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1/12 axelite confidential materials, do not copy or distribute without written consent . rev.1.1 aug.26, 2010 AX3518 1mhz, 3.2a synchronous step-down converter ? general description the AX3518 is a 1mhz constant frequency cu rrent mode pwm step- down converter. it is ideal for portable equipment requiring very high current up to 3.2a from single-cell lithium-ion batteries while still achieving ov er 90% efficiency during peak load conditions. the AX3518 also can run at 100% duty cycle for low dropout operation, extending battery life in portable systems while light load operation provides very low output ripple for noise sensitive applications. the AX3518 can supply up to 3.2a output load curr ent from a 2.8v to 5.5v input voltage and the output voltage can be regulated as low as 0.8v. the high switching frequency minimizes the size of ex ternal components while keeping switching losses low. the internal slope compensation setting allows the device to operate with smaller inductor values to optimize size and provide efficient oper ation. the AX3518 is available in adjustable (0.8v to vin) output voltage. the device is av ailable in sop-8l-ep and tdfn-8l pb-free packages. ? features - 2.8v to 5.5v input voltage range - output voltages from 0.8v to vin - high efficiency: up to 94 % - 1mhz constant frequency operation - with power saving mode - up to 3.2a output current - no schottky diode required - current mode operation for excell ent line and load transient response - current limit and therma l shutdown protection - short circuit protect (scp) - build-in soft start function - sop-8l-ep and tdfn-8l pb-free packages
2/12 axelite confidential materials, do not copy or distribute without written consent . rev.1.1 aug.26, 2010 AX3518 ? block diagram fb en sgnd svin pvin sw pgnd over-temperature and short - circuit protection ref soft-star + - - + icomp - + isense amp osc slope comp pwm logic - + izero comp non - overlap control reset set enable logic ref svin i ? pin assignment the packages of AX3518 are sop-8l-ep and tdfn -8l; the pin assignment is given by: name description svin signal supply input pin ref reference output pin sgnd signal ground pin fb output feedback pin en enable pin, inte rnal pull-high function. h normal operation l shutdown pgnd power ground pin sw switch output pin. connect external inductor here. minimize trace area at this pin to reduce emi. pvin power supply input pin
3/12 axelite confidential materials, do not copy or distribute without written consent . rev.1.1 aug.26, 2010 AX3518 ? order/marking information order information package type es: sop-8l-ep AX3518 packing a : taping xx x j8: tdfn-8l (3*3) top marking (sop-8l-ep) top marking (tdfn-8l 3*3) y ax logo part number id code: internal ww: 01 ~ 52 year: 10=2010 11=2011 y w w x 3518 y y w w x 3 part number id code: internal ww: 01~52 year : 10=2010 11=2011 5 1 8 ? absolute maximum ratings (at t a =25c) characteristics symbol rating unit pvin, svin, ref pin voltage v ss - 0.3 to v ss + 6.5 v feedback pin voltage v fb v ss - 0.3 to v in + 0.3 v en pin voltage v run v ss - 0.3 to v in + 0.3 v switch pin voltage v sw v ss - 0.3 to v in + 0.3 v power dissipation pd ( t j -t a ) / ja mw storage temperature range t st -40 to +150 c operating temperature range t op -40 to +85 c junction temperature t j +125 c thermal resistance from junction to case sop-8l-ep jc 15 c/w tdfn-8l thermal resistance from junction to ambient sop-8l-ep ja 40 c/w tdfn-8l 45 note: ja is measured with the pcb copper area of approximately 1 in 2 (multi-layer). that need connect to exposed pad.
4/12 axelite confidential materials, do not copy or distribute without written consent . rev.1.1 aug.26, 2010 AX3518 ? electrical characteristics (v in = v en =3.3v, t a =25c, unless otherwise specified) characteristics symbol conditions min typ max units supply voltage range v in 2.8 - 5.5 v feedback voltage v fb 0.784 0.800 0.816 v feedback bias current i fb v fb = 0.85v - - 30 na ref voltage v ref i ref =0, v in =2.8v to 5.5v - 0.8 - v quiescent current i ccq v fb = 1v - 250 400 ua shutdown supply current i sd v en =0v - 4 10 ua switching current limit i limit 3.5 4 - a line regulation v out /v out v in = 3v~5.5v - 0.04 0.4 %/v load regulation v out /v out i out = 0.01 to 3a - 0.5 1 % oscillation frequency f osc sw pin 0.8 1 1.2 mhz r ds(on) of p-ch mosfet r dson v fb =0v, i out =1a - 100 - m ? r ds(on) of n-ch mosfet r dson (note1) - 80 - m ? en pin lo g ic input threshold voltage v enl - - 0.4 v v enh 1.5 - - en pin input current i enh v en =v in - 0.1 1 ua i enl v en =0v - 4 8 ua efficiency effi v in =5v, v out =3.3v,i out =2a - 92 - % thermal shutdown t sd - 150 - c thermal shutdown h y steresis t sh - 40 - c note1: guaranteed by design.
5/12 axelite confidential materials, do not copy or distribute without written consent . rev.1.1 aug.26, 2010 AX3518 ? application circuit v out = 0.8 * (1+r2/r3) r3 600k c1 22uf c2 22uf vout=1.2v vin=2.8 to 5.5v r2 = 300k~820k l1 2.2uh r2 300k AX3518 pvin en sgnd sw fb c5 15pf svin pgnd r1 10r c3 100nf c4 100nf ref ? function descriptions operation AX3518 is a monolithic switching mode step-do wn dc-dc converter. it utilizes internal mosfets to achieve high efficiency and can generate very low output voltage by using internal reference at 0.8v. it operates at a fixed switching frequency, and uses the slope compensated current mode architecture. this step-down dc-dc converter supplies 3200ma output current at input vo ltage range from 2.8v to 5.5v. current mode pwm control slope compensated current mode pwm c ontrol provides stable switching and cycle-by-cycle current limit for excellent load and line responses and protection of the internal main switch (p-ch mosfet) and syn chronous rectifier (n-ch mosfet). during normal operation, the internal p-ch mosfet is turned on for a certain time to ramp the inductor current at each rising edge of the inte rnal oscillator, and switched off when the peak inductor current is above the error voltage. the current comparator, i comp , limits the peak inductor current. when the main switch is o ff, the synchronous rectifier will be turned on immediately and stay on until either the inductor current starts to reverse, as indicated by the current reversal comparator, i zero , or the beginning of the next clock cycle.
6/12 axelite confidential materials, do not copy or distribute without written consent . rev.1.1 aug.26, 2010 AX3518 ? application information setting the output voltage application circuit item shows the basic application circuit with AX3518 adjustable output version. the external resistor sets the output voltage according to the following equation: v out = 0.8v (1+ 3 r 2 r ) table 1 resistor select for output voltage setting v out r3 r2 1.2v 600k 300k 1.5v 380k 330k 1.8v 310k 390k 2.5v 220k 470k 3.3v 150k 470k inductor selection for most designs, the AX3518 operates with inductors of 2.2 h to 3.3h. low inductance values are physically smaller but require faster swit ching, which results in some efficiency loss. where is inductor ripple cu rrent. large value inductors lo wer ripple current and small value inductors result in high ripple currents. choose inductor ripple current approximately 15% of the maximum load current 3200ma, i l = 480ma. for output voltages above 3.0v, when li ght-load efficiency is important, the recommended inductor is 2.2h. for optimum vo ltage-positioning l oad transients, choose an inductor with dc series resistance in the 30m ? to 100m ? range. for higher efficiency at heavy loads (above 300ma), or minimal load regulat ion (but some transient overshoot), the resistance should be kept below 100m ? . the dc current rating of the inductor should be at least equal to the maximum load current plus half the ripple current to prevent core saturation (3200ma + 240ma). input capacitor selection the input capacitor reduces the surge cu rrent drawn from the input and switching noise from the device. the input capacitor impedance at the switching frequency shall be less than input source impedance to prevent hi gh frequency switching current passing to the input. a low esr input capacitor sized for maximum rms current must be used. ceramic capacitors with x5r or x7r dielectrics are highly recomm ended because of their low esr and small temperature coefficients. a 22f ce ramic capacitor for most applications is sufficient.
7/12 axelite confidential materials, do not copy or distribute without written consent . rev.1.1 aug.26, 2010 AX3518 output capacitor selection the output capacitor is required to keep the output voltage rippl e small and to ensure regulation loop stability. the output capacito r must have low impedance at the switching frequency. ceramic capacitors with x5r or x7 r dielectrics are re commended due to their low esr and high ripple current. a 22f ceramic capac itor for most applications is sufficient. compensation capacitor selection the compensation capacitor (c5) for incr easing phase margin provides additional stability. refer to demo board schematic, the c5 optimum value is 15pf for all conditions. layout guide AX3518 tdfn-8l and sop-8l- ep are the same layout. pvin fb sgnd lx ref svin pgnd AX3518 expose pad 8 1 2 3 4 5 6 7 en c1 c3 vin r1 l1 vout c2 r2 r3 c3 and c4 must be placed between pin and sgnd as closer as possible sw should be connected to inductor by wide and short trace, keep sensitive components away from this trace c1 must be placed between pvin and pgnd as closer as possible, vin trace should be wide and short. vin c5 gnd c4
8/12 axelite confidential materials, do not copy or distribute without written consent . rev.1.1 aug.26, 2010 AX3518 ? typical characteristics 0 100 200 300 400 500 2.5 3 3.5 4 4.5 5 5.5 v in (v) i ccq (ua) v in v.s. i ccq 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20 2.5 3 3.5 4 4.5 5 5.5 v in (v) frequency (mhz) v in v.s. frequency 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 2.5 3 3.5 4 4.5 5 5.5 v in (v) en threshold voltage (v) v in v.s. en threshold voltage enh enl 1.180 1.185 1.190 1.195 1.200 1.205 1.210 1.215 1.220 2.5 3 3.5 4 4.5 5 5.5 v in (v) v out (v) line regulation i out =300ma 1.180 1.185 1.190 1.195 1.200 1.205 1.210 1.215 1.220 00.511.522.53 i out (a) v out (v) load regulation v in =5v 0 40 80 120 160 200 2.5 3 3.5 4 4.5 5 5.5 v in (v) r ds(on) (m ? ) v in v.s. r ds(on) p-mos n-mos i out =1a
9/12 axelite confidential materials, do not copy or distribute without written consent . rev.1.1 aug.26, 2010 AX3518 ? typical characteristics (continues) 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40 1.50 -40 -25 0 25 50 75 100 125 temperature frequency (mhz) temperature v.s. frequency 1.16 1.17 1.18 1.19 1.20 1.21 1.22 1.23 1.24 -40 -25 0 25 50 75 100 125 temperature v out (v) temperature v.s. vout v in =5v i out =300ma 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 00.511.522.53 i out (a) efficiency efficiency (v in =3.3v) v out =1.2v v out =1.8v v out =2.5v 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 0.5 1 1.5 2 2.5 3 i out (a) efficiency efficiency (v in =5v) v out =1.2v v out =1.8v v out =2.5v v out =3.3v power on v in =5v, v out =1.2v, i out =2.9a (c ref =100nf) enable on v in =5v, v out =1.2v, i out =2.9a (c ref =100nf)
10/12 axelite confidential materials, do not copy or distribute without written consent . rev.1.1 aug.26, 2010 AX3518 ? typical characteristics (continues) power on v in =5v, v out =1.2v, i out =2.9a (c ref =10nf) enable on v in =5v, v out =1.2v, i out =2.9a (c ref =10nf) power on v in =5v, v out =3.3v, i out =2.9a (c ref =10nf) enable on v in =5v, v out =3.3v, i out =2.9a (c ref =10nf) load transient response v in =5v, v out =1.2v, i out = 0.3a~3.0a load transient response v in =5v, v out =3.3v, i out = 0.3a~3.0a
11/12 axelite confidential materials, do not copy or distribute without written consent . rev.1.1 aug.26, 2010 AX3518 ? package outlines (1) sop-8l-ep detail a l h e a a2 a1 e d 7 (4x) b y c detail a y x expose pad symbol dimensions in millimeters dimensions in inches min. nom. max. min. nom. max. a - - 1.75 - - 0.069 a1 0 - 0.15 0 - 0.06 a2 1.25 - - 0.049 - - c 0.1 0.2 0.25 0.0075 0.008 0.01 d 4.7 4.9 5.1 0.185 0.193 0.2 e 3.7 3.9 4.1 0.146 0.154 0.161 h 5.8 6 6.2 0.228 0.236 0.244 l 0.4 - 1.27 0.015 - 0.05 b 0.31 0.41 0.51 0.012 0.016 0.02 e 1.27 bsc 0.050 bsc y - - 0.1 - - 0.004 x - 2.34 - - 0.092 - y - 2.34 - - 0.092 - 0 o - 8 o 0 o - 8 o mold flash shall not exceed 0.25mm per side jedec outline: ms-012 ba
12/12 axelite confidential materials, do not copy or distribute without written consent . rev.1.1 aug.26, 2010 AX3518 (2) tdfn-8l (3*3 0.75mm) e d a a1 a3 d2 e e2 l (top view) (bottom view) b pin1 seating plane (side view) symbol dimensions in millimeters dimensions in inches min. nom. max. min. nom. max. a 0.70 0.75 0.80 0.028 0.030 0.031 a1 0.00 0.02 0.05 0.000 0.001 0.002 a3 0.203 ref. 0.008 ref. b 0.23 0.30 0.35 0.009 0.012 0.014 d 2.90 3.00 3.10 0.114 0.118 0.122 d2 2.30 2.40 2.50 0.091 0.094 0.098 e 2.90 3.00 3.10 0.114 0.118 0.122 e2 1.50 1.60 1.70 0.059 0.063 0.070 e 0.65 bsc. 0.026 bsc. l 0.25 0.38 0.50 0.012 0.015 0.020

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