power management 1 www.semtech.com SC2618 miniature pwm controller for buck regulator features applications revision: september 19, 2005 typical application circuit description the SC2618 is a hysteretic mode pwm controller de- signed for high efficiency, low cost ?point of load? appli- cations. the hysteretic control scheme allows the use of ceramic output capacitors without the need for compensation components and difficult calculations. an internal soft start prevents output voltage from start up overshoot. a ?hiccup? short circuit protection provides protection against output short circuits as well as ad- verse input supply sequencing. ? operating frequency up to 500khz ? input supply from 5v to 12v ? 0.5a gate drive capability ? internal soft start ? internal, trimmed bandgap reference ( 1%) ? hiccup mode short circuit protection ? input supply sequence protection ? 6 lead sot23 package gnd v cc b oost v in q2 g nd vo q1 SC2618 2 3 1 5 6 4 gnd vcc dl bst dh fb ? graphics ic power supplies ? embedded, low cost, high efficiency converters
2 ? 2004 semtech corp. www.semtech.com power management SC2618 note: (1) 1 square inch of fr4, double sided, 1oz, minimum copper weight absolute maximum ratings 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 g a t l o v y l p p u s c c vv cc 5 7 . 44 1v t n e r r u c t n e c s e i u q c c vi qvcc v c c v , v 0 . 5 = f e r v , v 0 = b f v m 0 0 1 =50 1a m t u o k c o l e g a t l o v r e d n u c c vv u vcc 0 . 42 . 45 . 4v e g a t l o v y l p p u s t s o o bv bst 9 2v t n e r r u c t n e c s e i u q t s o o bi t s b q v b f v 0 0 2 . 1 =3a m e m i t t r a t s t f o ss s t0 0 1s u e g a t l o v t u p t u o v o i o v ; a m 0 2 = b f v = o 7 3 2 . 10 5 2 . 13 6 2 . 1 v 5 2 2 . 15 7 2 . 1 n o i t a l u g e r d a o li o a 4 o t a 2 . 0 =1% n o i t a l u g e r e n i l 5 . 0 % e g a t l o v p i r t t i u c r i c t r o h sv sct 5 0 . 1v e m i t f f o / n o m u m i n i mt del 1s u t n e r r u c s a i b b fi fb v fb v 5 2 . 1 =1 -a u t n e r r u c e c r u o s / k n i s h d k a e p, v 5 . 4 = h d - t s bv 5 . 3 = d n g - h d v 5 . 1 = d n g - h d 5 . 0 0 5 a a m t n e r r u c e c r u o s / k n i s l d k a e p, v 5 . 4 = l d - t s bv 5 . 3 = d n g - l d v 5 . 1 = d n g - l d 5 . 0 0 5 a a m e m i t g n i p p a l r e v o n o n l d , h dc d a o l v 2 = l d / h d t a d e r u s a e m ; f p 0 0 0 1 =0 20 5s n unless specified: v cc = v in = 5v, v bst = 12v, v fb = v o ; t a = 25c. values in bold are over full operating temperature range. electrical characteristics r e t e m a r a p l 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 t u p n iv c c 5 1v e g a t l o v y l p p u s t s o o bv t s b 0 3v e g a t l o v b fv b f 8v d n g o t l dv l d v o t 1 - c c 3 . 0 +v d n g o t h dv h d v o t 1 - t s b 3 . 0 +v e g n a r e r u t a r e p m e t t n e i b m a g n i t a r e p ot a 5 8 + o t 0 4 -c e g n a r e r u t a r e p m e t n o i t c n u j g n i t a r e p ot j 5 2 1 o t 0 4 -c e g n a r e r u t a r e p m e t e g a r o t st g t s 0 5 1 o t 5 6 -c s 0 1 ) g n i r e d l o s ( e r u t a r e p m e t d a e lt d a e l 0 0 3c t n e i b m a o t n o i t c n u j e c n a t s i s e r l a m r e h t a j 6 9 ) 1 ( w / c e s a c o t n o i t c n u j e c n a t s i s e r l a m r e h t c j 2 6w / c ) l e d o m y d o b n a m u h ( g n i t a r d s ed s e2v k exceeding the specifications below may result in permanent damage to the device, or device malfunction. operation outside of th e parameters specified in the electrical characteristics section is not implied. exposure to absolute maximum rated conditions for extended periods of time may affect device reliability.
3 ? 2004 semtech corp. www.semtech.com power management SC2618 block diagram pin configuration ordering information pin descriptions notes: (1) only available in tape and reel packaging. a reel contains 3000 devices. (2) SC2618sktrt is the lead free version. this product is fully weee and rohs compliant. s r e b m u n t r a pe g a k c a p t r t k s 8 1 6 2 c s ) 2 ( ) 1 ( 6 - 3 2 t o s top view 3 1 gnd (sot-23 6l) 5 dh 6 4 bst 2 vcc dl fb # n i pe m a n n i pn o i t c n u f n i p 1l d. t u p t u o e v i r d t e f e d i s w o l 2d 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 3c c v. y l p p u s e v i r d t e f e d i s w o l d n a e g a t l o v t u p n i y l p p u s p i h c 4b f. t u p n i k c a b d e e f 5t s b. r e v i r d e t a g h g i h r o f e g a t l o v y l p p u s 6h d. t u p t u o e v i r d t e f e d i s h g i h level shift and high side drive shoot-thru control synchronous mosfet drive drive logic s q r q q s q r uvlo soft start vcc fb bst dh dl gnd vref 200mv ssreset ssbegin ssover restart latch fault latch vcc
4 ? 2004 semtech corp. www.semtech.com power management SC2618 theory of operation the SC2618 is a hysteretic mode pwm controller. it uses a comparator to generate pwm wave with minimum on time and off time control. as shown in the block diagram, the output voltage is fed back to the comparator nega- tive input and compared to a setting voltage. if the out- put voltage is below its set point, the top gate drive will turn on and remain on until the minimum on time has expired and the output voltage has risen above the set point. similarly, if the output voltage is above its set point, top gate drive will be turned off and bottom gate will turn on and stay on until the minimum off time has expired and the output is below its set point. because of this minimum time control scheme, the comparator hyster- esis is not required and is internally set to zero. switching f switching f switching f switching f switching f req req req req req uency vs. duty cy uency vs. duty cy uency vs. duty cy uency vs. duty cy uency vs. duty cy cle cle cle cle cle this control scheme will force a buck converter to oper- ate either at minimum on time, or minimum off time, or both. the SC2618 has minimum time of 1us. its switch- ing frequency, peaking at 500khz, can be found in fig. 1 as long as the voltage ratio of the buck converter has been decided. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 100 200 300 400 500 f (khz) duty cycle t o n = 1 u s t o f f = 1 u s fig. 1. switching frequency vs. duty cycle uvl uvl uvl uvl uvl o, star o, star o, star o, star o, star t up and shut do t up and shut do t up and shut do t up and shut do t up and shut do wn wn wn wn wn to initiate the SC2618, supply voltages are applied to vcc and bst pins. the top gate (dh) and bottom gate (dl) are held low until vcc exceed uvlo threshold, typi- cally 4.5v. at that point, the internal soft start capacitor begins to charge, the top gate remains low, and the bot- tom gate is pulled high to turn on the bottom fet. when the voltage at soft start cap reaches a setting level, the top and bottom gates of pwm controller begin to switch. the switching regulator output is slowly ramping up for a soft turn on. if, however, one or more supply voltage rails are late or absent, the output voltage will not rise and the part will behave as if there was a short circuit at the output, that is, it will go into hiccup mode. it will remain in this mode until all the necessary voltage rails are present, at which time normal operation will start. if the supply voltage vcc falls below uvlo threshold dur- ing normal operation, the soft start capacitor begins to discharge. when the voltage reaches the setting level, the pwm controller control the switching regulator out- put to ramp down slowly for a soft turn off. hiccup mode shor hiccup mode shor hiccup mode shor hiccup mode shor hiccup mode shor t cir t cir t cir t cir t cir cuit pr cuit pr cuit pr cuit pr cuit pr o o o o o t t t t t ection ection ection ection ection short circuit protection is implemented by comparing the feedback node with the soft start and reference volt- age. if the fb voltage falls more than about 200mv be- low it?s correct voltage the short circuit latch is set which immediately disables the top drive. if the short circuit occurred during soft start, the soft start capacitor con- tinues to charge to it?s full voltage before discharging towards zero. if the short occurred during steady state operation, the internal soft start capacitor begins to dis- charge immediately, at the same rate as it charged, to- wards zero. once the soft start capacitor reaches its minimum voltage, the latches are cleared and a normal soft start is attempted, if the short circuit condition has not been cleared, the fault latch will again be set once the soft start voltage reaches about 200mv and the cycle will repeat until the short is cleared. it is also possible, with large output capacitor values, that the output voltage will be unable to rise fast enough to accurately follow the soft start voltage and a short circuit trip may result. during soft start the bottom gate drive is disabled to prevent discharging the output ca- pacitor and so the output capacitor will retain the volt- age achieved before the trip and will charge to a higher voltage during the next hiccup cycle. in this way the out- put voltage will achieve the set point value but may take several hiccup cycles to do so.
5 ? 2004 semtech corp. www.semtech.com power management SC2618 component selection general design guideline of switching power supplies can be applied to the component selection for SC2618. output filter components output filter components output filter components output filter components output filter components the purpose of soft start is to control inductor current during start up, this in turn controls the amount of ex- cess charge the output capacitor will receive and there- fore the output voltage overshoot that will occur at the end of soft start. g nd l v in q2 2618-02 co vo q1 gnd fig. 2. output filter components v c i l v o 0 0 t ss t p i l v p t v ref fig. 3. soft start, inductor current and overshoot normally, controllers with an external soft start capaci- tor, have soft start set so long that overshoot is unde- tectable. since the soft start for the SC2618 is internal, it is necessarily limited and the maximum values of l and co should meet the constraints of: 5 0 2 s ss o x t c l ? = ? where t ss is soft start time, 100us typically, and xs is the percentage overshoot allowable. induct induct induct induct induct or and mosfet or and mosfet or and mosfet or and mosfet or and mosfet s s s s s the selection of inductor and mosfets should meet ther- mal requirement because they are power loss dominant components. pick an inductor with as high inductance as possible if without adding extra cost and size. higher inductance, lower ripple current, smaller core loss and higher efficiency. however, too high inductance slows down output transient response. it is recommended to choose the inductance that gives the inductor ripple cur- rent to be approximate 20% of maximum load current. so choose inductor value from: ) 1 ( 5 in o o osc o v v v f i l ? ? ? ? = the mosfets are selected from their r dson , gate charge, and package. the SC2618 provides 0.5a gate drive cur- rent. to drive a 25nc gate charge mosfet gives 25nc/ 0.5a=50ns switching time. the switching time ts contrib- utes to the top mosfet switching loss: osc s in o s f t v i p ? ? ? = there is no switching loss for the bottom mosfet be- cause of its zero voltage switching. the conduction losses of the top and bottom mosfets are given by: d r i p dson o top c ? ? = 2 _ ) 1 ( 2 _ d r i p dson o bot c ? ? ? = if the requirement of total power losses for each mosfet is given, the above equations can be used to calculate the values of r dson and gate charge can be calculated using above equations, then the devices can be deter- mined accordingly. the solution should ensure the mosfet is within its maximum junction temperature at highest ambient temperature. output capacitor(s) output capacitor(s) output capacitor(s) output capacitor(s) output capacitor(s) the output capacitors should be selected to meet both output ripple and transient response criteria. the output capacitor esr causes output ripple v ripple during the inductor ripple current flowing in. to meet output ripple criteria, the esr value should be: ) 1 ( in o o ripple osc esr v v v v f l r ? ? ? ? <
6 ? 2004 semtech corp. www.semtech.com power management SC2618 the output capacitor esr also causes output voltage tran- sient v t during a transient load current i t flowing in. to meet output transient criteria, the esr value should be: t t esr i v r < to meet both criteria, the smaller one of above two esrs is required. the output capacitor value also contributes to load tran- sient response. based on a worst case where the induc- tor energy 100% dumps to the output capacitor during the load transient, the capacitance then can be calcu- lated by: 2 2 t t v i l c ? > input capacitor input capacitor input capacitor input capacitor input capacitor the input capacitor should be chosen to handle the rms ripple current of a synchronous buck converter. this value is given by: 2 2 ) ( ) 1 ( in o in rms i i d i d i ? ? + ? ? = where i o is the load current, i in is the input average cur- rent, and d is the duty cycle. choosing low esr input capacitors will help maximize ripple rating for a given size. bootstrap circuit bootstrap circuit bootstrap circuit bootstrap circuit bootstrap circuit the SC2618 uses an external bootstrap circuit to pro- vide a voltage at bst pin for the top mosfet drive. this voltage is held up by a bootstrap capacitor. typically, it is recommended to use a 1uf ceramic capacitor with 25v rating and a commonly available diode in4148 for the bootstrap circuit. filter for vcc supply power filter for vcc supply power filter for vcc supply power filter for vcc supply power filter for vcc supply power for the pin of vcc, it is recommended to use a 1uf/25v ceramic capacitor for decoupling. in addition, place a small resistor (10 ohm) in between vcc pin and the sup- ply power for noise reduction. divider ratio divider ratio divider ratio divider ratio divider ratio the top resistor of the voltage divider can be chosen from 5k to 15k. then the bottom resistor is found from top o bot r v v v r ? ? = 25 . 1 25 . 1 where 1.25v is the internal reference voltage of the SC2618. component selection (cont.)
7 ? 2004 semtech corp. www.semtech.com power management SC2618 layout guidelines careful attention to layout requirements are necessary for successful implementation of the SC2618 controller. high currents switching at up to 500khz 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. cin should consist of the bulk input capacitors and must be supplemented with one or more ceramic decoupling capacitors as close as possible to the fets 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 cout 10uf u1 SC2618 5 6 4 1 3 2 vcc dh bst fb dl gnd q1 10 vpwr in vout gnd 0.1uf cin q2 vbst in v cc in
8 ? 2004 semtech corp. www.semtech.com power management SC2618 layout guidelines (cont.) vout vin + + currents in power section
9 ? 2004 semtech corp. www.semtech.com power management SC2618 contact information semtech corporation power management products division 200 flynn road, camarillo, ca 93012 phone: (805)498-2111 fax (805)498-3804 outline drawing - sot23-6 minimum land pattern - sot23-6 .110 bsc .037 bsc detail aaa c seating ccc c 2x n/2 tips 2x e/2 6 see detail a1 a a2 bxn a .008 12 n e .060 .114 .063 .118 .010 - 6 a 0.20 1.60 3.00 2.80 bsc 0.95 bsc .069 1.50 2.90 .020 0.25 1.75 0.50 - ei l (l1) c 01 0.25 plane gage h 2.80 .110 bbb c a-b d 0 .008 - .004 .012 .003 (.024) .018 - .035 .000 .035 - - .045 0.10 0.20 10 0 - 10 1.15 (0.60) 0.45 .024 .009 0.30 0.08 .057 .051 .006 0.00 .90 0.90 0.22 0.60 - 0.15 1.45 1.30 - - 1.90 bsc .075 bsc a e1 d e b c h nom inches dimensions l1 aaa bbb ccc 01 n dim c e e1 l e1 e d a1 a2 b a min millimeters max min nom max plane d datums and to be determined at datum plane controlling dimensions are in millimeters (angles in degrees). dimensions "e1" and "d" do not include mold flash, protrusions 3. or gate burrs. n otes: 1. 2. -a- -b- -h- side view this land pattern is for reference purposes only. consult your manufacturing group to ensure your company's manufacturing guidelines are met. n otes: 1. dimensions inches y z dim g p x c millimeters p (c) z y g .043 .141 .055 (.098) .037 .024 1.40 (2.50) 0.95 0.60 1.10 3.60 x dimensions inches y z dim g p x c millimeters
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