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1 www.semtech.com SC4608 low input, mhz operation, high efficiency synchronous buck power management revision: june 19, 2006 description features applications typical application circuit ? asynchronous start up ? programmable switching frequency up to 1mhz ? bicmos voltage mode pwm controller ? 2.7v to 5.5v input voltage range ? output voltage as low as 0.5v ? +/-1% reference accuracy ? sleep mode (icc = 1a max) ? adjustable lossless short circuit current limiting ? combination pulse by pulse & hiccup mode current limit ? high efficiency synchronous switching ? 1a peak current driver ? external soft start ? power good signal ? 16-pin mlp lead-free package. this product is fully weee and rohs compliant ? distributed power architecture ? servers/workstations ? local microprocessor core power supplies ? dsp and i/o power supplies ? battery-powered applications ? telecommunications equipment ? data processing applications the SC4608 is a voltage mode step down (buck) regula- tor controller that provides accurate high efficiency power conversion from an input supply range of 2.7v to 5.5v. a high level of integration reduces external component count, and makes it suitable for low voltage applications where cost, size and efficiency are critical. the SC4608 is capable of producing an output voltage as low as 0.5v. it?s frequency of operation is programmable to 1mhz. the SC4608 drives external, n-channel mosfets with a peak gate current of 1a. a non-overlap protection is pro- vided for the gate drive signals to prevent shoot through of the mosfet pair. the SC4608 features lossless cur- rent sensing of the voltage drop across the drain to source resistance of the high side mosfet during its conduction period. the quiescent supply current in sleep mode is typically lower than 1 a. a external soft start is provided to pre- vent output voltage overshoot during start-up. the SC4608 is an ideal choice for converting 3.3v, 5v or other low input supply voltages. it?s available in 16 pin mlp package. r3 c3 1u c2 2.2n r1 14.3k c1 180p v out = 0.5v / 12a c17 1u d2 l1 1.8u c5 22u c4 22u m11 c10 220u m2 c9 4.7n r8 200 r7 10k v in =3v ? 3.6v r6 1 r5 1 c16 560pf avdd 1 iset 2 comp 3 fset 4 pvdd 16 bst 15 phase 12 ss 7 vsense 8 drvh 13 pgood 6 en 5 agnd 9 pgnd 10 drvl 11 SC4608 r2 10k c6 330u c13 22u c14 22u r13 1 22n css r3 c3 1u c2 2.2n r1 14.3k c1 180p v out = 0.5v / 12a c17 1u d2 l1 1.8u c5 22u c4 22u m11 c10 220u m2 c9 4.7n r8 200 r7 10k v in =3v ? 3.6v r6 1 r5 1 c16 560pf avdd 1 iset 2 comp 3 fset 4 pvdd 16 bst 15 phase 12 ss 7 vsense 8 drvh 13 pgood 6 en 5 agnd 9 pgnd 10 drvl 11 SC4608 r2 10k c6 330u c13 22u c14 22u r13 1 22n css
2 ? 2006 semtech corp. www.semtech.com SC4608 power management absolute maximum ratings 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 l l a r e v o e g a t l o v y l p p u s 5 . 5v p e e l s , t n e r r u c y l p p u sv 0 = n e1 . 01a g n i t a r e p o , t n e r r u c y l p p u sv a d d =v 5 . 525 7 . 3a m v a d d d l o h s e r h t n o - n r u tt a c 5 8 o t c 0 4 - =7 . 2v v a d d s i s e r e t s y h f f o - n r u t5 7 20 5 35 2 4v m r e i f i l p m a r o r r e e g a t l o v b f ) e c n e r e f e r l a n r e t n i ( v a d d , v 5 . 5 o t v 7 . 2 = t a c 5 8 o t c 0 4 - = 9 4 . 05 . 01 5 . 0 v v a d d t , v 5 . 5 o t v 7 . 2 = a c 5 2 =3 9 4 . 05 . 07 0 5 . 0 t a c 5 8 o t c 0 4 - =2 9 4 . 05 . 08 0 5 . 0 t a c 5 2 =5 9 4 . 05 . 05 0 5 . 0 t n e r r u c s a i b e s n e s vv e s n e s v 5 . 0 =0 0 2a n n i a g p o o l n e p o ) 1 ( v p m o c v 5 . 2 o t 5 . 0 =0 9b d h t d i w d n a b n i a g y t i n u ) 1 ( 8z h m r e t e m a r a pl o b m y sm u m i x a ms t i n u v a ( e g a t l o v y l p p u s g o l a n a d d )6v v p ( e g a t l o v y l p p u s r e w o p d d )v a d d 3 . 0 - / +v d n g p 3 . 0 v s t n e r r u c ) l v r d , h v r d ( s r e v i r d t u p t u o s u o u n i t n o c k a e p 5 2 . 0 a 0 0 . 1 a ) s s , t e s i , t e s f , p m o c , e s n e s v ( s t u p n i v a o t 3 . 0 - d d 3 . 0 +v t s b v p d d 6 +v e s a h p v p o t 3 . 0 - d d 3 . 0 +v s n 0 5 < e s l u p t e s l u p e s a h p v p o t 2 - d d 1 +v 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 e r u t a r e p m e t n o i t c n u j m u m i x a mt j 0 5 1 +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 ) 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 all voltages with respect to agnd. currents are positive into, negative out of the specified terminal. unless otherwise specified, av dd = 3.3v, av dd = pv dd , ct = 270pf, t a = -40 c to 85 c , t a =t j 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.
3 ? 2006 semtech corp. www.semtech.com SC4608 power management electrical characteristics (cont.) 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 ) . t n o c ( r e i f i l p m a r o r r e e t a r w e l s ) 1 ( 4 . 2s / v h g i h t u o vi p m o c a m 5 . 5 - = v a d d 5 . 0 -v a d d 3 . 0 - v w o l t u o vi p m o c a m 5 . 5 =3 . 05 4 . 0 r o t a l l i c s o y c a r u c c a l a i t i n it a c 5 2 =5 2 55 7 55 2 6z h k n o i t a l u g e r e n i l c s ot a v a , c 5 2 = d d v 5 . 5 o t v 7 . 2 =15 . 2v / % t n e i c i f f e o c e r u t a r e p m e tt a c 5 8 o t c 0 4 - =2 0 . 0c / % y c n e u q e r f n o i t a r e p o m u m i n i m ) 1 ( 0 5z h k y c n e u q e r f n o i t a r e p o m u m i x a m ) 1 ( m 1z h y e l l a v o t k a e p p m a r 1v e g a t l o v k a e p p m a r 3 . 1v e g a t l o v y e l l a v p m a r 3 . 0v t i m i l t n e r r u c , t r a t s t f o s e m i t t r a t s t f o s e l b a m m a r g o r p ) 1 ( f n 2 2 = c2s m t n e r r u c e g r a h c t r a t s t f o st a c 5 2 =4 -5 2 . 5 -5 . 6 -a t n e r r u c s a i b t e s it j c 5 2 =5 4 -0 5 -5 5 -a t e s i f o t n e i c i f f e o c e r u t a r e p m e t 3 . 0c / % e m i t k n a l b t i m i l t n e r r u c ) 1 ( 0 3 1s n e v i r d e t a g e m i t f f o m u m i n i m h v r d ) 1 ( t a c 5 2 =0 6 1s n ) h v r d ( e c r u o s k a e pi , v 3 . 3 = s g v e c r u o s a m 0 0 1 =5 . 36 ? ) h v r d ( k n i s k a e pi , v 3 . 3 = s g v k n i s a m 0 0 1 =35 ? ) l v r d ( e c r u o s k a e p ) 1 ( i , v 3 . 3 = s g v e c r u o s a m 0 0 1 =2 . 2 ? ) l v r d ( k n i s k a e pi , v 3 . 3 = s g v k n i s a m 0 0 1 =24 ? e m i t e s i r t u p t u oc , v 3 . 3 = s g v t u o f n 7 . 4 =5 3s n e m i t l l a f t u p t u oc , v 3 . 3 = s g v t u o f n 7 . 4 =7 2s n p a l r e v o - n o n m u m i n i m ) 1 ( 0 4s n unless otherwise specified, av dd = 3.3v, av dd = pv dd , ct = 270pf, t a = -40 c to 85 c , t a =t j
4 ? 2006 semtech corp. www.semtech.com SC4608 power management 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 d o o g r e w o p w o l e g a t l o v d o o g pi d o o g p v a , a m 1 = d d v 5 . 5 =5 1 . 03 . 0v t n e r r u c e g a k a e l d o o g pv a d d v 5 . 5 =1a d l o h s e r h t r e p p u d o o g p 5 . 75 . 1 15 . 5 1% d l o h s e r h t r e w o l d o o g p 5 . 4 1 -5 . 0 1 -5 . 6 -% y a l e d d o o g p 4 1s m e l b a n e d l o h s e r h t l e v e l h g i h v a * 7 . 0 d d v d l o h s e r h t l e v e l w o l v a * 3 . 0 d d v t n e r r u c s a i b t u p n i n ev 0 = n e v0 1 -a n note: (1). guaranteed by design. unless otherwise specified, av dd = 3.3v, av dd = pv dd , ct = 270pf, t a = -40 c to 85 c , t a =t j
5 ? 2006 semtech corp. www.semtech.com SC4608 power management ordering information pin descriptions r e b m u n t r a pe c i v e d ) 1 ( t r t l m 8 0 6 4 c s ) 2 ( 6 1 - p l m b v e 8 0 6 4 c sd r a o b n o i t a u l a v e pin configuration # n i pe m a n n i pn o i t c n u f n i p 1d d v a. r e l l o r t n o c e h t f o n o i t c e s g o l a n a e h t r o f e g a t l o v y l p p u s r e w o p 2t e s i e h t s e s u 8 0 6 4 c s e h t . t e f s o m e d i s h g i h e h t n i t n e r r u c t i m i l o t d e s u s i n i p t e s i e h t v e h t s s o r c a e g a t l o v n i t i m i l t n e r r u c e h t . t i m i l t n e r r u c e h t t e s o t r e d r o n i s n i p t e s i d n a t i u c r i c n o i t a c i l p p a l a c i p y t e h t n i 3 r ( r o t s i s e r l a n r e t x e n a f o e u l a v e h t y b t e s s i d l o h s e r h t e s n e s e h t s s o r c a p o r d e g a t l o v e h t g n i r a p m o c y b d e m r o f r e p s i g n i t i m i l t n e r r u c . ) m a r g a i d e d i s h g i h e h t f o e c n a t s i s e r e c r u o s o t n i a r d e h t s s o r c a p o r d e g a t l o v e h t h t i w r o t s i s e r o t n i a r d e h t s s o r c a p o r d e g a t l o v e h t . d o i r e p n o i t c u d n o c s ? t e f s o m e h t g n i r u d t e f s o m v e h t m o r f d e n i a t b o s i t e f s o m e d i s h g i h e h t f o e c n a t s i s e r e c r u o s n i . n i p e s a h p d n a 3p m o c d e t r e v n i s i t u p t u o s i h t t a e g a t l o v e h t . r e i f i l p m a r o r r e e g a t l o v e h t f o t u p t u o e h t s i s i h t g a l - d a e l a . r o t a r a p m o c m w p e h t f o t u p n i g n i t r e v n i - n o n e h t o t d e t c e n n o c d n a y l l a n r e t n i r e t l i f c l e l o p o w t e h t r o f s e t a s n e p m o c n i p e s n e s v e h t o t n i p p m o c e h t m o r f k r o w t e n r e d r o n i d e r i u q e r s i k r o w t e n g a l - d a e l e h t . l o r t n o c e d o m e g a t l o v o t t n e r e h n i s c i t s i r e t c a r a h c . p o o l l o r t n o c e d o m e g a t l o v e h t f o e c n a m r o f r e p c i m a n y d e h t e z i m i t p o o t 4t e s f g n i m i t l a n r e t x e n a h g u o r h t y c n e u q e r f r o t a l l i c s o m w p e h t s t e s o t d e s u s i n i p t e s f e h t e b n a c 8 0 6 4 c s e h t . n i p d n g e h t o t n i p t e s f e h t m o r f d e t c e n n o c s i t a h t r o t i c a p a c r o t i c a p a c g n i m i t e h t n e e w t e b s e i r e s n i r o t s i s e r a g n i c a l p y b e d o m s u o n o r h c n y s n i d e t a r e p o t e s f e h t o t d e t c e n n o c n i a m e r l l i w r o t i c a p a c g n i m i t e h t f o l a n i m r e t r e h t o e h t . d n u o r g d n a . n i p 5n e e v o b a d l e h d n a d e l l u p s i n e n e h w t e s f y b t e s s i 8 0 6 4 c s e h t f o y c n e u q e r f r o t a l l i c s o e h t v a * 7 . 0 d d v a * 3 . 0 w o l e b d l e h d n a d e l l u p s i n e f i d e k o v n i s i e d o m n w o d t u h s s t i . . d d 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. top view (16 pin mlp)
6 ? 2006 semtech corp. www.semtech.com SC4608 power management pin descriptions (cont.) # n i pe m a n n i pn o i t c n u f n i p 6d o o g p d l o h s e r h t d o o g r e w o p e h t w o l e b s i t u p t u o e h t n e h w w o l . t u p t u o n i a r d n e p o d o o g r e w o p . l e v e l 7s s t n e d n e p e d n i s i e m i t t r a t s t f o s e h t . e m i t t r a t s t f o s e h t s t e s d n u o r g o t r o t i c a p a c a . t r a t s t f o s l a n r e t x e e h t s i c e r e h w s a d e n i f e d s i d n a y c n e u q e r f g n i h c t i w s f o . s m n i e m i t t r a t s t f o s e h t s i s s d n a f n n i r o t i c a p a c 8e s n e s v e g a t l o v t u p t u o e h t s a s e v r e s d n a r e i f i l p m a e g a t l o v e h t f o t u p n i g n i t r e v n i e h t s i n i p s i h t e u l a v e c n e r e f e r l a n r e t n i n a o t d e r a p m o c s i e s n e s v . r e t r e v n o c k c u b e h t r o f t n i o p k c a b d e e f . d e r i s e d s i v 5 . 0 f o t u p t u o n a n e h w e g a t l o v t u p t u o e h t o t d e r i w d r a h s i e s n e s v . v 5 . 0 f o l a c i p y t e h t n i 9 r d n a 7 r ( y r a s s e c e n s i k r o w t e n r e d i v i d r o t s i s e r a , s e g a t l o v t u p t u o r e h g i h r o f . ) m a r g a i d t i u c r i c n o i t a c i l p p a 9d n g a. d n u o r g g o l a n a 0 1d n g p. d n u o r g r e w o p 1 1l v r d s r e v i r d t u p t u o e h t . t e f s o m ) r e i f i t c e r s u o n o r h c n y s ( e d i s w o l e h t f o e t a g e h t s e v i r d l v r d o t s l a n g i s e v i r d y r a t n e m e l p m o c s e d i v o r p y r t i u c r i c m w p e h t . s t n e r r u c k a e p a 1 r o f d e t a r e r a y b d e t n e v e r p s i s t e f s o m l a n r e t x e e h t f o n o i t c u d n o c s s o r c e h t . s e g a t s t u p t u o e h t e m i t a h t i w n o i t c n u j n o c n i r i a p t e f s o m e h t f o s n i p r e v i r d e h t n o e g a t l o v e h t g n i r o t i n o m . s c i t s i r e t c a r a h c f f o - n r u t t e f r o f d e z i m i t p o y a l e d 2 1e s a h p e h t s e s u 8 0 6 4 c s e h t . t e f s o m e d i s h g i h e h t n i t n e r r u c t i m i l o t d e s u s i n i p e s a h p e h t v e h t s s o r c a e g a t l o v n i t i m i l t n e r r u c e h t . t i m i l t n e r r u c e h t t e s o t r e d r o n i n i p t e s i d n a t i u c r i c n o i t a c i l p p a l a c i p y t e h t n i 3 r ( r o t s i s e r l a n r e t x e n a f o e u l a v e h t y b t e s s i d l o h s e r h t e s n e s e h t s s o r c a p o r d e g a t l o v e h t g n i r a p m o c y b d e m r o f r e p s i g n i t i m i l t n e r r u c . ) m a r g a i d e d i s h g i h e h t f o e c n a t s i s e r e c r u o s o t n i a r d e h t s s o r c a p o r d e g a t l o v e h t h t i w r o t s i s e r o t n i a r d e h t s s o r c a p o r d e g a t l o v e h t . d o i r e p n o i t c u d n o c s ? t e f s o m e h t g n i r u d t e f s o m v e h t m o r f d e n i a t b o s i t e f s o m e d i s h g i h e h t f o e c n a t s i s e r e c r u o s n i . n i p e s a h p d n a 3 1h v r d d e t a r e r a s r e v i r d t u p t u o e h t . t e f s o m ) h c t i w s n i a m ( e d i s h g i h e h t f o e t a g e h t s e v i r d h v r d e h t o t s l a n g i s e v i r d y r a t n e m e l p m o c s e d i v o r p y r t i u c r i c m w p e h t . s t n e r r u c k a e p a 1 r o f g n i r o t i n o m y b d e t n e v e r p s i s t e f s o m l a n r e t x e e h t f o n o i t c u d n o c s s o r c e h t . s e g a t s t u p t u o y a l e d e m i t a h t i w n o i t c n u j n o c n i r i a p t e f s o m e h t f o s n i p r e v i r d e h t n o e g a t l o v e h t . s c i t s i r e t c a r a h c f f o - n r u t t e f r o f d e z i m i t p o 4 1c n. n o i t c e n n o c o n 5 1t s b o t s t c e n n o c t s b . t e f s o m e d i s h g i h l e n n a h c - n n a e v i r d o t r e t r e v n o c e h t s e l b a n e n i p s i h t a o t e g a t l o v n i p t s b e h t s t s o o b t i u c r i c p m u p e g r a h c e h t . t i u c r i c p m u p e g r a h c l a n r e t x e e h t . t e f s o m e d i s h g i h e h t f o e t a g e h t g n i v i r d r o f l e v e l e g a t l o v e c r u o s - o t - e t a g t n e i c i f f u s 6 1d d v p. t e f s o m e d i s w o l r o f e g a t l o v y l p p u s r e w o p d a p l a m r e h t 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 . c 09 . 0 ss ? =
7 ? 2006 semtech corp. www.semtech.com SC4608 power management block diagram
8 ? 2006 semtech corp. www.semtech.com SC4608 power management application information enable the SC4608 is enabled by applying a voltage greater than 2.7 volts to the av dd pin. the SC4608 is disabled when av dd falls below 2.35 volts or when sleep mode opera- tion is invoked by clamping the en pin to a voltage below 0.3*av dd . 0.1 a is the typical current drawn through the av dd pin during sleep mode. during the sleep mode, the high side and low side mosfets are turned off and the internal soft start voltage is held low. oscillator the fset pin is used to set the pwm oscillator frequency through an external timing capacitor that is connected from the fset pin to the gnd pin. the resulting ramp waveform ion the fset pin is a triangle at the pwm fre- quency with a peak voltage of 1.3v and a valley voltage of 0.3v. 160ns minimum off time for the top switch allows the bootstrap capacitor to be charged during each cycle. the capacitor tolerance adds to the accuracy of the oscillator frequency. the approximate operating fre- quency and soft start time are both determined by the value of the external timing capacitor as shown in table 1. g n i m i t l a n r e t x e ) f p ( e u l a v r o t i c a p a c ) z h k ( y c n e u q e r f 0 2 10 0 0 1 0 7 25 7 5 0 7 40 5 3 0 6 55 9 2 table 1. operating frequency value based on the value of the external timing capacitor placed across the fset and gnd pins synchronous mode operation is invoked by using a sig- nal from an external clock. a low value resistor (100 ? typical) must be inserted in series with the timing capaci- tor between the timing capacitor and the gnd pin. the other terminal of the timing capacitor will remain con- nected to the fset pin. the transformed external clock signal is then connected to the junction of the external timing capacitor and the added resistor r sync as shown in figure 1. the maximum frequency of the external clock signal can be higher than the natural switching frequency by about 10%. c fset r sync 100 SC4608 fset r 1k d c 56pf external clock signal a c fset r sync 100 SC4608 fset r 1k d c 56pf external clock signal a figure 1 uvlo when the en pin is not pulled and held below 0.3*av dd , the voltage on the av dd pin determines the operation of the SC4608. as av dd increases during start up, the uvlo block senses av dd and keeps the high side and low side mosfets off and the internal soft start voltage low until av dd reaches 2.7v. if no faults are present, the SC4608 will initiate a soft start when av dd exceeds 2.7v. a hyster- esis (350mv) in the uvlo comparator provides noise im- munity during its start up. power good indicator the pgood pin is the open-drain output of the power good comparators. these comparators are incorporated with small amount of hysteresis. a pull-up resistor from the pgood pin to the input supply or the output sets the logic high level of the pgood signal. the vsense low-to- high trip voltage of the power good comparator is 89% of the final regulation voltage. the power good compara- tor output becomes valid provided that v o is within about 11% of the programmed output voltage. in shutdown mode the power good output is actively pulled low. the pgood signal delay depends on its operating frequency fs, for example, about 14ms@ fs=575khz and 24ms @ fs=330khz. soft start the soft start function is required for step down control- lers to prevent excess inrush current through the dc bus during start up. generally this can be done by sourcing a controlled current into a timing capacitor and then using the voltage across this capacitor to slowly ramp up the error amp reference. the closed loop creates narrow
9 ? 2006 semtech corp. www.semtech.com SC4608 power management application information (cont.) width driver pulses while the output voltage is low and allows these pulses to increase to their steady state duty cycle as the output voltage reaches its regulated value. with this, the inrush current from the input side is con- trolled. the duration of the soft start in the SC4608 is controlled by an external capacitor. ss, the start-up time is defined as: c 09 . 0 ss ? = where, c is the value of the external capacitor in nf, and ss is the start-up time in ms. over current protection the SC4608 detects over current conditions by sensing the voltage across the drain-to-source of the high side mosfet. the SC4608 determines the high side mos- fet current level by sensing the drain-to-source conduc- tion voltage across the high side mosfet via the v in (see the typical application circuit on page 1) and phase pin during the high side mosfets conduction period. this voltage value is then compared internally to a user pro- grammed current limit threshold. note that user should place kelvin sensing connections directly from the high side mosfet source to the phase pin. the current limit threshold is programmed by the user based on the rds(on) of the high side mosfet and the value of the external set resistor rset (where rset is represented by r3 in the applications schematics of this document). the SC4608 uses an internal current source to pull a 50a current from the input voltage to the iset pin through external resistor rset. the current limit threshold resistor (rset) value is calcu- lated using the following equation: a 50 r i r ) on ( ds max set ? = the r ds(on) sensing used in the SC4608 has an addi- tional feature that enhances the performance of the over current protection. because the r ds(on) has a positive temperature coefficient, the 50 a current source has a positive coefficient of about 0.3%/c providing first or- der correction for current sensing vs temperature. this compensation depends on the high amount of thermal transferring that typically exists between the high side n- mosfet and the SC4608 due to the compact layout of the power supply. when the converter detects an over current condition (i > i max ) as shown in figure 2, the first action the SC4608 takes is to enter the cycle by cycle protection mode (point b to point c), which responds to minor over current cases. then the output voltage is monitored. if the over current and low output voltage (set at 70% of nominal output voltage) occur at the same time, the hiccup mode op- eration (point c to point d) of the SC4608 is invoked and the internal soft start capacitor is discharged. this is like a typical soft start cycle: figure 2. over current protection characteristic of SC4608 nom o v ? ? 6 . 0 nom o v ? o v o i i max d c b a nom o v ? ? 7 . 0 nom o v ? o v o i i max d c b a nom o v ? ? 6 . 0 nom o v ? o v o i i max d c b a nom o v ? ? 7 . 0 nom o v ? o v o i i max d c b a power mosfet drivers the SC4608 has two drivers which are optimized for driv- ing external power n-channel mosfets. the driver block consists two 1 amp drivers. drvh drives the high side n-mosfet (main switch), and drvl drives the low side n-mosfet (synchronous rectifier transistor). the output drivers also have gate drive non-overlap mechanism that provides a dead time between drvh and drvl transitions to avoid potential shoot through problems in the external mosfets. by using the proper design and the appropriate mosfets, the SC4608 is capable of driving a converter with up to 12a of output current. as shown in figure 3, t d1 , the delay from the top mosfet off to the bottom mosfet on is adaptive by detecting the voltage of the phase node. t d2 , the delay from the bottom mosfet off to the top mosfet on is fixed, is 40ns for the SC4608. this control scheme guar- antees avoidance of cross conduction or shoot through
10 ? 2006 semtech corp. www.semtech.com SC4608 power management between the upper and lower mosfets and also mini- mizes the conduction loss in the body diode of the bot- tom mosfet for high efficiency applications. bottom mosfet gate drive top mosfet gate drive phase node ground t d1 t d2 bottom mosfet gate drive top mosfet gate drive phase node ground t d1 t d2 figure 3. timing waveforms for gate drives and phase node inductor selection the factors for selecting the inductor include its cost, efficiency, size and emi. for a typical SC4608 applica- tion, the inductor selection is mainly based on its value, saturation current and dc resistance. increasing the in- ductor value will decrease the ripple level of the output voltage while the output transient response will be de- graded. low value inductors offer small size and fast tran- sient responses while they cause large ripple currents, poor efficiencies and more output capacitance to smooth out the large ripple currents. the inductor should be able to handle the peak current without saturating and its copper resistance in the winding should be as low as possible to minimize its resistive power loss. a good trade- off among its size, loss and cost is to set the inductor ripple current to be within 15% to 30% of the maximum output current. the inductor value can be determined according to its operating point and the switching frequency as follows: omax s in out in out i i f v ) v v ( v l ? ? ? ? ? ? = where: f s = switching frequency and ? i = ratio of the peak to peak inductor current to the maximum output load current. the peak to peak inductor current is: omax p p i i i ? ? = ? after the required inductor value is selected, the proper selection of the core material is based on the peak in- ductor current and efficiency requirements. the core must be able to handle the peak inductor current i peak without saturation and produce low core loss during the high frequency operation is: 2 i i i p p omax peak ? + = the power loss for the inductor includes its core loss and copper loss. if possible, the winding resistance should be minimized to reduce inductor?s copper loss. the core loss can be found in the manufacturer?s datasheet. the inductor? copper loss can be estimated as follows: winding lrms 2 copper r i p ? = where: i lrms is the rms current in the inductor. this current can be calculated as follow is: 2 omax lrms i 3 1 1 i i ? ? + ? = output capacitor selection basically there are two major factors to consider in se- lecting the type and quantity of the output capacitors. the first one is the required esr (equivalent series re- sistance) which should be low enough to reduce the volt- age deviation from its nominal one during its load changes. the second one is the required capacitance, which should be high enough to hold up the output voltage. before the SC4608 regulates the inductor current to a new value during a load transient, the output capacitor delivers all the additional current needed by the load. the esr and esl of the output capacitor, the loop parasitic inductance between the output capacitor and the load combined with inductor ripple current are all major contributors to the output voltage ripple. surface mount speciality poly- mer aluminum electrolytic chip capacitors in ue series from panasonic provide low esr and reduce the total capacitance required for a fast transient response. poscap from sanyo is a solid electrolytic chip capacitor that has a low esr and good performance for high fre- quency with a low profile and high capacitance. above mentioned capacitors are recommended to use in SC4608 application. input capacitor selection the input capacitor selection is based on its ripple cur- rent level, required capacitance and voltage rating. this capacitor must be able to provide the ripple current by the switching actions. for the continuous conduction application information (cont.)
11 ? 2006 semtech corp. www.semtech.com SC4608 power management application information (cont.) mode, the rms value of the input capacitor can be cal- culated from: in 2 out in out omax ) rms ( cin v ) v v ( v i i ? ? ? = this current gives the capacitor?s power loss as follows: ) esr ( cin ) rms ( cin 2 cin r i p ? = this capacitor?s rms loss can be a significant part of the total loss in the converter and reduce the overall con- verter efficiency. the input ripple voltage mainly depends on the input capacitor?s esr and its capacitance for a given load, input voltage and output voltage. assuming that the input current of the converter is constant, the required input capacitance for a given voltage ripple can be calculated by: ) r i v ( fs ) d 1 ( d i c ) esr ( cin omax i omax in ? ? ? ? ? ? ? = where: d = v o /v i , duty ratio and ? v i = the given input voltage ripple. because the input capacitor is exposed to the large surge current, attention is needed for the input capacitor. if tantalum capacitors are used at the input side of the converter, one needs to ensure that the rms and surge ratings are not exceeded. for generic tantalum capaci- tors, it is wise to derate their voltage ratings at a ratio of 2 to protect these input capacitors. boost capacitor selection the boost capacitor selection is based on its discharge ripple voltage, worst case conduction time and boost current. the worst case conduction time t w can be esti- mated as follows: max s d f 1 tw ? = where: f s = the switching frequency and dmax = maximum duty ratio. the required minimum capacitance for boost capacitor will be: w d b boost t v i c ? = where: i b = the boost current and v d = discharge ripple voltage. with f s = 300kh, v d =0.3v and ib = 50ma, the required capacitance for the boost capacitor is: nf 528 95 . 0 k 300 1 3 . 0 05 . 0 d f 1 v i c max s d b boost = ? ? = ? ? = power mosfet selection the SC4608 can drive an n-mosfet at the high side and an n-mosfet synchronous rectifier at the low side. the use of the high side n-mosfet will significantly re- duce its conduction loss for high current. for the top mosfet, its total power loss includes its conduction loss, switching loss, gate charge loss, output capacitance loss and the loss related to the reverse recovery of the bot- tom diode, shown as follows: s i rr oss s gate gt 2 gs gd g gate s i peak _ top on _ top rms _ top 2 total _ top f v ) q q ( f v q ) q q ( r v f v i r i p ? ? + + ? ? + + ? ? ? + ? = where: r g = gate drive resistor, q gd = the gate to drain charge of the top mosfet, q gs2 = the gate to source charge of the top mosfet, q gt = the total gate charge of the top mosfet, q oss = the output charge of the top mosfet and q rr = the reverse recovery charge of the bottom diode. for the top mosfet, it experiences high current and high voltage overlap during each on/off transition. but for the bottom mosfet, its switching voltage is the bottom diode?s forward drop during its on/off transition. so the switching loss for the bottom mosfet is negligible. its total power loss can be determined by: f avg d s gate gb on _ bot rms _ bot 2 total _ bot v _ i f v q r i p ? + ? ? + ? = where: q gb = the total gate charge of the bottom mosfet and v f = the forward voltage drop of the bottom diode.
12 ? 2006 semtech corp. www.semtech.com SC4608 power management c2 r1 c1 c4 r v out l1 c9 r8 r7 r9 comp vsense phase s c 4608 c2 r1 c1 c4 r v out l1 c9 r8 r7 r9 comp vsense phase s c 4608 figure 4. compensation network provides 3 poles and 2 zeros. for voltage mode step down applications as shown in figure 4, the power stage transfer function is: 4 1 2 1 4 c i vd c l s r l s 1 c r 1 s 1 v ) s ( g + + ? + = where: r = load resistance and r c = c 4 ?s esr. the compensation network will have the characteristic as follows: 2 p 2 z 1 p 1 z i comp s 1 s 1 s 1 s 1 s ) s ( g + ? + ? + + ? = where ) c c ( r 1 2 1 7 i + ? = 2 1 1 z c r 1 ? = 9 8 7 2 z c ) r r ( 1 ? + = 2 1 1 2 1 1 p c c r c c ? ? + = application information (cont.) for a low voltage and high output current application such as the 3.3v/1.5v@12a case, the conduction loss is of- ten dominant and selecting low r ds(on) mosfets will no- ticeably improve the efficiency of the converter even though they give higher switching losses. the gate charge loss portion of the top/bottom mosfet?s total power loss is derived from the SC4608. this gate charge loss is based on certain operating conditions (f s , v gate , and i o ). the thermal estimations have to be done for both mosfets to make sure that their junction temperatures do not exceed their thermal ratings according to their total power losses p total , ambient temperature t a and their thermal resistance r ja as follows: ja total a (max) j r p t t + < loop compensation design for a dc/dc converter, it is usually required that the converter has a loop gain of a high cross-over frequency for fast load response, high dc and low frequency gain for low steady state error, and enough phase margin for its operating stability. often one can not have all these properties at the same time. the purpose of the loop compensation is to arrange the poles and zeros of the compensation network to meet the requirements for a specific application. the SC4608 has an internal error amplifier and requires the compensation network to connect among the comp pin and vsense pin, gnd, and the output as shown in figure 4. the compensation network includes c1, c2, r1, r7, r8 and c9. r9 is used to program the output voltage according to: ) r r 1 ( 5 . 0 v 9 7 o + ? =
13 ? 2006 semtech corp. www.semtech.com SC4608 power management 9 8 2 p c r 1 ? = after the compensation, the converter will have the fol- lowing loop gain: c l s r l s 1 c r 1 s 1 s 1 s 1 s 1 s 1 s v v 1 ) s ( g ) s ( g g ) s ( t 1 2 1 4 c 2 p 2 z 1 p 1 z i i m vd comp pwm + + ? + ? + ? + ? + + ? ? ? = ? ? = where: g pwm = pwm gain v m = 1.0v, ramp peak to valley voltage of SC4608 the design guidelines for the SC4608 applications are as following: 1. set the loop gain crossover corner frequency c for given switching corner frequency s = 2 fs, 2. place an integrator at the origin to increase dc and low frequency gains. 3. select z1 and z2 such that they are placed near o to damp the peaking and the loop gain has a -20db/dec rate to go across the 0db line for obtaining a wide bandwidth. 4. cancel the zero from c 4 ?s esr by a compensator pole p1 ( p1 = esr = 1/( r c c 4 )). 5. place a high frequency compensator pole p 2 ( p 2 = f s ) to get the maximum attenuation of the switch- ing ripple and high frequency noise with the adequate phase lag at c . the compensated loop gain will be as given in figure 5: - 0db gvd t z1 z2 p1 p2 c esr o loo p g ain power stage - figure 5. asymptotic diagrams of power stage and its loop gain application information (cont.) layout guidelines in order to achieve optimal electrical, thermal and noise performance for high frequency converters, special at- tention must be paid to the pcb layouts. the goal of lay- out optimization is to identify the high di/dt loops and minimize them. the following guideline should be used to ensure proper functions of the converters. 1. a ground plane is recommended to minimize noises and copper losses, and maximize heat dissipation. 2. start the pcb layout by placing the power compo- nents first. arrange the power circuit to achieve a clean power flow route. put all the connections on one side of the pcb with wide copper filled areas if possible. 3. the av dd bypass capacitor should be placed next to the av dd and agnd pins. 4. the trace connecting the feedback resistors to the output should be short, direct and far away from the noise sources such as switching node and switching components. 5. minimize the traces between drvh/drvl and the gates of the mosfets to reduce their impedance to drive the mosfets. 6. minimize the loop including input capacitors, top/bot- tom mosfets. this loop passes high di/dt current. make sure the trace width is wide enough to reduce copper losses in this loop. 7. iset and phase connections to the top mosfet for current sensing must use kelvin connections. 8. maximize the trace width of the loop connecting the inductor, bottom mosfet and the output capacitors. 9. connect the ground of the feedback divider and the compensation components directly to the agnd pin of the SC4608 by using a separate ground trace. then connect this pin to the ground of the output capacitor as close as possible
14 ? 2006 semtech corp. www.semtech.com SC4608 power management application information (cont.) design example 1. 3v to1.5v @10a application with SC4608 r3 c3 1u c2 2.2n r1 14.3k c1 1.8n v out =1.5v/ 10a c17 1u d2 l1 2.3u c5 22u c4 22u m11 c10 220u m2 c9 8.2n r8 107 r7 5.76k r9 2.87k v in =3v - 5.5v r6 0 r5 0 c16 470pf avdd 1 iset 2 comp 3 fset 4 pvdd 16 bst 15 phase 12 ss 7 vsense 8 drvh 13 pgood 6 en 5 agnd 9 pgnd 10 drvl 11 SC4608 r2 10k c7 330u c13 22u c14 22u r13 1 22n css r3 c3 1u c2 2.2n r1 14.3k c1 1.8n v out =1.5v/ 10a c17 1u d2 l1 2.3u c5 22u c4 22u m11 c10 220u m2 c9 8.2n r8 107 r7 5.76k r9 2.87k v in =3v - 5.5v r6 0 r5 0 c16 470pf avdd 1 iset 2 comp 3 fset 4 pvdd 16 bst 15 phase 12 ss 7 vsense 8 drvh 13 pgood 6 en 5 agnd 9 pgnd 10 drvl 11 SC4608 r2 10k c7 330u c13 22u c14 22u r13 1 22n css
15 ? 2006 semtech corp. www.semtech.com SC4608 power management m e t iy t qe c n e r e f e re u l a vr e r u t c a f u n a m / . o n t r a p 111 cf n 8 . 1 212 cf n 2 . 2 317 1 cf u 1 44 4 1 c , 3 1 c , 5 c , 4 c6 0 2 1 , f u 2 2m 6 2 2 j 0 r 5 x 5 2 2 3 c : n / p k d t 517 c0 7 8 2 , f u 0 3 3l m 0 3 3 b p t 6 : n / p o y n a s 619 cf n 2 . 8 716 1 cf p 0 7 4 812 d1 t l 0 2 5 0 r b m1 t l 0 2 5 0 r b m : n / p i m e s n o 911 lh u 3 . 2 c i n o r t c e l e r e p o o c 3 r 2 - 1 c h : n / p 0 12 2 m , 1 m8 - o s , k c a p r e w o pp d 2 8 8 7 i s : n / p y a h s i v 1 111 rk 3 . 4 1 2 113 rk 3 3 . 1 3 117 rk 6 7 . 5 4 118 r7 0 1 5 119 rk 7 8 . 2 6 113 1 r1 7 113 c5 0 8 0 , f u 1 8 110 1 c0 7 8 2 , f u 0 2 2l m 0 2 2 b p t 6 : n / p o y n a s 9 11s s cf n 2 2 0 211 u8 0 6 4 c st r t l m i 8 0 6 4 c s : n / p h c e t m e s 1 212 rk 0 1 . e g a k c a p 3 0 6 0 h t i w n o i s i c e r p % 1 e v a h s r o t s i s e r l l a , d e i f i c e p s s s e l n u % 0 2 - / + e r a s r o t i c a p a c l l a d n a % 1 - / + e r a s r o t s i s e r bill of materials
16 ? 2006 semtech corp. www.semtech.com SC4608 power management pcb layout component side (top) component side (bottom)
17 ? 2006 semtech corp. www.semtech.com SC4608 power management (inter layer 1) (inter layer 2) pcb layout (cont.) (top) (bottom)
18 ? 2006 semtech corp. www.semtech.com SC4608 power management .003 .010 .100 16 .012 .106 - .000 .031 (.008) 0.08 0.30 16 .014 .110 0.25 2.55 .040 - .002 - 0.00 0.80 2.80 0.35 2.70 - 0.05 1.00 (0.20) .004 0.10 0.65 bsc .026 bsc 0.30 .012 .020 .016 0.40 0.50 d/2 2 a a1 1 bbb c a b a2 bxn e seating plane c e/2 n e/2 aaa c controlling dimensions are in millimeters (angles in degrees). coplanarity applies to the exposed pad as well as the terminals. 1. 2. notes: a d e b - - - - indicator (laser mark) pin 1 dimensions nom inches n bbb aaa a2 a1 e1 d1 dim l e e d a b min max millimeters min max nom .153 .157 .161 3.90 4.00 4.10 .153 .157 .161 3.90 4.00 4.10 lxn d1 e1 .100 .106 .110 2.55 2.70 2.80 outline drawing - mlp-16
19 ? 2006 semtech corp. www.semtech.com SC4608 power management semtech corporation power management products division 200 flynn road, camarillo, ca 93012 phone: (805)498-2111 fax (805)498-3804 land pattern - mlp-16 contact information this land pattern is for reference purposes only. consult your manufacturing group to ensure your company's manufacturing guidelines are met. notes: 1. 2x g h 2x (c) 2x z x p y k c z p y x g k h .189 .026 .016 .033 .122 .106 .106 4.80 0.40 0.85 0.65 2.70 2.70 3.10 dim (3.95) millimeters dimensions (.156) inches

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