v v 1 ( l f v v out s esr in out s out out + ? =
eup3472 ds3472 ver 1.0 jul. 2010 11 when using tantalum or electrolytic capacitors, the esr dominates the impedance at the switching frequency. for simplification, the output ripple can be approximated to: esr in out s out out r ) v v 1 ( l f v v ? = the characteristics of the outpu t capacitor also affect the stability of the regulation system. the eup3472 can be optimized for a wide range of capacitance and esr values. compensation components eup3472 employs current mode control for easy compensation and fast transi ent response. the system stability and transient resp onse 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: out fb vea cs load vdc v v a g r a = where v fb is the feedback voltage (0.8v), a vea is the error amplifier voltage gain, g cs is the current sense transconductance and r load is the load resistor value. the system has two poles of importance. one is due to the compensation capacitor (c c ) 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: vea c ea 1 p a c 2 g f = load out 2 p r c 2 1 f = where g ea is the error amplif ier transconductance. the system has one zero of importance, due to the compensation capacitor (c c ) and the compensation resistor (r c ). this zero is located at: c c 1 z r c 2 1 f = 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: esr out esr r c 2 1 f = in this case, a third pole set by the compensation capacitor (c 2 ) and the compensation resistor (r c ) 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 function 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 the system instability. a good standard is to set the crossover frequency below one-tenth of the switching frequency. to optimize the compensation components, the following procedure can be used: 1. choose the compensa tion resistor (r c ) to set the desired crossover frequency. determine r c by the following equation: where f c is the desired crossover frequency, which is typically below one tenth of the switching frequency. 2. choose the compensation capacitor (c c ) to achieve 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 c c by the following equation: c c c f r 2 4 c > where r c is the compensation resistor. 3. determine if the second compensation capacitor (c 2 ) is required. 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: 2 f r c 2 1 s esr out < if this is the case, then add the second compensation capacitor (c 2 ) to set the pole f p3 at the location of the esr zero. determine c 2 by the equation: c esr out 2 r r c c = table 1. recommended component selection 4.5v v in < 15v v out (v) r 1 (k ? )r 2 (k ? )r c (k ? ) c c (nf) c out ( f) l( h) 1.2 5 10 5 2.2 22 3 1.5 8.75 10 5 2.2 22 3 1.8 12.5 10 8 2.2 22 4.7 2.5 21.25 10 10 2.2 22 4.7 3.3 31.25 10 10 2.2 22 6.8 5 52.5 10 15 2.2 22 6.8 8 90 10 20 2.2 22 6.8 10 115 10 20 2.2 22 6.8 fb out cs ea s out fb out cs ea c out c v v g g f 1 . 0 c 2 v v g g f c 2 r < = c 2 3 p r c 2 1 f =
eup3472 ds3472 ver 1.0 jul. 2010 12 table 2. recommended component selection 15 v in 28v v out (v) r 1 (k ? ) r 2 (k ? ) r c (k ? )c c (nf) c 2 (pf) c out ( f) l( h) 1.2 5 10 5 2.2 20 22 3 1.5 8.75 10 5 2.2 20 22 3 1.8 12.5 10 8 2.2 20 22 4.7 2.5 21.25 10 10 2.2 20 22 4.7 3.3 31.25 10 10 2.2 20 22 6.8 5 52.5 10 15 2.2 20 22 6.8 8 90 10 20 2.2 20 22 6.8 10 115 10 20 2.2 20 22 6.8
eup3472 ds3472 ver 1.0 jul. 2010 13 packaging information sop-8 millimeters inches symbols min. max. min. max. a 1.35 1.75 0.053 0.069 a1 0.10 0.25 0.004 0.010 d 4.90 0.193 e 5.80 6.20 0.228 0.244 e1 3.90 0.153 l 0.40 1.27 0.016 0.050 b 0.31 0.51 0.012 0.020 e 1.27 0.050

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