power management 1 www.semtech.com SC2612E 500khz step-down dc/dc converter features applications revision: october 12, 2004 typical application circuit description the SC2612E is a voltage mode switcher designed for low cost, ?point of use? voltage conversion. SC2612E is available with fixed switching frequencies of 500khz. the SC2612E has soft start and enable functions and is short circuit protected. the output of the switcher may be set anywhere between 0.8v and 75% of vin. short circuit protection is disabled during start-up to allow the output capacitors time to fully charge. operating frequency of 500khz input supply of 4.5v to 15v 0.5a drive current for up to 10a output output voltages down to 0.8v overcurrent protection and soft start so-8 package q3 r6 l1 r2 u2 SC2612E 2 5 6 1 3 8 7 4 vcc dh bst fb dl ss/en comp gnd c2 r3 r1 r9 c7 c9 c3 r10 c1 1.5v out q2 12v in 5v in c10 c5 graphics ic power supplies embedded, low cost, high efficiency converters
2 ? 2004 semtech corp. www.semtech.com power management SC2612E absolute maximum ratings r e t e m a r a pl o b m y sm u m i x a ms t i n u e g a t l o v y l p p u s c c vv cc 8 1v e g a t l o v n i p t s o o bv bst 0 2v d n g o t l d ) 1 ( d n g o t h d , ) 1 ( v dlo v , dhi 0 2 + o t 1 -v t ( e s l u p e v i t a g e n d n g o t h d e s l u p ) s n 0 1 3 ? 2004 semtech corp. www.semtech.com power management SC2612E notes: (1) see gate resistor selection recommendations. (2) 1square inch of fr4, double sided, 1oz. minimum copper weight. (3) guaranteed by design, not tested in production. electrical characteristics unless specified: v cc = 4.5v to 12v; v fb = v o ; bst = vcc+5v; t a = 0 to 70c r e t e m a r a pl o b m y ss n o i t i d n o cn i mp y tx a ms t i n u e c n a t c u d n o c s n a r t r e i f i l p m a r o r r e ) 3 ( g m 8 . 0s m n i a g r e i f i l p m a r o r r e ) 3 ( a a e r p m o c n e p o =5 4b d t n e r r u c k n i s / e c r u o s r e i f i l p m a r o r r e 0 6 a n i a g r o t a l u d o m ) 3 ( a m v c c v 5 =9 1b d e m i t d a e d 0 5s n
4 ? 2004 semtech corp. www.semtech.com power management SC2612E block diagram pin configuration ordering information pin descriptions note: (1) only available in tape and reel packaging. a reel contains 2500 devices. (2) lead free product. this product is fully weee and rohs compliant. s r e b m u n t r a p ) 1 ( y c n e u q e r fe g a k c a p t r t s e 2 1 6 2 c s ) 2 ( z h k 0 0 58 - o s ss/en gnd comp dh bst dl vcc 1 2 3 4 top view ( so-8 ) 5 6 7 8 fb # n i pe m a n n i pn o i t c n u f n i p 1b f. t u p n i k c a b d e e e f n o i t c e s r e h c t i w s 2c c v. e g a t l o v t u p n i y l p p u s p i h c 3l d. t u p t u o e v i r d t e f e d i s w o l r e h c t i w s 4d n g . s e n i l e d i u g t u o y a l e e s , e n a l p d n u o r g o t y l t c e r i d t c e n n o c , d n u o r g r e w o p d n a g o l a n a 5h d. t u p t u o e v i r d t e f e d i s h g i h r e h c t i w s 6t s b. s e v i r d t e f r o f e g a t l o v y l p p u s 7p m o c. r e i f i l p m a r o r r e e g a t l o v n o i t c e s r e h c t i w s e h t f o t u p t u o 8n e / s s . e t a r p m a r e g a t l o v t u p t u o r e h c t i w s e h t s l o r t n o c , n i p e l b a n e d n a t r a t s t f o s dh ss/en comp + - synchronous mosfet drive dl r s q level shift and high side drive + - uvlo bst os cil lat or + - + - 25ua vref ssover vcc vref uvlo & ref fb shdn s hoot -t hru cont rol + - r s q gnd
5 ? 2004 semtech corp. www.semtech.com power management SC2612E theory of operation the SC2612E is a step down dc/dc controller designed for minimum cost and size without sacrificing accuracy and protection. overcurrent protection is implemented by a simple undervoltage detection scheme and is dis- abled until soft start has been completed to eliminate false trips due to output capacitor charging. the ss/en pin is held low, as are the dh and dl pins, until the undervoltage lockout points are exceeded. once the vcc and bst pins both rise above their undervoltage lockout points, the ss capacitor begins to charge, controlling the duty cycle of the switcher, and therefore slowly ramping up the switcher output voltage. once the ss capacitor is charged, the current limit circuitry is enabled. if a short circuit is applied , the output will be pulled down below it?s trip point and shut down. the device may be restarted by either cycling power, or momentarily pulling ss/en low. component selection output inductor output inductor output inductor output inductor output inductor - a good starting point for output filter component selection is to choose an inductor value that will give an inductor ripple current of approximately 20% of max. output current. inductor ripple current is given by:- osc in o o ripple l f l v v v i 1 so choose inductor value from:- osc o in o o f i v v v l 1 5 output cap output cap output cap output cap output cap a a a a a cit cit cit cit cit or(s) or(s) or(s) or(s) or(s) - the output capacitors should be selected to meet output ripple and transient response criteria. output ripple voltage is caused by the inductor ripple current flowing in the output capacitor?s esr (there is also a component due to the inductor ripple current charging and discharging the output capacitor itself, but this component is usually small and can often be ignored). given a maximum output voltage ripple requirement, esr is given by:- osc in o ripple o esr f l v v v v r 1 output voltage transient excursions are a function of load current transient levels, input and output voltages and inductor and capacitor values. capacitance and r esr values to meet a required tran- sient condition can be calculated from:- release) (load transients positive for v v and n) applicatio (load transients negative for v v v where v v i l c i v r o a o in a a t t t t esr 2 2 values for positive and negative transients must be cal- culated seperately and the worst case value chosen. for capacitor values, the calculated value should be doubled to allow for duty cycle limitation and voltage drop issues.
6 ? 2004 semtech corp. www.semtech.com power management SC2612E compensa compensa compensa compensa compensa tion components tion components tion components tion components tion components - once the filter com- ponents have been determined, the compensation com- ponents can be calculated. the goal of compensation is to modify the frequency response characteristics of the error amplifier to ensure that the closed loop feedback system has the highest gain and bandwidth possible while maintaining stability. a simplified stability criteria states that the open loop gain of the converter should fall through 0db at 20db/ decade at a frequency no higher than 20-25% of the switching frequency. this objective is most simply met by generating asymp- totic bode plots of the small signal response of the vari- ous sections of the converter. l vout co SC2612E and fets fb out comp ra modulat or ref + - ea rb resr zp zf zs it is convenient to split the converter into two sections, the error amp and compensation components being one section and the modulator, output filter and divider be- ing the other. first calculate the dc filter+modulator+divider gain the dc filter gain is always 0db, the modulator gain is 19db at 5v in and is proportional to vin, so modulator gain at any input voltage is. 5 20 19 in mod v log g the divider gain is given by 8 5 8 20 r r r log g div so the total filter+modulator+divider dc gain is b a b in fmd r r r log v log g 20 5 20 19 calculate the filter double pole frequency (fp(lc)) lco ) lc ( fp 2 1 and calculate esr zero frequency (fz(esr)) sr re co ) esr ( fz 2 1 choose an open loop crossover frequency (fco) no higher than 20% of the switching frequency (fs). the proximity of fz(esr) to the crossover frequency fco determines the type of compensation required, if fz(esr)>fco/4, use type 3 compensation, otherwise use type 2. type 1 compensation is not appropriate and is not discussed here. type 2 example as an example of type 2 compensation, we will use the evaluation board schematic. 3.3uh vout 3000uf SC2612E and fets fb out comp 6.98k modulat or ref + - ea 8.06k 22mohm cs cp rs vin=5v the total filter+modulator+divider dc gain is db . . . . log log g fmd 6 13 06 8 98 6 06 8 20 5 5 20 19 this is drawn as the line a-b in fig2 khz . . lco ) lc ( fp 6 1 10 3000 10 3 3 2 1 2 1 6 6 this is point b in fig2. khz . ) esr ( fz 4 2 10 22 10 3000 2 1 3 6 this is point c in fig2., the line joining b-c slopes at - 40db/decade, the line joining c-d slopes at -20db/de- cade. for 500khz switching frequency, crossover is designed for 100khz. since fz(esr)< 7 ? 2004 semtech corp. www.semtech.com power management SC2612E having plotted the line abcd, and confirmed the type of compensation necessary, compensation component val- ues can be determined. at fco, the line abcd shows a gain of -27.5db and a slope of -20db/decade. in order for the total open loop gain to be 0db with a -20db/decade slope at this frequency, the compensated error amp gain at fco must be +27.5db with a 0db slope. this is the line fg on the plot below. since open loop dc gain should be as high as possible to minimize errors, a zero is placed at f and to minimize high frequency gain and switching interference a pole is placed at g. the zero at f should be no higher than fco/4 and the pole at g no lower than 4*fco. the equations to set the gain and the pole and zero locations are: db) (in fco at gain a where gm rs a 20 10 rs fz cs 1 2 1 rs fp cp 1 2 1 for this example, this results in the following values. k k . . rs . 30 6 29 8 0 10 20 5 27 nf . cs 22 0 10 30 10 25 6 1 3 3 ) rolloff ea to due y (unecessar pf cp 14 10 30 10 400 6 1 3 3 fco fz(esr) fp(lc) -60 -40 -20 0 20 40 60 80 100 100.0e+0 1.0e+3 10.0e+3 100.0e+3 1.0e+6 frequency (hz) gain (db) a b c d e f g h fp1 fz1 filter+modulator +divider gain compensated error amp gain total open loop gain fig2: type 2 error amplifier compensation
8 ? 2004 semtech corp. www.semtech.com power management SC2612E layout guidelines careful attention to layout requirements are necessary for successful implementation of the SC2612E pwm con- troller. high currents switching at high frequency are present in the application and their effect on ground plane voltage differentials must be understood and minimized. 1). the high power parts of the circuit should be laid out first. a ground plane should be used, the number and position of ground plane interruptions should be such as to not unnecessarily compromise ground plane integrity. isolated or semi-isolated areas of the ground plane may be deliberately introduced to constrain ground currents to particular areas, for example the input capacitor and bottom fet ground. 2). the loop formed by the input capacitor(s) (cin), the top fet (q1) and the bottom fet (q2) must be kept as small as possible. this loop contains all the high current, fast transition switching. connections should be as wide and as short as possible to minimize loop inductance. minimizing this loop area will a) reduce emi, b) lower ground injection currents, resulting in electrically ?cleaner? grounds for the rest of the system and c) minimize source ringing, resulting in more reliable gate switching signals. 3). the connection between the junction of q1, q2 and the output inductor should be a wide trace or copper region. it should be as short as practical. since this con- nection has fast voltage transitions, keeping this con- nection short will minimize emi. the connection between the output inductor and the output capacitors should be a wide trace or copper area, there are no fast voltage or current transitions in this connection and length is not so important, however adding unnecessary impedance will reduce efficiency. l 12v in cout 10uf u1 SC2612E 4 7 8 3 6 2 1 5 vcc dh bst fb dl ss/en comp gnd q1 10 vin vout gnd 0.1uf cin q2
9 ? 2004 semtech corp. www.semtech.com power management SC2612E layout guidelines (cont.) 4) the output capacitor(s) (cout) should be located as close to the load as possible, fast transient load cur- rents are supplied by cout only, and connections between cout and the load must be short, wide copper areas to minimize inductance and resistance. 5) the SC2612E is best placed over a quiet ground plane area, avoid pulse currents in the cin, q1, q2 loop flowing in this area. pgndh and pgndl should be returned to the ground plane close to the package. the agnd pin should be connected to the ground side of (one of) the output capacitor(s). if this is not possible, the agnd pin may be connected to the ground path between the out- put capacitor(s) and the cin, q1, q2 loop. under no cir- cumstances should agnd be returned to a ground in- side the cin, q1, q2 loop. 6) vcc for the SC2612E should be supplied from the 5v supply through a 10 ? resistor, the vcc pin should be decoupled directly to agnd by a 0.1 f ceramic capaci- tor, trace lengths should be as short as possible. vout vin + + currents in power section
10 ? 2004 semtech corp. www.semtech.com power management SC2612E typical characteristics v in = 5v 12v 70% 75% 80% 85% 90% 95% 100% 0246810 output current (a) efficiency (%) v bst = 12v for v in = 5v v bst = 18v for v in = 12v typical efficiency 0% 20% 40% 60% 80% 100% 0.00.20.40.60.81.01.2 ss/en voltage (v) duty cycle (%) (no feedback) ss/en control of duty cycle 1.490 1.492 1.494 1.496 1.498 1.500 456789101112 v in (v) v o (v) i o = 2.00a; v bst = 18v typical line regulation 5v v in = 12v -2.0% -1.5% -1.0% -0.5% 0.0% 0246810 i o (a) v o (v) v bst = 12v for v in = 5v v bst = 18v for v in = 12v typical load regulation
11 ? 2004 semtech corp. www.semtech.com power management SC2612E land pattern - soic-8 semtech corporation power management products division 200 flynn rd., camarillo, ca 93012 phone: (805)498-2111 fax (805)498-3804 contact information outline drawing - soic-8 see detail detail a a .050 bsc .236 bsc 8 .010 .150 .189 .154 .193 .012 - 8 0.25 1.27 bsc 6.00 bsc 3.90 4.90 - .157 .197 3.80 4.80 .020 0.31 4.00 5.00 0.51 bxn 2x n/2 tips seating aaa c e/2 2x 12 n a d a1 e1 bbb c a-b d ccc c e/2 a2 (.041) .004 .008 - .028 - - - - 0 .016 .007 .049 .004 .053 8 0 0.20 0.10 - 8 0.40 0.17 1.25 0.10 .041 .010 .069 .065 .010 1.35 (1.04) 0.72 - 1.04 0.25 - - - 1.75 1.65 0.25 0.25 - .010 .020 0.50 - c l (l1) 01 0.25 gage plane h h 3. dimensions "e1" and "d" do not include mold flash, protrusions or gate burrs. -b- controlling dimensions are in millimeters (angles in degrees). datums and to be determined at datum plane notes: 1. 2. -a- -h- side view a b c d e h plane reference jedec std ms-012, variation aa. 4. l1 n 01 bbb aaa ccc a b a2 a1 d e e1 l h e c dim min millimeters nom dimensions inches min max max nom e (.205) (5.20) z g y p (c) 3.00 .118 1.27 .050 0.60 .024 2.20 .087 7.40 .291 x inches dimensions z p y x dim c g millimeters this land pattern is for reference purposes only. consult your manufacturing group to ensure your company's manufacturing guidelines are met. notes: 1. reference ipc-sm-782a, rlp no. 300a. 2.
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