power management 1 www.semtech.com SC4508 buck or buck-boost(inverting) current mode controller description revision: november 30, 2004 features typical application circuits applications ? wide input voltage range 2.7v to 15v ? programmable output voltage ? programmable switching frequency up to 1.5mhz ? buck or buck boost(inverting) configuration ? current mode control with slope compensation ? very low quiescent current in shutdown mode ? accessible reference voltage ? mlpq-12 package ? low power point of use converters ? single or multiple output low power converters ? positive and/or negative output voltage ? dsl cards ? graphic cards ? i/o cards ? negative bias supplies the SC4508 is a low voltage current mode switching regu- lator controller that drives a p-channel power mosfet with programmable switching frequency. it can be con- figured in either buck or buck boost (inverting) convert- ers. the converters can be operated from 2.7v to 15v input voltage range. the typical operating supply current is 3ma and a shutdown pin allows the user to turn the controller off reducing it to less than 200 a. the output voltage can adjusted by external resistor divider. the switching frequency is programmable up to 1.5mhz, allowing small inductor and capacitor values to minimize pcb space. the operating current level is programmable via an external sense resistor. accessible reference volt- age allows users to make output voltage as low as they want. vref cs out vdd fb+ fb- comp ss/shdn osc SC4508 vin gnd cs osc vdd vin ss/shdn fb- gnd SC4508 out fb+ vref comp
2 ? 2004 semtech corp. www.semtech.com power management SC4508 electrical characteristics absolute maximum rating unless specified: v dd = 2.7v to 15v , t a = t j = -40c to 125c r e t e m a r a pl o b m y sm u m i x a ms t i n u d n g o t d d v 6 1 o t 3 . 0 -v d n g o t n d h s / s s 2 . 3v d n g o t c s o , p m o c , - b f , + b f 5v t n e r r u c f e r v 1a m k n i s r o e c r u o s t u o 0 5 2a m 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 8 4w / 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 3w / 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 0 6 -c e g n a r e r u t a r e p m e t n o i t c n u jt j 0 5 1 + o t 0 4 -c s 0 4 - 0 1 e r u t a r e p m e t w o l f e r r i k a e pt g k p 0 6 2c r e t e m a r a ps n o i t i d n o c t s e tn i mp y tx a mt i n u y l p p u s r e w o p e g n a r e g a t l o v t u p n i 7 . 25 1v t n e r r u c t n e c s e i u qw o l = n d h s0 0 20 0 5a t n e r r u c g n i t a r e p od a o l o n , h g i h = n d h s3a m t u o k c o l e g a t l o v r e d n u d l o h s e r h t t r a t sv d d g n i s i r5 . 25 5 . 2v s i s e r e t s y h o l v u 0 0 1v m r o t a l l i c s o e g n a r y c n e u q e r f 0 0 10 0 5 1z h k y c n e u q e r fc t f p 0 0 3 =0 5 40 0 50 5 5z h k t n e r r u c e g r a h c 0 0 1a r e i f i l p m a r o r r e e g a t l o v t u p n i k c a b d e e f 5 7 . 0v t n e r r u c s a i b t u p n i 0 0 1a n e c n a t c u d n o c s n a r t 5s m t n e r r u c k n i s r o e c r u o s t u p t u o 0 50 0 1a 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 SC4508 electrical characteristics r e t e m a r a ps n o i t i d n o c t s e tn i mp y tx a mt i n u r o t a r a p m o c m w p t u p t u o o t y a l e d 0 5s n e c n e r e f e r f e r v e g a t l o v t u p t u o 1 3 2 . 15 2 . 19 6 2 . 1v t n e r r u c t u p t u o 1a m n o i t a l u g e r e n i lv d d i , v 5 1 o t 7 . 2 = f e r v a m 1 =4v m n o i t a l u g e r d a o lv d d i , v 5 = f e r v a m 1 o t 0 =4v m n w o d t u h s / t r a t s t f o s t n e r r u c e g r a h c 0 2a t n e r r u c e g r a h c s i d 2 1a m e g a t l o v t h g i h c i g o l n d h s 2v e g a t l o v w o l c i g o l n d h s 5 2 . 0v t i m i l t n e r r u c d l o h s e r h t e l c y c y b e l c y cv d d v 5 =0 90 1 10 3 1v m d l o h s e r h t n w o d t u h s 6 1 . 0v t u p t u o o t y a l e d 0 5s n t u p t u o ) h ( e c n a t s i s e r - n o e v i r d e t a gv d d i , v 5 = g n i c r u o s a m 0 5 2 =0 2m h o ) l ( e c n a t s i s e r - n o e v i r d e t a gv d d i , v 5 = k n i s a m 0 5 2 =0 2m h o e m i t e s i rc t u o f p 0 0 2 =0 2s n e m i t l l a fc t u o f p 0 0 2 =0 2s n unless specified: v dd = 2.7v to 15v , t a = t j = -40c to 125c note: (1) this device is esd sensitive. use of standard esd handling precautions is required.
4 ? 2004 semtech corp. www.semtech.com power management SC4508 pin descriptions notes: (1) only available in tape and reel packaging. a reel contains 3000 devices. (2) lead free product. this product is fully weee and rohs compliant. e c i v e de g a k c a pt ( e g n a r . p m e t j ) t r t l m i 8 0 5 4 c s ) 2 ( 2 1 - q p l m ) 1 ( c 0 5 1 o t c 0 4 - ordering information pin configuration top view (mlpq-12 4x4) 1 out 2 cs 3 ss/shdn 9 fb+ 8 fb- 7 pgnd 4 vref 5 nc 6 agnd 12 vdd 11 10 osc comp # n i pe m a n n i pn o i t c n u f n i p 1t u o . d n g p o t d d v m o r f s g n i w s t u o . t e f s o m - p l a n r e t x e r o f t u p t u o r e v i r d e t a g 2s c . s c d n a d d v n e e w t e b r o t s i s e r e s n e s t n e r r u c a t c e n n o c . n i p t u p n i e s n e s t n e r r u c 3n d h s / s s p u p m a r e h t . d n g a d n a n i p s i h t n e e w t e b r o t i c a p a c l a n r e t x e n a s t c e n n o c . n i p t r a t s t f o s d e l l u p s i n d h s / s s v n e h w n w o d t u h s o t n i s e o g e c i v e d e h t . r o t i c a p a c e h t y b d e n i f e d s i e m i t . v 5 2 . 0 w o l e b 4f e r v c i m a r e c f u 1 . 0 a h t i w s s a p y b . a m 1 o t p u e c r u o s n a c f e r v . t u p t u o e c n e r e f e r v 5 2 . 1 . d n g a o t f e r v m o r f r o t i c a p a c 5c n. n o i t c e n n o c o n 6d n g a. d n u o r g g o l a n a 7d n g p. d n u o r g r e w o p 8- b f. t u p n i g n i t r e v n i r e i f i l p m a r o r r e 9+ b f. t u p n i g n i t r e v n i - n o n r e i f i l p m a r o r r e 0 1p m o c p o o l t c e n n o c . r e i f i l p m a r o r r e e c n a t c u d n o c s n a r t l a n r e t n i e h t r o f n i p n o i t a s n e p m o c . d n g a o t p m o c m o r f k r o w t e n n o i t a s n e p m o c 1 1c s o e h t t e s o t d n g a o t c s o m o r f r o t i c a p a c c i m a r e c a t c e n n o c . t u p n i t e s y c n e u q e r f r o t a l l i c s o e h t t e s o t n o i t a u q e e s u . z h m 5 . 1 o t z h k 0 0 1 m o r f y c n e u q e r f r o t a l l i c s o l a n r e t n i . d n g a o t c s o m o r f r o t i c a p a c e h t s i c . y c n e u q e r f r o t a l l i c s o 2 1d d v . d n g p o t d d v m o r f r o t i c a p a c c i m a r e c f u 1 a s s a p y b . e g a t l o v y l p p u s -l a m r e h t d a p d e t c e n n o c t o n . s a i v e l p i t l u m g n i s u e n a l p d n u o r g o t t c e n n o c . s e s o p r u p g n i k n i s t a e h r o f d a p . y l l a n r e t n i 65 . 0 c a 100 f ? =
5 ? 2004 semtech corp. www.semtech.com power management SC4508 block diagram marking information top view yyww = date code (example: 0012)
6 ? 2004 semtech corp. www.semtech.com power management SC4508 application information the SC4508 is designed to control buck (step down) or buck-boost (inverting) converter with p-channel mosfet as a switch using current mode, programmable switching frequency architecture. during steady state operation, the switch is turned on each cycle and turned off when the voltage across current sense resistor exceeds the voltage level at comp pin set by voltage loop error amplifier. a fixed 0.5v artificial ramp is added internally to the sensed current signal for operations when duty- cycle is larger than 50%. in over load or output shortage condition, if the sensed current signal reaching typical 120mv, the switch is turned off immediately in the same cycle. if the sensed current signal further increases to typical 160mv, not only the switch is turned off but also the soft start capacitor is discharged by a internal mosfet to ground then charging back to threshold 250mv during which the switch is held off. with the ?hiccup? mode over current protection, the thermal stress is reduced in the faulty conditions. current sense and current limit the typical cycle-by-cycle current limit threshold in the current sense pin of the SC4508 is 120mv. the over current limit is assumed typical 120% of full load current. then the current sense resistor can be calculated by the following equation: rs = ) pk ( i % 120 mv 120 l ? i l (pk) = ) v v v v ( l f v v 2 1 i d in d o s o in o + + ? ? + for buck boost buck for ) v v v v v ( l f v 2 1 v v v v i ) pk ( i d o in d o s in in d o in o l ? + + + ? + + + = i o - full load current v o - output voltage v in - input voltage v d - diode forward voltage drop f s - switching frequency l - inductor inductor selection a inductor is chosen based on the required output ripple current which is usually 20-30% of load current. the ripple current ? i decreases with higher inductance and increases with higher input voltage v in or output voltage v o. ) v v v v ( i f v v l d in d o l s o in + + ? ? ? = for buck ) v v v v v ( i f v l d o in d o l s in + + + ? ? = for buck-boost specify the maximum inductor current larger than i l (pk) set by the current sense resistor r s to avoid the inductor core saturation. input capacitor c in and output capacitor c out selection both input and output capacitors need to be sized to handle the ripple current safely. buck converters have high ripple current in the input side while buck-boost converters have high ripple current in the both input and output capacitors. therefore, the rms value of the current must be less than the high frequency ripple rating of the capacitors. in continuous mode operation, in o in o o cap _ input _ rms v ) v v ( v i i ? for buck in d o o cap _ rms v v v i i + for buck-boost input and output capacitors. the c out is selected for buck converter is by required output ripple voltage and converter loop stability. the output ripple is determined by: ) c f 8 1 esr ( i v out s l o + ? ? where f s is converted switching frequency, c out is output capacitance and ? i l is inductor ripple current.
7 ? 2004 semtech corp. www.semtech.com power management SC4508 application information (cont.) in buck converter, the worst case input rms ripple current occurs at 50% duty cycle, or at v in = 2v out . under this condition, the input capacitors rms ripple current is about half of the load current. mosfet selection the selection criteria for the power mosfet is its operating junction temperature not exceeding the maximum junction temperature. therefore, from a specified ambient temperature, the maximum junction to ambient temperature rise has to be determined, which is related to the mosfet power dissipation or power loss. the power loss includes conduction loss and switching loss. the conduction loss is given by: ) on ( ds 2 o d in d o on r i ) v v v v ( p + + = for buck ) on ( ds 2 o d o in d o in on r i ) v v )( v v v v ( p + + + = for buck-boost freewheeling diode selection the schottky diode is recommended as freewheeling diode in the both buck and buck-boost applications. the diode conducts during the off-time. the diode voltage and current ratings are selected based upon the peak reverse voltage, the peak current and average power dissipation. buck for v v v v i i , 2 i i i , v v d in o in o ) avg ( d l o ) peak ( d in ) rev ( d + ? = ? + = = boost buck for i i , 2 i ) v v v v ( i i , v v v o ) avg ( d l in d o in o ) peak ( d o in ) rev ( d ? = ? + + + = + = the most stressful condition for the diode occurs when the output is shorted. under this condition, due to the v out = 0, the diode conducts at close to 100% duty cycle. therefore, attention should be paid to the thermal condition when laying out a board. layout guidelines careful attention to layout requirements are necessary for successful implementation of the SC4508 pwm con- troller. high switching currents are present in the appli- cation and their effect on ground plane voltage differen- tials 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 output capacitor ground. 2). the loop formed by the input capacitor(s) (cin), the mosfet and the schottky diode or inductor must be kept as small as possible. this loop contains all the high cur- rent, fast transition switching. connections should be as wide and as short as possible to minimize loop induc- tance 3). the connection between the junction of mosfet, schottky diode and the output inductor should be a wide trace or copper region. it should be as short as practical. since this connection has fast voltage transitions, keep- ing this connection short will minimize emi. 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) a separate analog ground plane connects to the SC4508 agnd pin. all analog grounding path including decoupling capacitors, feedback resistors, compensation components, and current-limit setting resistors should be connected to this plane.
8 ? 2004 semtech corp. www.semtech.com power management SC4508 evaluation board schematic, buck bill of materials m e t iy t i t n a u qe c n e r e f e rt r a pr e r u t c a f u n a m 12 2 c , 1 cv 6 1 / f u 7 4m 7 4 b p t 6 1 : n / p o y n a s 213 cf n 1 31, 4 cf u 1 42 7 c , 5 cf u 1 . 0 516 cf p 0 3 3 62 9 c , 8 cv 3 . 6 / f u 0 7 4m 0 7 4 b p t 6 1 : n / p o y n a s 710 1 cf n 2 811 1 cf n 7 . 2 911 lh u 0 1 0 111 r9 0 . 0 1 112 r0 1 2 114 rk 0 2 3 115 rk 5 7 . 4 4 116 rk 4 2 . 1 5 117 rk 0 1 6 111 u8 0 5 4 c s. p r o c h c e t m e s 7 112 ua 2 0 1 p 2 s f d f: n / p d l i h c r i a fa 2 0 1 p 2 s f d f gnd 12v gnd tp4 tp3 tp8 1 2 3 4 5 6 7 8 u2 fdfs2p102a r1 0.09 c5 0.1uf r7 10k c4 1uf r5 3.74k c1 47uf/16v c8 470uf/6.3v l1 10uh tp1 tp7 comp 10 nc 5 ss/shdn 3 osc 11 fb+ 9 fb- 8 vref 4 out 1 cs 2 vdd 12 agnd 6 pgnd 7 u1 SC4508 u1 tp6 r6 1.24k r4 20k tp5 tp2 c11 2.7nf c9 470uf/6.3v c3 1nf c2 47uf/16v c7 0.1uf c6 330pf r2 10 c10 2nf fs=440khz 6v/1 a
9 ? 2004 semtech corp. www.semtech.com power management SC4508 evaluation board schematic, buck-boost m e t iy t i t n a u qe c n e r e f e rt r a pr e r u t c a f u n a m 13 8 c , 2 c , 1 cv 6 1 / f u 7 4m 7 4 b p t 6 1 : n / p o y n a s 213 cf n 1 314 cf u 1 42 7 c , 5 cf u 1 . 0 516 cf p 0 8 6 619 cv 3 . 6 / f u 0 0 1 710 1 cf p 0 0 1 811 1 cf n 7 . 2 916 dw h 9 1 8 5 n 1 0 111 lh u 3 . 3 1 111 r1 . 0 2 112 r0 0 1 3 114 rk 0 2 4 115 rk 4 0 . 6 5 12 8 r , 6 r9 4 2 6 117 rk 0 1 7 119 rk 0 . 1 9 111 u8 0 5 4 c s. p r o c h c e t m e s 0 212 uy d 1 3 8 4 i sy a h s i v bill of materials gnd c8 47uf/16v r1 0.1 c6 680pf tp8 tp1 c4 1uf l1 33uh tp7 tp6 c10 100pf 1 2 3 4 5 6 7 8 u2 si4831dy c2 47uf/16v c11 2.7nf c1 47uf/16v c5 0.1uf tp3 r5 6.04k d1 1n5819hw c7 0.1uf comp 10 nc 5 ss/shdn 3 osc 11 fb+ 9 fb- 8 vref 4 out 1 cs 2 vdd 12 agnd 6 pgnd 7 u1 SC4508 u1 r2 100 r6 240 r7 10k tp5 c3 1nf tp4 r4 20k c9 100uf/6.3v tp10 fs=250k h z r8 249 r9 1.0k tp9 12v gnd -5v/0.1 a
10 ? 2004 semtech corp. www.semtech.com power management SC4508 contact information semtech corporation power management products division 200 flynn road, camarillo, ca 93012 phone: (805)498-2111 fax (805)498-3804 outline drawing - mlpq-12, 4 x 4 0.25 0.00 - 0.30 - a d e b controlling dimensions are in millimeters (angles in degrees). coplanarity applies to the exposed pad as well as the terminals. 1. 2. notes: - - - - indicator (laser mark) pin 1 nom dimensions inches aaa bbb d1 e1 a2 a1 dim d l n e e min b a max millimeters min max nom 4.10 4.00 3.90 .161 .157 .153 .153 .157 .161 3.90 4.00 4.10 seating plane bbb c a b e1 aaa c e a a1 d/2 1 a2 c bxn lxn d1 n 2 e/2 0.08 .003 .004 0.10 .010 .074 .000 .031 .018 .074 - .031 bsc .085 12 .022 .085 .089 .026 .089 (.008) .012 .040 .014 .002 - 0.35 0.05 1.00 2.25 2.25 0.65 0.80 bsc 1.90 0.45 1.90 2.15 0.55 12 2.15 (0.20) 0.80 p y z p y x h k k dim c .091 .091 2.30 2.30 dimensions millimeters inches this land pattern is for reference purposes only. consult your manufacturing group to ensure your company's manufacturing guidelines are met. notes: 1. .031 .189 .041 .016 .106 (.148) 0.80 4.80 1.05 0.40 (3.75) 2.70 g h 2x (c) 2x g 2x z x land pattern - mlpq-12, 4 x 4
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