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e c t3408 1.2a, 1.5mhz step-down converter e-cmos corp. ( www .ecmos.com.tw ) page 1 of 17 2007/1 1 /19 description the ect34 0 8 is a 1.5m hz co nsta nt freque ncy cu rre nt mode pwm step-do wn co nverter. it is ideal for po rtable equipm ent re quirin g very high cu rrent up to 1.2a from singl e-cell lit h ium-i on batt e ries whil e st ill achieving over 90% efficien cy during pe ak load con d itio ns. the ect 3 408 also can run at 100% duty cycl e for lo w drop out ope ration, extending bat tery life in p o r t able system s while lig ht l oad operation provides very lo w output ripple for noise sen s itive appl ication s . the ect3 40 8 can supply up to 1.2a o u tput load cu rre nt from a 2.5v t o 5.5v input voltage an d the outp u t voltage can be regul ated as lo w as 0.6v. the high swit chin g freque ncy mi nimize s the size of external co mpon e n ts while keeping switching l o sses low. t he internal slope comp en satio n setting all o ws the device to operate with smalle r indu ctor value s to optimize size and provide efficient ope ration. the ect 340 8 is availabl e in adju s table (0.6v to vin) and fixed (1.8v) output voltage versio n s . the d e vice is available i n a pb-fre e, 3x 3 mm tdfn-1 0 pa ckage a nd is rated ove r the -40 c to +8 5c temp erature rang e. features z input voltage rang e: 2.5v to 5.5v z output voltag es from 0.6v to v in z 1.2a output curre n t z high effic i ency: up to 95% z 1.5mhz co nstant switchin g freq uen cy z low r ds(on) internal s w itches: 0.15 ? z allows us e of cera mic cap a citor s z curre n t mode operatio n for excellent lin e and load t r an sie nt respon se z short-circuit and the r mal fault prote c tion z soft start z low dro pout operation: 10 0% duty cycl e z low shutd o wn curre n t: i sh ut do w n < 1a z available is the lea d fre e packag e applications z cellular phones z digital c a me r a s z dsp core supplies z pdas z portable in struments z smart phone s block diagram http://
e c t3408 1.2a, 1.5mhz step-down converter e-cmos corp. ( www .ecmos.com.tw ) page 2 of 17 2007/1 1 /19 pin# symbol functio n 1 en enab le p i n. active hi gh. in shu t do w n , all fu nctions ar e disa ble d dra w i ng < 1a su ppl y curr ent. do not leave en floating. 2 in po w e r su ppl y i nput pi n. must be clos el y d e coup led to agnd with a 2.2f or greater ceramic capacitor. 3 ain analog supply input pin. provides bias for internal circuitry. 4,6 agnd analog ground pin 5 fb/out f b (ec t 3408- adj): adjustab le versi on fee d back in put pi n. con nect f b to the center p o in t of the extern a l resistor divid e r. t he fe edb ack thresh old vo ltag e is 0 . 6v. out (ec t 3408-b): f i xed v e r sion fe edb ack i nput. co nnect out to the output volt age, v ou t . 7,8 lx s w itc h i ng n ode pin. co nn ect the o u tput in du ctor to this pin. 9,10 pgnd power ground pin ? ep power ground exposed pad. must be connected to bare copper ground plane. packag e v out v o lt a g e part numb er marking marking info rmation adj ECT3408-adj-ff tdfn -10 1.8v ECT3408-b-ff 3408x yy ww x is the output volt age of p r oduct i on. example: blank adjust able, b 1.8v . y y is the ye ar of p r oductio n . 07 means t he product is manuf actured in y e ar o f 2007. ww is the w eek of production. 25 means the product is manufactured in the 25 th w eek. pin descriptions ordering information marking in formation pin assignment tdfn -10 (to p vie w ) e c t 3 408 - xxx - xx f o u t p ut v o l t ag e: v ou t = ad j : ad j u s t a b l e b : 1 . 8 v p a c k a g e : f = td fn - 1 0 f : le ad - f r e e ci r c u i t t y p e
e c t3408 1.2a, 1.5mhz step-down converter e-cmos corp. ( www .ecmos.com.tw ) page 3 of 17 2007/1 1 /19 absolute maximum rating (1) parameter sy mbol v a lue units input sup p l y v o ltag es v in ,v ain -0.3 to 6.0 v fb, lx voltages v fb ,v lx -0.3 to v in + 0.3 v en voltage v en -0.3 to v in + 0.3 v ground vo ltag es pgnd,agnd -0.3 to 6.0 v operatin g t e mperatur e ra ng e t a -40 to + 85 c storage t e mperature t st g -65 to 150 c lea d t e mperature (sol deri ng, 10s) t lead 300 c recommended operating conditions (3) parameter sy mbol v a lue unit s t hermal resist ance (2 ) ja 45 c/w maximum po w e r dissipation at t a = 25c p d 2.2 w note : (1). absolute ma ximum ratings a r e those values be y ond w h ich th e life of a device ma y be imp a ired . (2). t j is calculat ed from t he ambi ent tempe r ature t a a nd po wer di ssip a tion p d according to th e follow i ng formula: t j = t a + p d x ja . (3). the r mal resi st ance is specifie d w i th app roxima tely 1 squa re inch of 1 oz coppe r .
e c t3408 1.2a, 1.5mhz step-down converter e-cmos corp. ( www .ecmos.com.tw ) page 4 of 17 2007/1 1 /19 electrical characteristics 1 v in = 3.6v , t a = - 40c to +85 c u n less otherw i se noted; t y pical values are t a = 25 c. description symbol conditions min typ max units input voltage range 2 v in ? 2.5 ? 5.5 v output voltage range v out ? 0.6 ? v in v active mode v fb = 0.5v ? 300 500 a input dc sup p l y curr ent shutdo w n mo d e i q v fb = 0v, v ain =5.5v ? 0.1 1 a feedback input bias current i fb v fb = 0.65v ? ? 30 na t a = 25c 0.5880 0.6000 0.6120 t a = 0c t a 85c 0.5865 0.6000 0.6135 reg u late d f e e dback vo ltag e 3 v fb t a = -40c t a 85c 0.585 0 0.600 0 0.615 0 v l i ne r e gu la ti on v l i ne re g / v in v in = 2.5v to 5.5v, i out = 10ma ? 0.10 0.20 %/v loa d re gul atio n v l o a dre g / i out i out = 10ma to 1200ma ? 0.50 ? %/a output voltage accuracy v fb v out v in = 2.5 to 4.2 v , i out = 10 to 1200ma -3 ? +3 % oscillator frequency f osc v fb = 0.6v 1.2 1.5 1.8 mhz startup t i me t s f r om enabl e to output reg u lati on ? 1.3 ? ms over-temperature shutdown threshold t sd ? ? 150 ? c over-temperature shutdown hysteresis t hys ? ? 15 ? c peak switch current i lim ? ? 2.5 ? a p-ch mosfet v in = 3.6v ? 135 200 n-ch mosfet r ds (on) v in = 3.6v ? 95 150 m ? enable threshold low v en(l) ? ? ? 0.3 v enable threshold high v en(h) ? 1.5 ? ? v input low current i en v in = v en = 5.5v -1.0 ? 1.0 a note : 1. the ect 3408 is guaranteed to meet pe rformanc e specifications o v er the -40 c to +85c op erating temperatu r e rang e and is assu r e d by de sig n , characterization, and correlation with statistical process controls. 2. v in should be not less than v out + v dropou t , w h ere v dropout = i out x (r ds(on)p m os + esr induct or ), typically v dro pout = 0.3v. 3. the r egulated feedback voltage is tested in an internal test mode that connects v fb to the o utput of the erro r amplifier.
e c t3408 1.2a, 1.5mhz step-down converter e-cmos corp. ( www .ecmos.com.tw ) page 5 of 17 2007/1 1 /19 t y pical performance characteristics
e c t3408 1.2a, 1.5mhz step-down converter e-cmos corp. ( www .ecmos.com.tw ) page 6 of 17 2007/1 1 /19 t y pical performance characteristics (continued)
e c t3408 1.2a, 1.5mhz step-down converter e-cmos corp. ( www .ecmos.com.tw ) page 7 of 17 2007/1 1 /19 functional description the ect34 0 8 is a high ou tput current m onolithi c sw itc h - m o de s t e p - d o w n dc -dc c o n v er ter . t h e devi c e ope rates at a fixed 1.5mhz switchin g fre quen cy , and use s slop e compen sa te d curre n t mod e archite c ture. this step do wn dc-dc conve r ter can supply up to 1200ma out put cu rre nt at v in = 3v a n d has an input volt age rang e fr o m 2 . 5 v to 5 . 5 v . it minimizes external com p o nent si ze an d optimizes ef fi cien cy at the heavy lo ad ra nge. t h e slop e com pen sation allows the de vice to remai n st able ove r a wide r ran g e of inductor v a lue s so that smalle r value s (1 h to 4.7 h) with lowe r dcr can be u s e d t o achieve hi g her ef ficie n cy . ap art from the small byp a ss i nput cap a citor , o n ly a small l-c filter is req u ired at the out put. the fixe d outp u t vers ion requi re s o n ly three external power co mpone nt s (c in , c out , and l). t he a d just able v e r s ion c an be p r og ramm ed w i t h ex t e r nal f eedba c k t o any v o lt age, ran g ing f r om 0. 6 v to near the input volt age. it use s i n ternal m o s f et s to a c hie v e hi gh ef ficie n cy and ca n gene rate very low output volt ages by usin g an internal refe ren c e of 0.6v . at dropout, the conv erte r du ty cycle incre a se s to100% and the outp u t voltage tracks the i n p u t volt age mi nus th e lo w r ds (on) d r op of the p-cha n nel hig h -side mosfet an d the indu cto r dcr. t h e i n t e r n a l e r r o r a m p l i f i e r a n d compensation p r o v i d e s e x c e l l e n t t r a n s i e n t r e sp on se, lo ad and line re gulation . internal sof t st art eliminates any output volt age over shoot whe n the enabl e or the input volt age is ap plied. current mode pwm control slope compe n sate d cu rre n t mode pwm control provides st able switching a nd cycl e-by-cycle cu rre nt limit for excelle nt loa d and li ne re spo n se an d p r otectio n of t he in te rnal m a in switch (p -cha nnel m o s f et) an d syn c hrono us rectifie r (n-ch annel m o sf et). du ring n o rmal ope rati on, the in te rn al p-chan nel mosfet is t u rne d on for a sp ecifie d time to ram p the indu ctor curre n t at ea ch risin g ed g e of the inte rnal o scill ator, and i s switched off when the pea k indu ctor curre n t is above th e error voltag e. the cu rren t compa r ato r , i co mp , limits the pea k ind u ctor curre n t. whe n the main s w it c h is off, the s y nc hronous rec t ifier turns on immediately and s t ay s on un til either the induc t or c u rrent s t art s to reverse, as indicate d by the curre n t re versal com p a r ator, i zero , or the beginni n g of the next clo ck cycle. control loop the ect3 40 8 is a pea k current mode step- do wn co nverter. the curre n t throu gh the p-cha nnel mosfe t (high si de ) is sen s e d for current loop control, as well as sho r t circ uit and ove r load p r ote c tion. a slope compen satio n sign al i s adde d to the sen s ed current to maintain stability for dut y cycles greate r than 50%. the pe ak current m ode loop appe ars a s a voltage-p r o g rammed cu rre n t sou r ce in p a rallel with th e output cap a c itor. th e out put of the voltage e r ror amplifier p r og ram s the current mode lo o p for the ne ce ssary pe ak switch cu rre nt to force a co n s tant output v o ltage for all load a nd l i ne condition s. internal l o op compe n sation termi n a t es the t r an con d u c tan c e voltage erro r amplifier output. for fixed voltage versio ns, the error a m plifie r refe ren c e v o ltage is i n ternally set to p r og ram the converte r output voltag e. for the adj ustabl e outpu t, the e rror a m plifier refe re nce i s fixed at 0.6v. soft start / enable sof t st art limit s the curre nt surge seen at the input a n d e l i m i n a t e s o u t p u t v o l t a g e o v e r s h o o t . t h e enabl e pin is active high. when p u lled low , the enable input (en) forces t h e ect3 408 into a low- po wer , non-swit ching st ate. the tot a l input current during shut do wn is less tha n 1 a.
e c t3408 1.2a, 1.5mhz step-down converter e-cmos corp. ( www .ecmos.com.tw ) page 8 of 17 2007/1 1 /19 current limit and over-temperature protection for overl oad con d ition s , the peak inp u t curre n t is limited to 2.5a. t o minimize p o we r dissipati on and stresses und er curre n t limit and sh ort-ci rcu i t condition s, swit chin g is term in ated after ente r ing cu rre nt limit for a se rie s of pu lse s . the t e rminat io n last s f o r sev e n con s e c ut iv e clo ck cy cle s af t e r a curre n t limit has be en se nsed d uring a se rie s of four con s e c ut iv e c l oc k cy cl es. therm a l protection com p l e tely disabl e s switchi ng whe n intern a l dissip a tio n becom es e x cessive. th e junctio n over-tem pe ra ture thre sh ol d is 150 c with 15 c of hyst ere s i s . once an over-tem pe rature or ove r -cu rre nt fault con d ition s is removed, the output volt age automatical ly recove rs. dropout operation whe n the bat tery input voltage de crea se s nea r the value of the outp u t voltage, the ect34 08 al lows the main switch to remain o n for more than one switching cycle an d increa se s the du ty cycle until it reach e s 1 0 0 % . the duty cycl e d of a step - down co nvert e r is defin ed as: whe r e t on is the main switch on time an d f osc is the oscillator frequency. the output voltage then is the input volt age minu s the voltage drop acro ss the main swit ch and the induct o r. at low input su pply voltage, the r ds (on) of the p-ch ann el m o sfet incre ase s an d t he efficien cy of the co nverte r d e c re a s e s . cautio n mu st be exe r ci sed to ensure th e heat di ssip ated do es not excee d the maximum jun c tion temp era t ure of the ic. maximum load current the ect340 8 will o perate with a n in put su pply voltage a s lo w a s 2.5v; however, the maximum lo ad curren t decrea s e s at lowe r inp u t due to the large i r d r o p on t he m a in switch an d syn c hrono us rectifie r. the sl op e comp en satio n sign al red u c e s the pea k inducto r cu rrent a s a fun c tion of the duty cycle to prevent sub - ha rmoni c oscillation s at duty cycles g r eate r t han 5 0 %. conversely, the curre n t limit increa se s as the d u t y cycle decre ase s . % 100 * % 100 * * in out osc on v v f t d =
e c t3408 1.2a, 1.5mhz step-down converter e-cmos corp. ( www .ecmos.com.tw ) page 9 of 17 2007/1 1 /19 applications inform ation
e c t3408 1.2a, 1.5mhz step-down converter e-cmos corp. ( www .ecmos.com.tw ) page 10 of 17 2007/1 1 /19 setting th e output v o ltage figure 1 sho w s the basic application circuit with the ec t3408 adj ust able o u tput version whil e figure 2 shows the application circuit with the ECT3408 fixed o u t p u t v e r s io n . f o r a p p l ic a t i o n s r e q u i r in g a n a d j u s ta b l e o u t p ut vo lt age , th e ECT3408 adjust able version can b e externally programmed. re si stors r1 and r2 in figure 1 progra m the output to regulate at a volt age higher than 0.6v . t o limit the bia s cu rr ent req u ired for the external feedback resisto r string while maint a ining good noise immunity , the minimum sugg ested value for r2 i s 59 k ? . al though a larger value will further reduce quiescent current, it will also increa se the im pedance of the feedback node, maki ng it more sensitive to external noise and interference. t able 1 summarize s the resi stor values for various output voltages with r2 set to either 59k ? for good noise immunity or 316k ? for red u ced n o load input current. the adj usta bl e versi on of t he ect 3408, combi ned wi th an exte rn al feed forward cap a cito r (c3 in figu re 1 ) , delivers enha nced tra n sie n t re spo n se fo r extre m e pul sed lo ad appli c atio ns. the a ddi tion of the feed forwa r d cap a cito r typically requi res a larger o u tput capa ci t or c2 for stab ility. the external re si st or sets the output voltage acco rding to the followin g equatio n: t able1. resistor sel e ctio n for out put voltage settin g ; standar d 1% r e sistor valu es substituted c l os est to the calc ulate d v a l u e s . v out (v) r1 =59 k r2 (k ? ) r1 =31 6 k r2 (k ? ) 0.8 19.6 105 0.9 29.4 158 1.0 39.2 210 1.1 49.9 261 1.2 59.0 316 1.3 68.1 365 1.4 78.7 422 1.5 88.7 475 1.8 118 634 1.85 124 655 2.0 137 732 2.5 187 1000 3.3 267 1430 ? ? o ? ? a 2 1 1 * 6 . 0 r r v v out 2 * 1 6 . 0 1 r v v r out ? ? o ? ? a ? 1 ? or
e c t3408 1.2a, 1.5mhz step-down converter e-cmos corp. ( www .ecmos.com.tw ) page 1 1 of 17 2007/1 1/19 inductor selection for mo st desi gns, the ect 3408 op erate s with indu cto r values of 1h to 4.7h. low indu ctan ce values are physi cally smalle r, but require faster swit chin g, wh ich re sults in some efficie n c y loss. the indu ctor value can be deriv ed from the followin g equatio n: whe r e i l is indu ctor ri ppl e cu rre nt. large value in du ctors lo we r ri pple current and small val ue indu cto r s result in high rip p le cu rre nts. ch oo se indu ctor ri p p le cu rrent ap proximately 3 0 % of the maximum load current 120 0m a, or i l = 360ma. for o u tput volt ages ab ove 2.0v , when li ght-loa d ef fici ency is import ant, the minimum recomm ende d indu ct or is 2.2 h manufa c turer' s spe c ificatio ns li st both th e indu cto r . dc cu rrent rating, which is a thermal limit a tion and the pea k cu rrent rating, which i s dete r mined by the saturatio n cha r a c teri stics. th e indu ctor should n o t sho w a n y appr eci able saturation u n der n o rm al l oad con d itio ns. some indu ctors ma y meet the pe ak and ave r a ge curre n t ratings yet result in excessive losse s due to a high dcr. always co nsi der the losse s asso ciated with the dcr and it s ef fe ct on the tot a l c onverte r ef fici ency whe n se lecting a n indu ctor . for optimum volt age- po sitioni ng load t r an si ent s, cho ose an indu cto r wi th dc serie s resi st ance in the 20m ? to 100m ? ra nge. f o r h i gher ef ficienc y at hea vy loads (above 200ma), or minimal load regulation (but som e transi ent oversho o t), the re sist a n ce sh ou ld be kept bel ow 1 00m ? . t he dc cu rre n t rating of the indu ctor shou ld be at least e qual to the maximu m load curre n t plus h a lf the ripple cu rre n t to prevent core satu ratio n (1200 ma + 36 0 m a) . t able 2 lists some typical surfa c e m oun t inductors th at meet targe t applicatio ns for the ect3 408. for exampl e, the 2.2 h sd31 18-2r2-r indu ctor sele ct ed fro m coilt roni cs h a s a 74m ? dcr a nd a 2.00adc c u r r e n t rating. at full load, the indu ctor dc lo ss is 106m w which give s a 5% loss in ef ficien cy for a 1200ma, 1.8 v output. table2. sugge sted component selection information manufac ture r part number induc t a nc e (h) max dc curre nt (a) dc r (m ? ) siz e ( mm) lxwxh ty p e sumida cdrh 2d1 4 np 2.2 1.5 75 3.2 x 3.2 x 1. 55 shie lde d sumida cdrh 3d1 4 3.3 1.35 80 4x4 x 1.5 shie lde d sumida cdrh 3d1 4 /h p 4.7 1.45 171 4x4 x 1.5 shie lde d coiltro nics sd31 18- 2r2- r 2.2 2.00 74 3.1 x 3.1 x 1. 8 shie lde d coiltro nics sd31 14- 2r2- r 2.2 1.74 11 0 3.1 x 3.1 x 1. 4 shie lde d coiltro nics sd31 18- 3r3- r 3.3 1.59 11 3 3.1 x 3.1 x 1. 8 shie lde d coiltro nics sd31 18- 4r7- r 4.7 1.31 162 3.1 x 3.1 x 1. 8 shie lde d manufac ture r part number va l u e v o lt age (v) te m p . c o . cas e murat a grm219 r60j 106ke 1 9 10f 6.3 x5 r 080 5 murat a grm21br 60j 226me 3 9 22f 6.3 x5 r 080 5 murat a grm155 1 x 1e 220jz 0 1b 22pf 25 jis 040 2 osc l in out in out f i v v v v l * * ) ( * '
e c t3408 1.2a, 1.5mhz step-down converter e-cmos corp. ( www .ecmos.com.tw ) page 12 of 17 2007/1 1 /19 input capacitor selection the input cap a citor reduces the surge current drawn from the inpu t and switching noise from the device. th e input cap acitor impedance at the switching frequency shoul d be l ess than the inpu t source impedance to prevent high frequency switching current p a ssing to th e input. t h e c a lc u l a t e d v a l u e v a r i e s w i th i nput voltage and i s a maximum whe n v in is double the o u tput voltage. a low esr input cap a citor size d for ma ximum rms curre n t must be use d . ceramic cap a cit o rs with x5r or x7r diele c trics a r e highly re co mmend ed be cau s e of their low esr an d small temp eratu r e coef fi cient s. a 22f ceramic cap a citor for most appli c ati ons i s suf f i c ie nt. a large va lue may be u s ed for im pro v ed input volt age filtering. the maximu m input cap a citor rms cu rrent is: the inpu t capacitor rms r i pple current varies with the input and output voltage a nd will a l ways be less than or equal to half of the to tal dc load cu rrent to minimi ze stray ind u ct a n ce, the cap a citor sho u ld b e pla c ed as closely a s p o ssible to th e ic. thi s keep s the high freque ncy co ntent of the input cu r- rent locali zed, mi nimizin g emi and inp u t volt age rip p le. a laboratory t e st set-u p typically con s ist s of two l ong wire s ru nnin g from the be n c h po we r sup p ly to the evaluation board i nput v o lt age pi ns. t he in du ct ance of these wires, alon g with the low-es r cerami c input cap a citor , ca n create a high q net - wo rk that may af fect converte r p e rf orma nce. this proble m of ten be com e s app arent in t he form of exce ssive rin g ing in the o u tput volt age durin g loa d trans i ent s. errors in the loo p phase and gain mea s u r e m ent s can als o result. since the in d u ct ance of a sho r t pcb tra c e feedi ng the input volt ag e is significa n t ly lower than the power le ads from the bench po wer supply , most appli c atio ns do not exh i bit this pro b l e m. in applications wh ere the input p o wer sou r ce lead ind u ct an ce cann ot be redu ced to a level that does not af fe ct the co nverte r perfo rman ce, a high esr t a n t a l u m or alumi num electrolytic sh ould be pla c e d in p a rallel with the low esr, esl byp a ss cerami c. this da mpe n s the high q netwo rk a nd stabili ze s the system. s o pp in o in o in f esr i v v v v v c * 1 * ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? = s o pp min in f esr i v c * 4 * 1 ) ( ? ? ? ? ? ? ? ? ? = 2 ) ( o max rmx i i =
e c t3408 1.2a, 1.5mhz step-down converter e-cmos corp. ( www .ecmos.com.tw ) page 13 of 17 2007/1 1 /19 output ca pacitor selection the fun c tion of output ca p a citan c e i s to store ene rgy to atte mpt to maintain a co nstant voltag e. the ene rg y is store d in the cap a cit o r?s el ect r ic fi eld due to the voltage appli ed. the value of output capa citan c e is ge nerally sel e ct ed to limit o u tput voltage ripple to the level require d by the spe c ification. since the ri pple current i n the output indu ctor i s u s ually dete r m i ned by l, v out and v in , the s e ries impeda nce of the ca pa citor prima r ily det ermin e s th e output vo ltag e rippl e. the three eleme n t s of the cap a citor that contri bute to its impedan ce (and outpu t voltage ripple) are equi v alent seri es resi stan ce (e sr), equival ent serie s indu ctan ce (e sl), and cap a citan c e (c). the output voltage dro op due to a load transie nt is dominated by the cap a cita n c e of the ceramic out- put capa citor. duri ng a step increa se in l oad current, the ce rami c o u tput cap a cit o r alo ne sup p lies the lo ad current u n til the loop respon ds. wi thin two swit chin g cycle s , the loop respond s and th e inducto r cu rre nt incre a ses to match the load curre n t demand. the relation ship o f the output voltage droo p during the two switchi ng cycle s to the output cap a cita nce can be e s timated by: in many practical desi g n s , to get the required esr, a cap a cito r with much more capa citan c e than is neede d must be sele ct ed. for b o th cont inuou s a nd d i scontinu o u s indu ctor cu rrent mod e op eration, th e esr of the c out n eed ed to limit the ripple to v o , v peak-to - pe ak is: rippl e curre n t flowing thro ugh a cap a cit o r?s esr cau s e s pow er di ssi pation in the cap a cito r. this po we r dissi pation cau s e s a tem peratu r e in crease intern al to the cap a citor . exce ssive te mp er a t u r e can seri ou sly sho r ten the expecte d life of a cap a citor . cap a cit o rs have rip p l ed curre n t ra ti ngs th at are depen dent o n ambie n t temperature an d sho u ld not be excee ded. the o u tput cap a citor ripple curre n t is the inducto r current, i l , minus the outpu t curre nt, i o . the rms value of the ripple current flowin g in th e output cap a cit ance (conti nuou s indu ct or cu rrent mo de op eratio n) is give n by: esl can be a problem by causi ng rin g in g in the low megah ert z re gion but can be co ntroll ed by choo sin g low esl cap a citors, limiting lead l ength (p cb and cap a cito r), and repla c ing one larg e device with several sm a ller one s connected in p a rallel. in con c lu sio n , in orde r to meet the requi rem ent of output volt age ripple small and re g u lation loop st ab i l i t y , c e r a m i c c a p a c i t o r s w i t h x 5 r o r x 7 r diele c trics are recomme n ded due to their lo w esr and high rip p le current rating s. the output rippl e v out is dete r mined by: a 22f cera mic cap a cito r can sati sfy most appli c atio ns. s droop load out f v i c * * 2 ' 289 . 0 * 6 3 ) ( l max rms i i ' l i v esr ' ' d 0 ? ? 1 ? d ' out osc osc in out in out out c f esr l f v v v v v * * 8 1 * * * *
e c t3408 1.2a, 1.5mhz step-down converter e-cmos corp. ( www .ecmos.com.tw ) page 14 of 17 2007/1 1 /19 slope compensation the ect34 0 8 step-d o wn conve r ter u s es pea k cu rrent mode co ntrol with a unique ada ptive slope co m pen sation scheme to m a intain stabilit y with lower value inductors fo r duty cycles greater than 50%. the slope com p ensation is set to maintain stability with lower value inductors wh ich provid e better overall efficien cy. the output inductor value must be sele cted so the i ndu ctor curre n t down slo p e meet s the i n ternal slop e compe n satio n requi rem e n t s. as an example, the value of the slope co mpe n sation is set to 1a /s whi c h i s large eno ug h to guarante e st ability when u s i n g a 2.2h indu ctor for all ou tput volt age levels from 0. 6v to 3.3v . the wo rst ca se external current slop e (m ) usin g the2 .2h indu ctor is whe n v out = 3.3v and i s : t o keep the p o we r su pply st able wh en the duty cycle is a bove 5 0 % , the intern al slo pe com pen sation (m a) sho u l d be: ther ef ore, t o guar ant ee cu rre nt loop st a b ilit y , t he slop e of t he com pen sat i on r a mp must be gr eater than one-half of the down sl o pe of the current waveform . so the internal slop e co mpen sated value of 1a/s will guarante e st ability usin g a 2.2 h indu ctor val ue for all outp u t volt ages from 0.6v to 3.3v . thermal calculation s there are three types o f lo sses associated with the ECT3408 step- down conver ter: switching losses, conduction losses , and qu iescent current losses. conductio n losses are associated with the r ds (o n) characteristics of the powe r ou tpu t switching de vices. switchin g losses are dominated by the gate charge of the po wer output switching. at full load, assuming continuous condu ction mode (c cm), a s implified form o f th e losses is g i ven by i q is the ste p - down converter quiescent current. the term t sw is us ed to estima te the fu ll loa d step-down converter swit ching loss es. for the condition where the step-down converter is in dropout a t 10 0% du ty cycle , the tota l de vice dissipatio n reduces to: since r ds (on ) , quiescent current and switc h ing losses all vary w i th inp u t vo lta ge, the tota l losses should be in ve stigated over the complete inpu t voltage range. g i ven the to tal lo sses, the maximum junction temperatur e can be der ived from the ja for the tdfn-10 pa ckage which is 45c/w. [ ] () ? + = * * * ) ( ) ( 2 in q hs dson o total v i r i p * * ) ( 2 + = + = * ) ( s a l v m out / 5 . 1 2 . 2 3 . 3 = = = / 75 . 0 * 2 1 =
e c t3408 1.2a, 1.5mhz step-down converter e-cmos corp. ( www .ecmos.com.tw ) page 15 of 17 2007/1 1 /19 s t ep-dow n converter design example
e c t3408 1.2a, 1.5mhz step-down converter e-cmos corp. ( www .ecmos.com.tw ) page 16 of 17 2007/1 1 /19 s t ep-dow n converter design example (continued)
e c t3408 1.2a, 1.5mhz step-down converter e-cmos corp. ( www .ecmos.com.tw ) page 17 of 17 2007/1 1 /19 mechanical dimensions outli ne d r a wing t d f n -1 0 dimensio n s inches mm dimn min max min max a 0.1 16 0.1 19 2.95 3.05 b 0.1 16 0.1 19 2.95 3.05 c 0.065 0.069 1.65 1.75 d 0.093 0.096 2.35 2.45 b 0.007 0.01 1 0.18 0.28 e 0.020 0.500 bsc l 0.014 0.018 0.35 0.45 a1 0.028 0.031 0.70 0.80 a2 0.000 0.004 0.00 0.10 a3 0.008 0.203ref

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