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1.5mhz , 1.2a, synchronous step - dow n regulator dropout ec3211 e - c m o s c o r p . ( ww w . e c m o s . c o m . t w ) p a g e 1 o f 16 3 i12 n - rev. f 001 general description the ec3211 is a high effic i ency monolithic synchronous buck regulator using a constant frequ e ncy, current mode architecture. the device is available in an adjustable version . supply curr e nt during o pe ra t ion is only 20ma and drops to 1ma in shutdown. the 2.5v to 5.5v input voltage ran g e makes the ec32 11 ideally suited for single li - ion battery - po we red a p plication s . 100% duty cycle provides low dro p out ope r ation, extend i ng batt e ry life in portable systems. automatic burst mode op e ratio n increases efficiency at light loads, f u rther ext e nd i ng battery life. switching frequency is internally set at 1.5mhz, all o wing the use of small surface mount inductors and cap a citors. the internal synchron o us switch increases efficiency and eliminates t h e need for a n exter n al schottky diode. low output volta g es are easily supported w ith the 0.6v feedback reference voltage. the ec3211 is avail a ble in t sot23 - 5 package. featur e s high efficie n c y : up to 96% high efficie n cy at light lo a ds very low qu iescent current: max 7 0 ua during operati o n 1. 2 a output cur r ent 2.5v to 5.5v input voltage ran g e 1.5mhz co ns tant frequency operati o n no schottky diode r equi r ed low d r op o ut op eration: 1 00% duty cycle 0.6v refer e nce allows l ow output voltages shutdown m ode d r aws 1ua supp l y cur r ent cur r ent mode operation f or excellent line and l o a d transient r e sponse over - tem p er a ture protected t sot23 - 5 package is a v ailable applicati ons cellular tel e pho n es personal information appliances wireless and dsl modems digital still cameras mp3 players portable instruments package types t s o t23 - 5 figu r e 1. p ac k age t yp e s of ec3211
1.5mhz , 1.2a, synchronous step - dow n regulator dropout ec3211 e - c m o s c o r p . ( ww w . e c m o s . c o m . t w ) p a g e 2 o f 16 3 i12 n - rev. f 001 pin name description 1 run run c o ntrol in put. forcing this pin above 1.5v enables the part. forcing this pin below 0 . 3v shuts down the d e vice. in shutdown, all functions are disabled dr a wing < 1 a supp l y current. do n ot leave run floatin g . 2 gnd ground pin. 3 sw switch node connection to inducto r. this pin connects to the drains of the internal main and synchron o us power mosfet switches. 4 vin main supply pin. must be closely decoupl e d to gnd, pin 2, with a 2. 2 f or g re ater ceramic capacitor. 5 vfb feed b ack pin. receives the feedback vo l tage fr om an external resistive divider across the output. 5 vout output voltage feed b ack pin. an internal resistive divider d i vides the output volta g e down for comparison to the intern a l reference voltage. pin assignments t s o t2 3 \ 5
1.5mhz , 1.2a, synchronous step - dow n regulator dropout ec3211 e - c m o s c o r p . ( ww w . e c m o s . c o m . t w ) p a g e 3 o f 16 3 i12 n - rev. f 001 orderi n g information part number package marking marking information ec 3211 adj t2r tsot23 - 5 1 1 aj f 1. starting with underlined 1 , a bar is for production year 2012. the ne xt bar is mark on top of a is f or year 2013. the next bar is mark on bottom of a is for year 2014. the next bar is mark on top of j is year for 2015 . the naming pattern continues with consecutive characters for later years. 2. aj functional block diagram figu r e 2 : f u nctional block diag r am of ec3211
1.5mhz , 1.2a, synchronous step - dow n regulator dropout ec3211 e - c m o s c o r p . ( ww w . e c m o s . c o m . t w ) p a g e 4 o f 16 3 i12 n - rev. f 001 type applicatio n circuit figu r e 3. t y p e application circuit of ec32 1 1 absolute maximum ratings parameter value unit input supply voltage - 0.3 ~ 6 v run, vfb voltages - 0.3 ~ vin v sw voltage - 0.3v ~(vin+0.3) v p - cha n nel switch source curr e nt (d c ) 1 500 ma n - c h an n el switch sink curr e nt (dc) 1500 ma peak sw sink and sou r ce current 1.8 a operati n g temperatu r e r ange - 40~ + 85 oc junction temperatu r e 125 oc lead tem pe rature (sol de ring, 10 sec) 260 oc storage te m peratu r e ra n ge - 65~ 1 50 oc n o te 1 : str e s s es g re a ter t h an t h o s e l ist e d u n d e r ma x imum r a ti n g s m a y c au s e p e r m an e nt d a ma g e to t h e de v ice. t h i s i s a s tr es s ra t i n g o n ly and f u n c ti o n a l o p e r at io n of t h e d e v ice at th ese or any oth e r c on d it i o n s a b o v e t h ose i n di c a t ed i n t h e o p e r at i on i s n o t impl i ed. e x pos u re to a b s o l u te ma x i m u m r a t i n g c o n d i t i o n s f o r e x t en ded pe r i o ds m a y a f f e ct re l i a b il i t y .
1.5mhz , 1.2a, synchronous step - dow n regulator dropout ec3211 e - c m o s c o r p . ( ww w . e c m o s . c o m . t w ) p a g e 5 o f 16 3 i12 n - rev. f 001 electrical characteristics ( v in =3.6v, t a =25c , unless otherwise specified ) parameter symbol conditions min typ max unit feedback current infb --- --- 30 na regulator feedback voltage v fb t a =25 0.5880 0.6000 0.6120 v 0 Q t a Q 85 0.5865 0.6000 0.6135 v - 40 Q t a Q 85 0.5850 0.6000 0.6150 v reference voltage line regulator vfb vin=2.5v to 5.5v --- 0.04 0.4 %/v peak inductor current i pk vin=3v,vfb =0.5v or vout=90%,duty cycles <35% 1.55 1.6 1.7 a output voltage load regulator v loadreg ------ --- 0.5 --- % input voltage range vin 2.5 --- 5.5 v input dc bias current active mode is vfb=0.5v or vout=90%, i load =0a --- 300 400 ua sleep mode vfb= 0.62v or vout=103%, i load =0a --- 45 70 ua shut down vrun=0v,vin=4.2v --- 0.1 1 ua oscillator frequency f osc vfb=0.6v or vout=100% 1.2 1.5 1.8 mhz vfb=0v or vout=0v --- 400 --- khz rds(on) of p - channel fet rpfet i sw =100ma --- 0.3 0.4 rds(on) of n - channel fet rnfet i sw = - 100ma --- 0.3 0.4 sw leakage ilsw vrun=0v,vsw=0v or 5v, vin=5v --- 0.01 1 ua run threshold vrun 0.3 1 1.5 v run leakage current irun --- 0.01 1 ua
1.5mhz , 1.2a, synchronous step - dow n regulator dropout ec3211 e - c m o s c o r p . ( ww w . e c m o s . c o m . t w ) p a g e 6 o f 16 3 i12 n - rev. f 001 typical pe r formance characteristics refe r ence v oltage o scillator frequen c y oscillator frequen c y vs supp l y voltage rds(o n ) vs temperature
1.5mhz , 1.2a, synchronous step - dow n regulator dropout ec3211 e - c m o s c o r p . ( ww w . e c m o s . c o m . t w ) p a g e 7 o f 16 3 i12 n - rev. f 001 typical performance characteristics(cont.) rds(on) vs input voltage efficien c y vs output current efficien c y vs output current efficien c y vs output curr ent
1.5mhz , 1.2a, synchronous step - dow n regulator dropout ec3211 e - c m o s c o r p . ( ww w . e c m o s . c o m . t w ) p a g e 8 o f 16 3 i12 n - rev. f 001 typical performance characteristics( c o nt.) efficien c y vs output current output vol t age vs out p ut current efficien c y vs input v o ltage d y namic supp l y curre n t vs supp l y voltage
1.5mhz , 1.2a, synchronous step - dow n regulator dropout ec3211 e - c m o s c o r p . ( ww w . e c m o s . c o m . t w ) p a g e 9 o f 16 3 i12 n - rev. f 001 typical pe r formance characteristics ( c o nt.) p - fet leakage vs temperature n - fet lea k age vs temperature
1.5mhz , 1.2a, synchronous step - dow n regulator dropout ec3211 e - c m o s c o r p . ( ww w . e c m o s . c o m . t w ) p a g e 10 o f 16 3 i12 n - rev. f 001 function description main control loop the ec3211 uses a cons t ant frequ e ncy, current mode step - down architecture. both the main (p - chan n el mosfet) and synch r onous ( n - chan n el mosfet) switch es are internal. during no r mal operat i on, the internal top pow e r mosfet is turned on each cycle when the oscillator sets t he rs latch, and turned off when the curr e nt com p arator, icomp, resets the rs latch. the peak inductor current at w hich icomp resets the rs latch, is controlled by the output of error amplifier ea. when the load curr e nt increases, it causes a slight decrease in the fe e dback voltage, fb, relative to t h e 0 . 6v reference, which in turn, causes the ea amplifiers output volt ag e to increase until the avera g e inductor curr e nt matches the n e w load current. while the top mosfet is off, the b ottom mosfet is turned on until either the inductor current starts to reverse, as indic a ted by t h e current re versal comparator i rcmp, or the beg i nning of the next clock c y cle. burst m o de operation the ec3211 is capable of b u rst mode ope r ation in which the internal p ower mosfets operate intermittently based on lo a d demand. in burst mode ope r ation, the peak cur r ent of the inductor is set to appro ximately 2 0 0ma regard l ess of the output load. each burst event can last from a few cycles at light loads to almost continuously cycling with short sl e ep intervals at moderate lo a d s. in between these bu r st events, the power mosfets and any unne e ded circuitr y are turned off, reducing t h e quiescent current to 20ma. in this sleep st a te, the load curr e nt is being supplied solely from the output capacitor. as the output voltage d r oops, the ea amplifiers output r i ses above the sleep thr e shold signaling the burst comparator to trip and turn t h e top mosfet on. this process repeats at a rate that is dep e nd e nt on the load d e mand. short - circuit protection when the o u tput is shorted to ground, the frequen c y of the oscillator is reduced to about 400 k hz, 1/4 the nomin al fre q uency. this frequency fold - back ensures that the inductor current has more time to decay, ther e by preventing runaway. the oscillators f r equency will pro g ressively increase to 1.5mhz when vfb or vout rises above 0v. dropout operation as the inpu t supply volta g e decreases to a value app r oaching the output v o ltage, the duty cycle increases toward the m aximum on - t i me. further reduction of the supply volta g e forces the main switch to remain on for more than o ne cycle until it reaches 1 00% duty cycle. the output voltage will then be determined by the input voltage min u s the voltage drop across the p - chan n el mosfet a n d the inductor. an important detail to reme m ber is that at low input supply volta g es, the rds(on) of the p - channel switch increases (see t ypical per f ormance cha r acteristics). therefo r e, the user should calculate the po w er dissipation when the ec3211 is used at 100% duty cycle with low inp u t voltage (see thermal c onsideratio n s in the applications information section).
1.5mhz , 1.2a, synchronous step - dow n regulator dropout ec3211 e - c m o s c o r p . ( ww w . e c m o s . c o m . t w ) p a g e 11 o f 16 3 i12 n - rev. f 001 function description( cont.) low supply ope r ation the ec3211 will operate with input supply voltages as low as 2.5v, but the maximum allow a ble output current is reduced at this low voltage. figure 4 shows the re d uction in the maximum output current as a function of inpu t voltage for various output voltages. slope compensation and i n ductor peak current slope compensation prov i des stability in constant frequ e ncy a r chitectures b y pr e v enting subharmonic oscillations a t high duty c y cles. it is accomplished internally b y adding a c o mpensating r amp to the inductor current sign a l at duty cycles in excess of 40%. normally, this results in a reduction of maximum inductor pe a k current for duty cycles > 4 0 % . howev e r, the ec3211 uses a patent - p en ding scheme t hat counteracts this compensating ramp, which allows the maximum inductor peak current to remain u n affected through o ut all duty c y cles. figure 4. maximum o ut put current vs i npu t voltag e the basic ec3211 appli c ation circuit is shown in figure 3. external compon e nt selection is driven by the lo a d requi r ement and b e gins with the selection of l followed by cin and cout. inductor selection for most applications, the value of the inductor will fall in the ra n ge of 1u h to 4.7 u h. its value is chosen b a sed on the desired r i pple current. lar g e val u e inductors lower ripple curr e nt and small value inducto r s result in higher ripple curr e nts. higher vin or vout also increases the ripple curr e nt as shown in equation 1. a reasonab l e starting point for setting ripple current i s dil = 480 ma (40% of 12 0 0ma). the dc cur r ent rating of the inductor should be at least equal to the maximum load current p l us half the ripple current to p r event core saturation. t h us, a 1320ma rated inductor should be en o u g h f o r most a p plications (12 0 0ma + 120ma). for better efficie n cy, choose a low dc - r esistance inductor. the inductor value also h as an effect on burst m o de ope r ation. the transition to low curre n t operation b egins when the in d uctor current peaks fall to appr o x imat e ly 200ma. lower inductor values (h igher dil) will cause this to occur at lower load currents, which can cause a dip in efficie n cy in the upper ran g e of low current ope r ation. in burst mode operation, lower inducta nc e values will cause the bu r st frequency to increase.
1.5mhz , 1.2a, synchronous step - dow n regulator dropout ec3211 e - c m o s c o r p . ( ww w . e c m o s . c o m . t w ) p a g e 12 o f 16 3 i12 n - rev. f 001 function descriptio n(cont.) inductor cor e selection different core materials and shapes will change the size/current and price/cu r rent relation s hip of an inductor. toroid or sh i elded pot co r es in ferrite or permalloy materials are small and d ont radiate much ener gy , but gen e rally c o st more than pow d er e d ir o n core inductors with similar e lectrical characteristics. the choice of which style inductor to use often depe n ds more on the price vs size requir e ments and any rad i ated field/emi requi r ements than on wh a t the ec32 11 requires to ope r ate. table 1 shows some typical surface mount inductors that work w e ll in ec3211 a pp lications. table 1. repr e sentati v e sur f ace mount i n du ctors cin and cout selection in continuous mode, the s ource curr e nt of the top mosfet is a s quare w a ve of duty cycle vout/vin. to prevent large voltage tra n sients, a low esr input capacitor sized for the maximum rms current must be used. the maximum rms capacitor curr e nt is given by: this formula has a maxim u m at vin = 2vout, wh e re irms = io u t/2 . this simple worst - case condition is commonly used for design because even significant de v iations do not offer m uch relief. note that the capacitor manufacturers ripp l e current ratings are often based on 2 0 00 ho u rs of life. this makes it advisable to fur ther de r ate the capacitor, or choose a capacitor rated at a higher t e mperatu r e than re q uired. always consult the manufacturer if there is any question. the selecti o n of cout is driven b y the requi r ed e f fective series resistance (esr). typically, o n ce the e sr requi r ement for cout h a s been met, the rms current rating ge n e r ally far exc e eds the iripple(p - p) requi r ement. the output ripple dvo u t is determi n ed by: whe r e f = operating fre qu ency, cout = output capacitance and dil = ri p ple cur r ent in the inductor . for a fixed output voltage, the output ripple is highest at maximum input voltage since dil increases with input voltage. aluminum electrolytic and dry tantal u m capacitors are both avail a b l e in surface m ount configurations. in the case of tantalum, it is cri t ical that the capacitors are surge tested for use in switching power supplies. an excellent choice is the avx tps series of surface mount tantalum. these are specially constructed and tested for low esr so they give the lowest esr for a given v o lume. other capacitor types include sanyo poscap, k e met t510 and t 4 95 series, and sprag u e 593d a n d 59 5 d series. consult the manufacturer for other specific recommend a tions.
1.5mhz , 1.2a, synchronous step - dow n regulator dropout ec3211 e - c m o s c o r p . ( ww w . e c m o s . c o m . t w ) p a g e 13 o f 16 3 i12 n - rev. f 001 function description(cont.) using ceramic in p ut and output capacitors hig her valu es , lower cost ceramic ca p acitors are now becoming available in smaller case sizes. their high ripple curr e nt, high volta g e rating and l ow esr make them ideal for switching r egulator a pp l ications. because the ec3211 s control lo o p does not depe n d o n the output capacitors esr for stable operation, ceramic capacitors can be used freely to achieve very l o w output ripple and small circuit size. how e ver, c a re must be t a ken when c e ramic capacitors are used at the input and the output. when a cer a mic capa citor is used at the input and the pow e r is su p plied by a wall ad a pter throu g h l ong wires, a load step at the output can induce ringing at the input, vin. at best, this ringing can couple to the ou t put and b e mistaken a s loop instability. at worst, a sudde n inrush of current through the long wir e s can potentially cause a voltage spike at vin, large enough to da m age the part. when choos i ng the input and ou t put ceramic capacitors, choose the x5r or x7r d i ele c tric formulations. these dielectrics h a ve the be s t temperat u re and volt a ge characteristics of all the ceramics for a given va l ue and size. output vol t a ge programming in the adjustable v e rsion, the output voltage is set by a resistive divider accordi n g to the foll o wing formul a : the exter n al resistiv e divider is conn e cted to the o utput, allowing r emote voltage sen s ing as shown in figu r e5 . figure 5 :s e tting the o u t pu t vo l t a g e v out r1 r2 1.2v 150k 150k 1.5v 160k 240k 1.8v 150k 300k 2.5v 150k 470k 3.3v 150k 680k t able 2. v o ut v s. r1, r2, cf s e l e ct t a b l e efficiency considerations the efficiency of a switching re g ulator is equal to t h e output po w er divided by t h e input p o w e r times 100%. it is often useful to analyze i n divid u al losses to determine what is limiting the eff i ci e ncy and whi c h ch ange w o uld pro d uce the most impro v ement. efficiency can be exp r essed as: eff i ciency = 100% C (l1 + l2 + l3 + ...) whe r e l1, l 2 , etc. are the individ u al losses as a percenta g e o f input powe r . although all dissipative elements in t h e circuit produce losses, two main sources usu a lly account for most of the losses in ec3211 circuits: vin quiescent current and i2r losses. the vin quiesc e nt current loss dominates the efficiency loss at very low load currents wher e as the i2r loss dominates the efficiency loss at m edium to hi g h load currents. in a t y pical eff i c i ency plot, the efficiency curve at very low l o ad currents can be misleading since the actual power lost is o f no consequen c e as illustrated in figure 6 .
1.5mhz , 1.2a, synchronous step - dow n regulator dropout ec3211 e - c m o s c o r p . ( ww w . e c m o s . c o m . t w ) p a g e 14 o f 16 3 i12 n - rev. f 001 function description(cont.) figure 6 : pow er l o st v s l o ad c u r r e n t 1. the vin q uiescent current is due to two components: the dc bias current as g i ven in the e l ectrical characteristics and the i n ternal main switch and synchron o us switch gate charge curr e nts. the gate charge curr e nt results from switch ing the gate capacitance of the intern a l power mosfet switches. each time the gate is switched from high to low to high again, a packet of charg e , dq, moves from vin to gro u nd. the resulting dq/dt is the current out of vin that is typically l a rger than the dc bias current. in c o ntinuous mode, igate c hg =f(qt + qb) whe r e qt a n d qb are the gate charg e s of the internal top and bott o m switches. both the dc bias and gate ch a rge losses are pro p ortio n al to vin and thus their effects will be m o re pronounced at highe r supply volta g es. 2. i2r losses are calculated from the resistances of the internal switches, rsw, a nd exte r nal inductor rl. in continuous mode, t he avera g e output current flowing throu g h ind u ctor l is choppe d betw e en the main switch and the synchro n ous switch. thus, the series resistance looking into the sw pin is a function of both top and bottom mosfet rds(on) and the d uty cycle (dc) as follows: rsw = (r d s(on)top) ( dc) + (rds ( on)bot)(1 C dc) the rds( o n) for both the top and bo t tom mosfets can be ob tained from the typical performance cha r ac teristics curves. t h us, to obtain i2r losses, simply add rsw to rl and multiply the result by the squa r e of the ave r age output curr e nt. other losses including cin and cout e sr dissipati v e losses and inductor core l osses generally account for less than 2% total additional loss. thermal considerations in most applications t he ec3211 does not dissipate much heat due to its high eff i ciency. but, in applications whe r e the ec3211 is running at high ambient temperature with low supply volta g e a nd high duty cycles, such as in dropou t , the heat dissipated may exceed the maximum j u nction temperature of the part. if the junction temperat u re r eaches app r oximately 1 5 0c, both power switches will b e turned off and the sw node will become high impedance. to avoid the ec3211 from exceeding t he maximum junction temperatu r e, the user will need to do some thermal ana l ysis. the goal of the thermal analysis is to determine w hether the p ower dissipated exce e ds the maximum j u nction temp erature of the part. the temperature rise is given by: tr = (pd)( q ja) whe r e pd is the pow e r dissipated by the re g ulator and qja is the t h ermal resistance from the j u nction of t h e die to the ambient temperature. the j u nction temperature, tj, is given by: tj = ta + tr whe r e ta is the ambient temperat u re. as an example, consider t he ec3211 in dro p out at an input voltage of 2.7v, a load current of 800ma and an ambient temperatu r e of 70c. from the typical perform a nce graph of switch resistance, the rds(on) of t he p - chan n el switch at 70 c is appr o x imately 0. 5 2w.
1.5mhz , 1.2a, synchronous step - dow n regulator dropout ec3211 e - c m o s c o r p . ( ww w . e c m o s . c o m . t w ) p a g e 15 o f 16 3 i12 n - rev. f 001 function description(cont.) therefo r e, pow e r dissipated by the p art is: pd = iload 2 ? rds(on) = 187.2 m w for the t sot - 23 package, the q j a is 250c/ w. thus, the j u nction temperatu r e of the re g ulator is: tj = 70c + (0.18 7 2)( 2 5 0 ) = 116.8c which is below the maxim u m junction temperat u re of 125c. note that at higher su p p ly voltages, the j u nction temperature is lower due to reduced switch resistance (rds( o n)). checking transient respo n se the regu l a t o r loop respo n se can be c h ecked by lo ok ing at the load transient response. switc h ing regu l ato r s take several cycles to respond to a step in load current. when a load step occurs, vout i mmediately shifts by an amount eq ua l to ( iload ? esr), whe r e esr is t he effective series resistance of cout. iload also begins to charge or discharge c out, which gen e rates a feedback er r or signal. the regulat o r loop then acts to return vout to its steady state value. during this recov e ry time vout can be monitored for over sho o t or ringing that would indica t e a stability p roblem. for a detailed exp l anation of swi t ching control loop theo r y. a second, more seve r e trans i ent is caused by switching in loads with large ( > 1 f ) supply bypass capacitors. the discharged b y pass capacit ors are effectively put in parallel with cout, causing a rapid dr op in vout. no regulat o r can deliver e n o u gh current to prev e nt this problem if the load switch resistance is low and it is driv e n quickly. the only solution is to l i mit the r i se t i me of the s witch drive so that t h e load rise time is limited to app r oximate l y (25 ? cl o ad).thus, a 1 0 f capacitor charging to 3 .3v would r e quire a 2 5 0 s rise t i me, l i miting the charging current to about 130ma.
1.5mhz , 1.2a, synchronous step - dow n regulator dropout ec3211 e - c m o s c o r p . ( ww w . e c m o s . c o m . t w ) p a g e 16 o f 16 3 i12 n - rev. f 001 package information t s o t23 - 5 p acka g e outline dimensions

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