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1 rev.1.2 parameter symbol limit unit vin power supply voltage vin -0.3 to +24 v apply voltage to sw vsw gnd-0.3 to vin+0.3 v apply voltage to bs vbs vsw-0.3 to vsw+6 v apply voltage to fb vfb -0.3 to +6 v apply voltage to comp vcomp -0.3 to +6 v apply voltage to en ven -0.3 to vin+0.3 v apply voltage to ss vss -0.3 to +6 v power dissipation pd 630 mw operating temperature range top -40 to +85 c storage temperature range tstg -65 to +150 c elm611d a 2a 23v high ef?ciency synchronous pwm step down dc/dc converter general description features maximum absolute ratings ? distributed power system ? network system ? fpga, dsp, asic power supply ? laptop ? domestic appliance application 11 - ELM611DA is 350khz fixed frequency pwm synchronous step-down regulator, whose input voltage can be set within the range from 4.75v to 23v and output one is adjustable within the range from 0.923v to 18v; maximum output current of ELM611DA can reach 2a. ELM611DA includes 2 switching mosfets whose on resistance is 85m. current mode control of ELM611DA makes it possible to provide fast transient response and current protection of cycle-by-cycle. shutdown current is typ.1a. soft start is programmable by external capacitor during start and limits inrush current to the appropriate value. ELM611DA is equipped with thermal shutdown protection. ? programmable soft start ? short circuit protection ? thermal shutdown protection ? input voltage : 4.75v to 23v ? output voltage : 0.923v to 18v ? output current : 2a ? high ef?ciency : max.93% ? power mosfet switches : 85m ? shutdown current : typ.1a ? fixed frequency : typ.350khz ? package : sop-8 caution:permanent damage to the device may occur when ratings above maximum absolute ones are used. * taping direction is one way. ELM611DA-n symbol a package d: sop-8 b product version a c taping direction n: refer to pkg file ELM611DA - n a b c selection guide
2 rev.1.2 block diagram 11 - standard circuit input cin= 10f/25v ceramic r4=22k r5= 10k c5= 0.1f 7 8 c2= 0.1f c3=10nf c4 option c1=3.3nf 3 5 cout= 22f/6.3v ceramic2 2 1 4 6 l=10h ELM611DA vin fb sw en gnd bs ss comp r1=26.1k 1% r2=10k 1% r3=2.2k output= 3.3v/2a + + - - + - + current sense ampli?er current comparator error ampli?er oscillator 100khz & 350khz shutdown comparator vin fb sw en gnd bs ss comp 5 8 6 7 2 1 3 4 1.5v 1.2v internal regulators 5v in in<4.10v en ovp m2 85m m1 85m 5v s r q q ovp 1.1v 6a 0.923v clk ramp 0.3v - - + + - + note: en is 5v logic input. when vin=12v, r4=22k, r5=10k is required; value of r5/r4= 1/2.2 sop-8(top view) pin configuration pin no. pin name pin description 1 bs high-side gate drive boost input 2 vin power input 3 sw power switching output 4 gnd ground 5 fb feedback input 6 comp compensation node 7 en enable input 8 ss soft start control input 4 3 2 1 5 6 7 8 elm611d a 2a 23v high ef?ciency synchronous pwm step down dc/dc converter
3 rev.1.2 11 - parameter symbol test condition min. typ. max. unit supply voltage vin 4.75 23.00 v output voltage vout 0.923 18.000 v shutdown current is ven = 0v 1 3 a supply current iss ven = 2.0v, vfb = 1.0v 1.3 1.5 ma feedback voltage vfb 4.75v vin 23v 0.900 0.923 0.946 v feedback over-voltage threshold vfbo-th 1.1 v error ampli?er voltage gain aea 400 v/v error ampli?er transconductance gea ?ic = 10a 800 a/v high-side switch-on resistance rds(on)1 85 m low-side switch-on resistance rds(on)2 85 m high-side switch leakage current ileak ven = 0v, vsw = 0v 10 a upper switch current limit ilim_usw minimum duty cycle 2.4 3.4 a lower switch current limit ilim_lsw from drain to source 1.1 a comp to current sense transconductance gcs 3.5 a/v oscillation frequency fosc1 350 khz short circuit oscillation frequency fosc2 vfb = 0v 100 khz maximum duty cycle dmax vfb = 1.0v 90 % minimum on time to 220 ns en shutdown threshold voltage vens_th ven rising 1.3 1.6 1.9 v en shutdown threshold voltage hysteresis vens_hys 10 mv input under voltage lockout threshold vth vin rising 3.80 4.10 4.40 v input under voltage lockout threshold hysteresis vth_hys 210 mv soft-start current isoft vss = 0v 6 a soft-start period psoft vss = 0.1f 15 ms thermal shutdown tsd 160 c electrical characteristics vin=+12v, top=+25c, unless otherwise noted. test circuits b s f b e n g n d e l m 6 1 1 d a s s s w c o m p v i n l = 1 0 h v o u t r l v e n = 3 v r 3 = 2 . 2 k ? v i n r 2 r 1 c i n = 1 0 f c 2 = 0 . 1 f c 3 = 1 0 n f c o u t = 2 2 f c 1 = 3 . 3 n f elm611d a 2a 23v high ef?ciency synchronous pwm step down dc/dc converter
4 rev.1.2 application notes 11 - ELM611DA is 350khz ?xed frequency pwm synchronous step-down regulator, whose input voltage can be set within the range from 4.75v to 23v and output one is adjustable within the range from 0.923v to 18v; maximum output current of ELM611DA can reach 2a. ELM611DA adopts current-mode control to regulate output voltage; error between voltage divided by resistive voltage divider from output voltage is input to fb and internal standard voltage is controlled by internal transconductance error ampli?er. cr circuit, which corrects the transmission function of error ampli?er to ensure stable operation, is connected to comp. ELM611DA in - cludes 2 n-channel mosfets which works as switches; it is required that gate voltage is higher than the input one in order to turn the nmos switch of power side on and this voltage can be generated by internal boost strap circuit. a boost capacitor between sw and bs to drive the high side gate. the boost capacitor is charged by the internal 5v line when sw is low. if fb voltage of ELM611DA is higher than 0.923v by 20% or more under monitoring, the over voltage comparator will activate, comp and ss are discharged to gnd, and forces mos switch to be off. in order to activate internal circuit under 5v, elm does not recommend adding voltage higher than 5v to fb, comp, ev and ss. 1. pin description bs: high side gate drive boost input bs supplies the drive for the high-side n-channel mosfet switch. connect a capacitor of 0.01f or greater between sw and bs to power the high side switch. vin: power input ELM611DA is powered by vin and input range is 4.75v to 23v. to absorb switch noise, connect the capacitor of suitable value between vin and gnd. sw: power switch output sw powers output by switching inductor current. connect lc ?lter between sw and output load. a capacitor between sw and bs is required. gnd: ground connect to pcb wiring which is lower than high frequency impedance. fb: control voltage feedback fb regulates voltage by detecting output voltage. feedback threshold is 0.923v. fb is connected through resis - tive voltage divider network between output and gnd. comp: compensation node comp is used to compensate regulation control loop. connect series rc between comp and gnd. in some cases, an additional capacitor from comp to gnd is required. en: enable input en is digital input that turns the regulator on/off. the regulator works when its high input by high enable and becomes standby when its low input. automatic startup would activate with pullup by 100k resistor. en threshold is 1.6v(typ.). it is possible to adjust automatic startup voltage using register divider (r4, r5) be - tween vin and gnd. ss: soft-start control input ss controls soft start period. by connecting to a 0.15f capacitor, soft-start period can be set to 15ms. soft- start function would be disabled when ss is open. elm611d a 2a 23v high ef?ciency synchronous pwm step down dc/dc converter
5 rev.1.2 2. setting output voltage it is possible to set output voltage by using a resistive voltage divider which divides output voltage and returns it to fb. the the relationship between vout and voltage of fb can be formulated as follows: vfb = vout r2 / (r1 + r2) when vfb is 0.923v: vout = 0.923 (r1 + r2) / r2 elm recommends using resistor of 10k; maximum value of r2 can be as high as 100k. when using 10k resistor, the value of r2 would be determined by r1 by following formula: r1 = 10.83 (v out ? 0.923) (k) 3. inductor the inductor is required to supply constant current to the output load while being driven by the switched input voltage. a larger value inductor would generate less ripple current thus results in smaller output ripple voltage. however, inductor with large value are usually with bigger size, higher series resistance, and/or lower saturation current. elm recommends determining the value of inductor by setting 30% of maximum switch current limit to be peak-to-peak ripple current. a maximum current of the inductor is required to be smaller than a maximum switch current of ELM611DA. the value of inductor can be calculated as follows: l = [ vout / (fs il) ] (1 ? vout/vin) vout=output voltage; vin=input voltage; fs=switching frequency; il=peak-to-peak inductor ripple current. the peak current of inductor can be calculated as follows: ilp = iload + [ vout / (2 fs l) ] (1 ? vout/vin) iload=load current; elm recommends choosing the shape of inductor by its price, size and emi require ments. 4. adding schottky diode body diode of mos switch of gnd would be conducted by inductor current during the transition from on to off of mos switches of power and gnd. the forward voltage of this body diode is high and would result in power loss. by connecting an additional schottky diode between sw and gnd in parallel arrangement, the low forward voltage would bypass the inductor current and improve the ovrall ef?ciency. table 1 are some schottky diodes recommended by elm. part number voltage and current rating vendor b130 30v, 1a diodes inc. sk13 30v, 1a diodes inc. mbrs130 30v, 1a international rectifier table 1: diode selection guide. 11 - 5. input capacitor because the input current to step-down converter is discontinuous, a capacitor is required to supply ac current to step-down converter while maintaining dc input voltage. for best performance, elm recommends using low esr capacitors, such as ceramic ones. tantalum or low-esr electrolytic capacitors may also be used. due to dielectric characteristic, it requires when using ceramic capacitors; make sure to con?rm temperature and volt - age characteristics in advance. x5r or x7r are preferable ceramic capacitors. adequate ripple voltage rating is elm611d a 2a 23v high ef?ciency synchronous pwm step down dc/dc converter
6 rev.1.2 11 - necessary since the input switching current is absorbed by input capacitor (cin). rms of input current can be calculated by following formula: icin = iload [ (v out/vin) (1 ? vout/vin) ] 1/2 in worst case, when vin = 2vout, icin = iload/2. it is necessary to select capacitors which tolerate rms ripple current that is half of maximum load current. for input capacitors, elm recommends using electrolytic, tan - talum or ceramic ones. when using electrolytic or tantalum capacitors, please connect the 0.1f one which is high quality with high frequency to the ic as close as possible. when using ceramic capacitors, it is necessary to provide suf?cient capacity to prevent ripple voltage of input . input voltage ripple for low esr capacitors can be calculated by following formula: vin = [ iload/(cin fs) ] (vout/vin) (1 ? vout/vin) cin=input capacitance value. 6. capacitor capacitors are used to ensure output voltage of dc; elm recommends using ceramic, tantalum, or low esr electrolytic ones. to keep output voltage ripple low, low esr capacitors are preferable. output voltage ripple can be calculated by following formula: vout = [ vout/(fs l) ] (1 ? vout/vin) [ resr + 1 / (8 fs cout) ] cout=output capacitance value; resr=equivalent series resistance (esr) value of the output capacitor . when using ceramic capacitors, please select by the high frequency impedance capacitance of switching fre - quency; output voltage ripple is mainly determined by capacitance. output voltage ripple can be calculated by following formula: vout = [ vout/(8 fs 2 l cout) ] (1 ? vout/vin) when using tantalum or electrolytic capacitors, please select by esr, which is mainly determined by imped - ance of switching frequency. output ripple can be calculated by following formula: vout = [ vout/(fs l) ] (1 ? vout/vin) resr stability of dc/dc converter would be effected by capacitance of output capacitor. ELM611DA is designed to provide wide range of capacitance and stable operation of esr. 7. compensation components ELM611DA realizes simple compensation and fast transient response by adopting current mode control; comp, which is output of internal transconductance error ampli?er, controls system stability and transient re - sponse. a capacitor and a resistor in series connection sets a pole-zero combination for compensation. dc gain of voltage feedback loop can be calculated by following formula: avdc = rload gcs aea vfb/vout aea=error ampli?er voltage gain; gcs=current sense transconductance; rload=load resistor value the control loop has two important poles; one is the product of compensation capacitor (c1) and output resis - tor of error ampli?er, and the other one is the product of output capacitor and load resistor. these poles are located at: fp1 = gea / (2 c1 aea), fp2 = 1 / (2 cout rload) gea=error ampli?er transconductance control system is produced by compensation capacitor (c1) and compensation resistor (r3), and has one zero. this zero is located at: fz1 = 1 / (2 c1 r3) elm611d a 2a 23v high ef?ciency synchronous pwm step down dc/dc converter
7 rev.1.2 there is another important zero which is produced by output capacitance and esr when output capacitor is with large capacitance with high esr. this zero is located at: fesr = 1 / (2 cout resr) under this situation, a third pole is produced by compensation capacitor (c4) and compensation resistor (r3) which are used to compensate the effect of esr zero. this pole is located at: fp3 = 1 / (2 c4 r3) the purpose of compensation is to stabilize transfer function of dc/dc converter. for crossover frequency, feedback loop with unity gain is important. low crossover frequency slows the re - sponse of line and load regulation, whereas high crossover frequency may result in unstableness of control sys - tem . to set cross frequency to be 1/10 of switching one is the easiest way. for best solution of compensation, please follow these steps: 1) select compensation resistor (r3) based upon the desired crossover frequency. the value of r3 can be calcu - lated by following formula: r3 = [ (2 cout fc) / (gea gcs) ] (vout/vfb) < [ (2 cout 0.1 fs) / (gea gcs) ] (vout/vfb) fc = desired crossover frequency, which is usually set to be 1/10 of switching frequency. 2) select compensation capacitor (c1) based upon the desired phase margin. for typical inductor values, set fz1 to be 1/10 of switching frequency so that fz1 is able to acquire suf?cient phase margin to produce compensation zero. c1 can be calculated by following formula: c1 > 4 / (2 r3 fc) r3 = compensation resistor. 3) select the second compensation capacitor (c4) if it is required. if esr zero is located in half of switching fre - quency because of output capacitor, or when the following relationship is satis?ed: 1 / (2 cout resr) < fs/2 the second compensation capacitor, c4 can be calculated by following formula: c4 = (cout resr) / r3 8. external bootstrap diode an external bootstrap diode may enhance the ef?ciency of dc/dc converter, applicable conditions of external bs diode are: vout = 5v or 3.3v, and duty cycle is high: d = vout/vin > 65% under this situation, elm recommends using an additional external bs diode for better solution. sw cout bs cbs 0.1 to 1f l ELM611DA external bs diode in4148 5v or 3.3v ? * add external bootstrap diode to enhance ef?ciency. 11 - elm611d a 2a 23v high ef?ciency synchronous pwm step down dc/dc converter
8 rev.1.2 9. pcb layout guide to stabilize the operation, pcb layout is very important. please take the following guidelines as reference. 1) keep the path of switching current as short as possible; minimize the loop area which is connected to input capacitor, high-side and low-side mosfets. 2) bypass ceramic capacitors are recommended to be connected as close to vin as possible. 3) please connect all feedback loop wire in the shortest way. locate feedback resistors and compensation components as close to the chip as possible. 4) locate wire of sw away from sensitive analog areas such as fb. 5) to cool down the operation temperature of the chip and gain higher long-term reliability, please connect vin, sw, and especially gnd respectively to a large copper area. 11 - for external bs diode, elm recommends in4148; bs capacitors between 0.1 to 1f are preferable. to improve ef?ciency when vin is 6v, it is possible to add an external schottky diode between in and bs. sw bs gnd vin ELM611DA schottky (b0520lw) 5v to 6v v out * add a schottky diode to improve ef?ciency when vin is 6v. sop-8 marking e l m 6 1 1 da a b c elm611d a 2a 23v high ef?ciency synchronous pwm step down dc/dc converter mark content ELM611DA product name abc assembly lot no.: 000 to 999 repeated
9 rev.1.2 11 - typical characteristics 0 4 8 12 16 20 0.4 0.8 1.2 1.6 2.0 2.4 iout=1ma 10ma 100ma 1a vin (v) vout (v) vout-vin 23 ? v=1.8v 0.1 1 10 100 1000 0 20 40 60 80 100 vin=5v vin=12v iout (ma) efficiency (%) efficiency-iout -40 -20 0 20 40 60 80 0.90 0.91 0.92 0.93 0.94 0.95 top ( ? ) vfb (v) vfb-top vin=12v, iout=0.1a 1 10 100 1000 1.7 1.8 1.9 vin=12v vin=5v iout (ma) vout (v) vout-iout 0 5 10 15 20 25 30 35 40 0 1 2 0 1 2 time (ms) start response vout (v) ven (v) vin=12v, no load 0 0.1 0.2 0.3 0.4 1.5 2 2.5 0 0.5 1 time (ms) load transient response vout (v) iout (a) vin=12v, iout=1ma ? 1a ? vout=1.8v : cin=10f, cout=22f, l=10h, r1=13.8k , r2=14.7k , top=25c elm611d a 2a 23v high ef?ciency synchronous pwm step down dc/dc converter
10 rev.1.2 11 - 0 4 8 12 16 20 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 iout=1ma 10ma 100ma 1a vin (v) vout (v) vout-vin ? v=3.3v 0.1 1 10 100 1000 0 20 40 60 80 100 vin=5v vin=12v iout (ma) efficiency (%) efficiency-iout -40 -20 0 20 40 60 80 0.90 0.91 0.92 0.93 0.94 0.95 top ( ? ) vfb (v) vfb-top vin=12v, iout=0.1a 1 10 100 1000 3.1 3.2 3.3 3.4 3.5 vin=12v vin=5v iout (ma) vout (v) vout-iout 0 5 10 15 20 25 30 35 40 0 1 2 3 0 1 2 time (ms) start response vout (v) ven (v) vin=12v, no load 0 0.1 0.2 0.3 0.4 2.5 3 3.5 4 0 1 2 time (ms) load transient response vout (v) iout (a) vin=12v, iout=1ma ? 1a ? vout=3.3v : cin=10f, cout=22f, l=10h, r1=38.3k , r2=15.2k , top=25c elm611d a 2a 23v high ef?ciency synchronous pwm step down dc/dc converter
11 rev.1.2 11 - ? v=5.0v 0 4 8 12 16 20 1.0 2.0 3.0 4.0 5.0 6.0 iout=1ma 10ma 100ma 1a vin (v) vout (v) vout-vin 23 -40 -20 0 20 40 60 80 0.90 0.91 0.92 0.93 0.94 0.95 top ( ? ) vfb (v) vfb-top vin=12v, iout=0.1a 1 10 100 1000 4.8 4.9 5 5.1 5.2 vin=12v iout (ma) vout (v) vout-iout 0 5 10 15 20 25 30 35 40 0 1 2 3 4 5 6 0 1 2 time (ms) start response vout (v) ven (v) vin=12v, no load 0 0.1 0.2 0.3 0.4 3 4 5 6 0 1 2 time (ms) load transient response vout (v) iout (a) vin=12v, iout=1ma ? 1a 0.1 1 10 100 1000 0 20 40 60 80 100 iout (ma) efficiency (%) efficiency-iout vin=12v ? vout=5.0v : cin=10f, cout=22f, l=10h, r1=6.4k , r2=1.47k , top=25c elm611d a 2a 23v high ef?ciency synchronous pwm step down dc/dc converter

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