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power management 1 www.semtech.com sc4517a 1.25mhz, 1.5a step-down switching regulator description features typical application circuit applications revision: december 13, 2006 the sc4517a is a current mode switching regulator with an integrated switch, operating at 1.25mhz with separate sync & enable functions. the integrated switch allows for cost effective low power solutions (peak switch current 1.5 amps). the sync function allows customers to synchronize to a faster clock in order to avoid frequency beating in noise sensitive applications. high frequency of operation allows for very small passive components. current mode operation allows for fast dynamic response & instantaneous duty cycle adjustment as the input varies (ideal for cpe applications where the input is a wall plug power). the low shutdown current makes it ideal for portable applications where battery life is important. the sc4517a is a 1.25mhz switching regulator synchronizable to a faster frequency from 1.6mhz to 2mhz. ? integrated 1.5 amp switch ? 1.25mhz frequency of operation ? current mode controller ? synchronizable to higher frequency up to 2mhz ? 6a low shutdown current ? msop-8 and mlpd-8 lead-free packages. this product is fully weee and rohs compliant ? xdsl modems ? cpe equipment ? dc-dc point of load applications ? portable equipment c1 vout d2 d1 r3 c2 l1 enable bst 1 in 2 sw 3 gnd 4 en 5 fb 6 comp 7 sync 8 vin c4 c3 sc4517axx r1 r2
? 2006 semtech corp. www.semtech.com power management sc4517a 2 r e t e m a r a pl o b m y ss t i m i ls t i n u e g a t l o v y l p p u s t u p n i ) 1 ( v n i 4 2 + o t 3 . 0 -v v e v o b a n i p t s o o b w s v ( t s b v - w s )6 1v e g a t l o v n i p t s o o bv t s b 2 3 + o t 3 . 0 -v e g a t l o v n i p n ev n e 6 1 + o t 3 . 0 -v e g a t l o v n i p b fv b f 6 + o t 3 . 0 -v t n e r r u c n i p b fi b f 1a m t n e r r u c n i p c n y si c n y s 1a m t n e i b m a o t n o i t c n u j e c n a d e p m i l a m r e h t ) 2 ( p o s m d p l m a j 5 8 1 3 5 w / c 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 g n i t a r e p oe 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 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 ) p o s m ( 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 ) d p l m ( 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 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 t u p n i g n i t a r e p o ) 1 ( v n i 6 1v t i m i l t n e r r u c h c t i w s m u m i x a mi w s 5 . 1a y c n e u q e r f r o t a l l i c s of c s o 9 . 05 2 . 16 . 1z h m p o r d e g a t l o v n o h c t i w sv ) w s ( d 0 3 30 5 5v m v n i t u o k c o l e g a t l o v r e d n uv o l v u 0 6 . 23v v n i t n e r r u c y b d n a t si q v b f v = ) m o n ( t u o % 7 1 +0 . 15 a m t n e r r u c n w o d t u h si ) f f o ( q v n e v , v 0 = n i v , v 6 1 = w s v 0 =5 4a electrical characteristics unless specified: v in = 12v, v comp = 0.8v, v bst = v in + 5v, en = tied to v in , sync = 0, sw = open. t a = t j = -40c to 125c. absolute maximum ratings 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. notes: (1) for proper operation of device, v in should be within maximum operating input voltage as defined in electrical characteristics. (2) minimum pad size.
3 ? 2006 semtech corp. www.semtech.com power management sc4517a 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 t n e r r u c t u p n i b fi b f ) j d a ( a 7 1 5 4 c s5 2 . 0 -0 5 . 0 -a e g a t l o v k c a b d e e f v < v 3 n i v 6 1 < ) 1 ( b f v = t u o v , ) j d a ( a 7 1 5 4 c s3 7 1 . 12 . 17 2 2 . 1 v v 8 . 1 - a 7 1 5 4 c s4 6 7 . 18 . 16 3 8 . 1 v 5 . 2 - a 7 1 5 4 c s5 4 . 25 . 25 5 . 2 v 3 . 3 - a 7 1 5 4 c s4 3 2 . 33 . 36 6 3 . 3 v 5 - a 7 1 5 4 c s9 . 451 . 5 e c n a t s i s e r t u p n i b fr b f v 8 . 1 - a 7 1 5 4 c s5 . 0 10 . 5 10 . 1 2k ? v 5 . 2 - a 7 1 5 4 c s7 . 4 10 . 1 20 . 0 3 v 3 . 3 - a 7 1 5 4 c s0 . 9 15 . 7 20 . 9 3 v 5 - a 7 1 5 4 c s0 . 9 20 . 2 40 . 0 6 v o t b f p m o c n i a g e g a t l o v ) 2 ( v 4 . 0 v p m o c v 9 . 00 5 10 5 3 v o t b f p m o c e c n a t c u d n o c s n a r t ) 2 ( ? i p m o c = a 0 1 0 0 50 5 80 0 3 1o h m v p m o c t n e r r u c e c r u o s n i pv b f v = ) m o n ( t u o % 7 1 -0 2 10 6 1a v p m o c t n e r r u c k n i s n i pv b f v = ) m o n ( t u o % 7 1 +0 1 10 8 1a v p m o c t n e r r u c h c t i w s o t n i p e c n a t c u d n o c s n a r t 5 . 2v / a v p m o c g n i h c t i w s m u m i x a m n i p d l o h s e r h t % 0 = e l c y c y t u d5 3 . 0v v p m o c d l o h s e r h t n i pi w s a 5 . 1 =9 . 0v e l c y c y t u d h c t i w s m u m i x a mv p m o c i , v 2 . 1 = w s a m 0 0 4 =0 80 9% e g a t l o v t s o o b m u m i n i m h c t i w s e v o b a i w s c 0 , a 5 . 1 = t a d n a c 5 2 1 i w s t , a 3 . 1 = a c 0 < 8 . 17 . 2v t n e r r u c t s o o bi w s a 5 . 0 =0 15 1a m i w s c 0 , a 5 . 1 = t a d n a c 5 2 1 i w s t , a 3 . 1 = a c 0 < 0 35 4 electrical characteristics (cont.) unless specified: v in = 12v, v comp = 0.8v, v bst = v in + 5v, en = tied to v in , sync = 0, sw = open. t a = t j = -40c to 125c.
? 2006 semtech corp. www.semtech.com power management sc4517a 4 marking information electrical characteristics (cont.) 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 d l o h s e r h t t u p n i e l b a n ev h i 2v v l i 4 . 0v t n e r r u c s a i b t u p n i e l b a n ei l i 0 1 -a i h i d l o h s e r h t w o l e b v m 0 0 1 = n e75 1a e g a t l o v d l o h s e r h t c n y s 5 . 1v y c n e u q e r f t u p n i c n y s ) 3 ( 6 . 12z h m e c n a t s i s e r n i p c n y sv c n y s v 5 . 0 =0 2k ? ap50 yyww apaj yyww part number code (example: ap50 = 5.0v (50) yyww = date code (example: 0012) xxxx = semtech lot no. (example: e901 xxxx 01-1) yyww = date code (example: 0012) xxxx = semtech lot no. (example: e901 xxxx 01-1) voltage options (msop) sc4517aimsxx adjustable options (msop) sc4517aims sc 4517 yyww adjustable options (mlpd) sc4517aiml yyww = date code (example: 0012) yyww xx 4517 part number code (example: xx = 5.0v (50) yyww = date code (example: 0012) voltage options (mlpd) sc4517aimlxx unless specified: v in = 12v, v comp = 0.8v, v bst = v in + 5v, en = tied to v in , sync = 0, sw = open. t a = t j = -40c to 125c. notes: (1) the device is not guaranteed to function outside of its operating condition. the required minimum input voltage for a regulated output depends on the output voltage and load condition. (2) guaranteed by design. (3) for sync applications, please contact factory.
5 ? 2006 semtech corp. www.semtech.com power management sc4517a pin configurations ordering information r e b m u n t r a p ) 3 ( ) 2 ( ) 1 ( e g a k c a p t r t x x s m i a 7 1 5 4 c s 8 - p o s m t r t s m i a 7 1 5 4 c s t r t x x l m i a 7 1 5 4 c s 8 - d p l m t r t l m i a 7 1 5 4 c s b v e a 7 1 5 4 c sd r a o b n o i t a u l a v e notes: (1) where xx denotes voltage options. available voltages are: 1.8v (18), 2.5v (25), 3.3v (33) and 5.0v (50). leave blank for adjustable voltage option. (2) only available in tape and reel packaging. a reel contains 2500 for msop and 3000 for mlp devices. (3) lead-free product. this product is fully weee and rohs compliant. 1 2 3 4 sync bst top view (8 pin msop) 5 6 7 8 comp in fb sw en gnd # n i pe m a n n i pn o i t c n u f n i p 1t s b s i h c t i w s s i h t r o f e g a t l o v n o n r u t m u m i n i m e h t . h c t i w s n p n l a n r e t n i e h t o t r e w o p s e d i v o r p n i p s i h t . v 7 . 2 2n i g n i r u d t d / i d h g i h s e e s n i p s i h t . y r t i u c r i c r e w o p d n a l o r t n o c y b d e r i u q e r r e w o p l l a s r e v i l e d n i n i p s a e s o l c s a n i p s i h t o t d e h c a t t a e b d l u o h s r o t i c a p a c g n i l p u o c e d a . h c t i w s e h t f o s n o i t c a g n i h c t i w s . e l b i s s o p 3w s s a d e t c e n n o c e b d l u o h s e d o i d g n i l e e h w e e r f l a n r e t x e e h t . h c t i w s l a n r e t n i e h t f o r e t t i m e e h t s i w s n i p . n i p s i h t o t e l b i s s o p s a e s o l c 4d n g g n i l e e h w e e r f e h t d n a r o t i c a p a c g n i l p u o c e d e h t . n i p s i h t o t t c e p s e r h t i w d e r u s a e m e r a s e g a t l o v l l a . e l b i s s o p s a t r o h s s a d n g o t d e t c e n n o c e b d l u o h s e d o i d 5n e e h t . w o l s i n e f i f f o s i t i d n a , h g i h s i n e f i n o d e h c t i w s s i r o t a l u g e r e h t . t u p n i e l b a n e p i h c e h t s i s i h t w e f a o t d e c u d e r s i t n e r r u c y l p p u s t u p n i e h t d n a , w o l s i n e n e h w e d o m y b d n a t s n i s i r o t a l u g e r . d e s u t o n f i n i v o t p u d e l l u p e b o t s d e e n t i . s e r e p m a o r c i m 6b f y l t c e r i d e b d l u o h s n i p s i h t , s r e l l o r t n o c t u p t u o d e x i f r o f . s r e l l o r t n o c t u p t u o e l b a t s u j d a r o f t u p n i k c a b d e e f - a 7 1 5 4 c s ( s p i h c e h t o t n i d e t a r g e t n i n e e b e v a h s r e d i v i d e g a t l o v e h t e c n i s t u p t u o e h t o t d e t c e n n o c . ) x x 7p m o c a . r o t a r a p m o c t n e r r u c k a e p e h t f o t u p n i d n a r e i f i l p m a r o r r e l a n r e t n i e h t f o t u p t u o e h t s i s i h t . e c n a m r o f r e p d e i f i c e p s e h t e v e i h c a o t n i p s i h t o t d e t c e n n o c s i k r o w t e n n o i t a s n e p m o c 8c n y s l o r t n o c e s l u p l a n r e t x e n a o t r o t a l l i c s o l a n r e t n i e h t e z i n o r h c n y s o t d e s u n i p l o r t n o c s u o n o r h c n y s s i s i h t . d n g o t d e t c e n n o c e b d l u o h s t i , d e s u t o n n e h w . l a n g i s l a m r e h t d a p ) y l n o d p l m ( d e t c e n n o c y l l a c i r t c e l e 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 . 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 pin descriptions (8 pin mlpd) top view
? 2006 semtech corp. www.semtech.com power management sc4517a 6 block diagram fb comp r q s pwm - + power in transistor slope + + + isen 40m sw bst oscillator clk frequency slope comp sync slope ol fb en reference 1v uvlo 0.7v ea soft start hiccup is is gnd fb comp r q s pwm - + power in transistor slope + + + isen 40m sw bst oscillator clk frequency slope comp sync slope ol fb en reference 1v uvlo 0.7v ea soft start hiccup is is gnd
7 ? 2006 semtech corp. www.semtech.com power management sc4517a application information general overview the sc4517a is a high frequency current mode pwm buck regulator. it has an internal clock with fixed- frequency. the sc4517a uses two feedback loops (voltage loop and current loop) that control the duty cycle of the internal power switch. the error amplifier functions like the one of the voltage mode controller. the output of the error amplifier provides a switch current reference. this technique effectively removes one of the double poles in the output lc filter stage. with this, it is easier to compensate a current mode converter for better performance. a minimum 2.7v voltage is required to saturate the npn power switch when it is ?on? to reduce its conduction loss. current limit and overcurrent protection the current sense amplifier in the sc4517a monitors the switch current during each cycle. overcurrent protection (ocp) is triggered when the current limit exceeds the upper limit of 1.5a, detected by a voltage on comp being greater than about 2v. when an ocp fault is detected, the power switch is turned off and the external comp capacitor is quickly discharged using an internal small signal npn transistor. once the comp voltage has fallen below 250mv the power switch is turned off, control circuit is held off for 50 s determined by a internal timer. when the 50s time is up, an internal timer prevents any operation for 50s, the part enters a normal startup cycle. in the case of sustained overcurrent or dead-short, the part will continually cycle through the retry sequence, at a rate dependent on the value of ccomp. during start up, the voltage on comp rises roughly at the rate of dv/dt = 120 a/c comp . c comp is the total capacitance value attached to comp. therefore, the retry time for a sustained overcurrent can be approximately calculated as: 50us 120ua 2v c t comp retry + ? = figure 1 shows the voltage on comp during a sustained overcurrent condition. 2v 250mv figure 1. voltage on comp for startup and ocp enable pulling and holding the en pin below 0.4v activates the shut down mode of the sc4517a which reduces the input supply current to less than 10 a. during the shut down mode, the switch is turned off. the sc4517a is turned on if the en pin is pulled high. oscillator its internal free running oscillator sets the pwm frequency at 1.25mhz for the sc4517a without any external components to program the frequency. an external clock with a duty cycle from 20% to 80% connected to the sync pin activates synchronous mode. the frequency of the external clock can be from 1.6mhz to 2mhz. uvlo when the en pin is pulled and held above 1.8v, the voltage on pin in determines the operation of the sc4517a. as vin increases during power up, the internal circuit senses vin and keeps the power transistor off until vin reaches 2.6v. load current the peak current i peak in the switch is internally limited. for a specific application, the allowed load current i omax will change if the input voltage drifts away from the original design as given for current continuous mode: s o omax f l 2 ) d 1 ( v 5 . 1 i ? ? ? ? ? = where: fs = switching frequency, v o = output voltage; d = duty ratio, v o /v i ; v i = input voltage figure 2 shows the theoretical maximum load current for the specific cases. in a real application, however, the allowed maximum load current also depends on the layout and the air cooling condition. therefore, the maximum load current may need to be degraded according to the
? 2006 semtech corp. www.semtech.com power management sc4517a 8 thermal situation of the application. for example, the sc4517a with edp package is able to handle higher current than the sc4517a with msop package if their working conditions are same. maximum load current vs input voltage l=4.7uh 1.200 1.250 1.300 1.350 1.400 4 6 8 1012141618 vi (v) iomax (a) vo=2.5v vo=3.3v vo=5v figure 2. theoretical maximum load current curves. inductor selection the factors for selecting the inductor include its cost, efficiency, size and emi. for a typical sc4517a application, the inductor selection is mainly based on its value, saturation current and dc resistance. increasing the inductor value will decrease the ripple level of the output voltage while the output transient response will be degraded. low value inductors offer small size and fast transient responses while they cause large ripple currents, poor efficiencies and more output capacitance to filter 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 under its continuous mode and the switching frequency as follows: omax s i o i o i f v ) v v ( v l ? ? ? ? ? = application information (cont.) where: fs = switching frequency, = ratio of the peak to peak inductor current to the output load current and v o = output voltage. the peak to peak inductor current is: omax p p i i ? = ? 2 i i i p p omax peak ? + = after the required inductor value is selected, the proper selection of the core material is based on the peak inductor current and efficiency specifications. 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. 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 must be able to handle the peak inductor current i peak without saturation and produce low core loss during the high frequency operation. 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?s 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 follows: 2 omax lrms 12 1 1 i i ? + ? = output capacitor selection basically there are two major factors to consider in selecting the type and quantity of the output capacitors. the first one is the required esr (equivalent series resistance) which should be low enough to reduce the output voltage deviation during load changes. the second one is the required capacitance, which should be high enough to hold up the output voltage. before the sc4517a regulates the inductor current to a new value during a load transient, the output capacitor delivers all
9 ? 2006 semtech corp. www.semtech.com power management sc4517a 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 ceramic capacitors are recommended. input capacitor selection the input capacitor selection is based on its ripple current level, required capacitance and voltage rating. this capacitor must be able to provide the ripple current by the switching actions. for the continuous conduction mode, the rms value of the input capacitor current i cin(rms) can be calculated from: i 2 o i o omax ) rms ( cin v ) v v ( v i i ? ? ? = this current gives the capacitor?s power loss through its r cin(esr) as follows: ) esr ( cin ) rms ( cin 2 cin r i p ? = 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: ? 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 capacitors, it is suggested to derate their voltage ratings at a ratio of about two to protect these input capacitors. boost capacitor and its supply source 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 estimated as follows: max s w d f 1 t ? = where: f s = the switching frequency and dmax = maximum duty ratio, 0.9 for the sc4517a. the required minimum capacitance for the 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 = 1.2mhz, v d = 0.5v and i b =0.045a, the required minimum capacitance for the boost capacitor is: nf 5 . 67 9 . 0 m 2 . 1 1 5 . 0 045 . 0 d f 1 v i c max s d b boost = ? ? = ? ? = the internal driver of the switch requires a minimum 2.7v to fully turn on that switch to reduce its conduction loss. if the output voltage is less than 2.7v, the boost capacitor can be connected to either the input side or an independent supply with a decoupling capacitor. but the pin bst should not see a voltage higher than its maximum rating. freewheeling diode selection this diode conducts during the switch?s off-time. the diode should have enough current capability for full load and short circuit conditions without any thermal concerns. its maximum repetitive reverse block voltage has to be higher than the input voltage of the sc4517a. a low forward conduction drop is also required to increase the overall efficiency. the freewheeling diode should be turned on and off fast with minimum reverse recovery because the sc4517a is designed for high frequency applications. ss13 schottky rectifier is recommended for certain applications. the average current of the diode, i d _ avg can be calculated by: ) d 1 ( i i max o avg _ d ? ? = application information (cont.)
? 2006 semtech corp. www.semtech.com power management sc4517a 10 application information (cont.) thermal considerations there are three major power dissipation sources for the sc4517a. the internal switch conduction loss, its switching loss due to the high frequency switching actions and the base drive boost circuit loss. these losses can be estimated as: ) v ( d i 500 10 v i 10 5 . 22 d r i p boost o i o 3 on 2 o total ? ? ? + ? ? ? + ? ? = ? where: i o = load current; r on = on-equivalent resistance of the switch; v boost = input voltage or output based on the boost circuit connection. the junction temperature of the sc4517a can be further decided by: total ja a j p t t ? + = ja is the thermal resistance from junction to ambient. its value is a function of the ic package, the application layout and the air cooling system. it is recommended that a big copper area attached to pin 4 or the thermal pad be used for better cooling condition. the freewheeling diode also contributes a significant portion of the total converter loss. this loss should be minimized to increase the converter efficiency by using schottky diodes with low forward drop (v f ). ) d 1 ( i v p o f diode ? ? ? = loop compensation design the sc4517a has an internal error amplifier and requires a compensation network to connect between the comp pin and gnd pin as shown in figure 3. the compensation network includes c4, c5 and r3. r1 and r2 are used to program the output voltage according to: ) r r 1 ( 2 . 1 v 2 1 o + ? = assuming the power stage esr (equivalent series resistance) zero is an order of magnitude higher than the closed loop bandwidth, which is typically one tenth of the switching frequency, the power stage control to output transfer function with the current loop closed (ridley model) for the sc4517a will be as follows: c r 1 s 1 r 5 . 2 ) s ( g l l vd ? + ? = where: r l ? load and c ? output capacitor. the goal of the compensation design is to shape the loop to have a high dc gain, high bandwidth, enough phase margin, and high attenuation for high frequency noises. figure 3 gives a typical compensation network which offers 2 poles and 1 zero to the power stage: sc4517 bst 1 in 2 sw 3 gnd 4 en 5 fb 6 comp 7 sync 8 r2 r1 r3 c4 c d2 l1 c5 vout figure 3. compensation network provides 2 poles and 1 zero. the compensation network gives the following characteristics: 2 1 2 m 2 p z 1 comp r r r g ) s 1 ( s s 1 ) s ( g + ? ? + ? + ? = where: 5 4 1 c c 1 + = 4 3 z c r 1 ? = 5 4 3 5 4 2 p c c r c c ? ? + =
11 ? 2006 semtech corp. www.semtech.com power management sc4517a application information (cont.) the loop gain will be given by: ) s 1 ( ) s 1 ( s 1 s 1 r r r c r 10 125 . 2 ) s ( g ) s ( g ) s ( t 2 p 1 p z 2 1 2 4 l 3 vd comp + ? + + ? + ? ? ? = ? = ? where: c r 1 l 1 p ? = for 1.8v, 2.5v, 3.3v and 5v out applications, their respective fixed output parts can be used. the fb pins are connected directly to the outputs. the voltage dividers (r 1 and r 2 ) have been integrated into the sc4517a controllers. for other output cases, the adjustable sc4517a should be used with an external voltage divider. one integrator is added at origin to increase the dc gain. z is used to cancel the power stage pole p1 so that the loop gain has ?20db/dec rate when it reaches 0db line. p2 is placed at half switching frequency to reject high frequency switching noises. figure 4 gives the asymptotic diagrams of the power stage with current loop closed and its loop gain. c p2 z p1 loop gain t(s) power stage figure 4. asymptotic diagrams of power stage with current loop closed and its loop gain. the design guidelines for the sc4517a applications are as following: 1. set the loop gain crossover corner frequency c for given switching corner frequency c = 2 f c . 2. place an integrator at the origin to increase dc and low frequency gains. 3. select z such that it is placed at p1 to obtain a -20db/dec rate to go across the 0db line. 4. place a high frequency compensator pole p2 ( p2 = f s ) to get the maximum attenuation of the switching ripple and high frequency noise with the adequate phase lag at c. layout guidelines: in order to achieve optimal electrical and thermal performance for high frequency converters, special attention must be paid to the pcb layouts. the goal of layout optimization is to identify the high di/dt loops and minimize them. the following guidelines should be used to ensure proper operation of the converters. 1. a ground plane is suggested to minimize switching noises and trace losses and maximize heat transferring. 2. start the pcb layout by placing the power components first. arrange the power circuit to achieve a clean power flow route. put all power connections on one side of the pcb with wide copper filled areas if possible. 3. the v in bypass capacitor should be placed next to the v in and gnd pins. 4. the trace connecting the feedback resistors to the output should be short, direct and far away from any noise sources such as switching node and switching components. 5. minimize the loop including input capacitor, the sc4517a and freewheeling diode d 2 . this loop passes high di/dt current. make sure the trace width is wide enough to reduce copper losses in this loop. 6. maximize the trace width of the loop connecting the inductor, freewheeling diode d 2 and the output capacitor. 7. connect the ground of the feedback divider and the compensation components directly to the gnd pin of the sc4517a by using a separate ground trace. 8. connect pin 4 to a large copper area to remove the ic heat and increase the power capability of the sc4517a. a few feedthrough holes are required to connect this large copper area to a ground plane to further improve the thermal environment of the sc4517a. the traces attached to other pins should be as wide as possible for the same purpose.
? 2006 semtech corp. www.semtech.com power management sc4517a 12 c1 0.1u c3 2.2u sc4517a bst 1 in 2 sw 3 gnd 4 en 5 fb 6 comp 7 sync 8 d3 r2 10k r1 17.4k r3 4.75k c4 1.5n c2 2.2u d2 l1 4.7uh r4 4.75k c5 100p v i =12v v o =3.3v c1 0.1u c3 2.2u sc4517a bst 1 in 2 sw 3 gnd 4 en 5 fb 6 comp 7 sync 8 d3 r2 10k r1 17.4k r3 4.75k c4 1.5n c2 2.2u d2 l1 4.7uh r4 4.75k c5 100p v i =12v v o =3.3v design example 1. 12v to 3.3v. 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 cr 7 x , 5 0 8 0 , v 5 2 , f u 1 . 0 22 3 c , 2 cf u 2 . 2k 5 2 2 e 1 r 7 x 6 1 2 3 c : n / p k d t 314 cf n 5 . 1 415 cf p 0 0 1 513 d3 2 - d o s , s w 8 4 1 4 n 1 612 d3 1 s s3 1 s s : n / p d l i h c r i a f 711 l, h u 7 . 47 r 4 - 3 7 r d : n / p r e p o o c 811 rk 4 . 7 1 912 rk 0 1 0 12 4 r , 3 rk 5 7 . 4 1 111 ua 7 1 5 4 c s : n / p h c e t m e s t r t l m i a 7 1 5 4 c s . 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 application information (cont.)
13 ? 2006 semtech corp. www.semtech.com power management sc4517a (component - top) (component - bottom) (pcb - top) (pcb - bottom) pcb layout
? 2006 semtech corp. www.semtech.com power management sc4517a 14 outline drawing - msop-8 land pattern - msop-8 0 .010 .004 - .016 .003 .024 (.037) - .000 .030 - - - - 0.25 0.10 8 0 - 8 0.60 (.95) .032 .009 0.40 0.08 .043 .006 .037 0.75 0.00 - 0.80 0.23 - 0.95 1.10 0.15 - - - 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 e h e/2 d plane detail .193 bsc .026 bsc aaa c seating ccc c 2x n/2 tips indicator e/2 pin 1 2x 8 bbb c a-b d see detail a1 a a2 bxn d 0.25 a plane gage .005 e1 12 n .114 .114 .118 .118 .009 - 8 01 c (l1) l a 0.13 3.00 3.00 4.90 bsc 0.65 bsc .122 .122 2.90 2.90 .015 0.22 3.10 3.10 0.38 - reference jedec std mo-187 , variation aa. 4. dim ccc a1 e bbb aaa 01 l1 n l d e1 e a2 b c a millimeters nom dimensions inches min nom max min max e this land pattern is for reference purposes only. consult your manufacturing group to ensure your company's manufacturing guidelines are met. notes: 1. p x (c) y z g .224 .063 .016 .026 (.161) .098 (4.10) 5.70 1.60 0.40 0.65 2.50 millimeters dimensions dim inches x z y c p g
15 ? 2006 semtech corp. www.semtech.com power management sc4517a contact information semtech corporation power management products division 200 flynn road, camarillo, ca 93012 phone: (805)498-2111 fax (805)498-3804 land pattern - mlpd-8, 3 x 3mm outline drawing - mlpd-8, 3 x 3mm pin 1 indicator (laser mark) notes: controlling dimensions are in millimeters (angles in degrees). coplanarity applies to the exposed pad as well as terminals. 2. 1. .003 .010 .059 8 .012 .065 - .000 .031 .001 .035 (.008) 0.08 0.30 8 .014 .069 0.25 1.50 .039 - .002 - 0.00 0.80 1.75 0.35 1.65 - 0.05 1.00 (0.20) 0.02 0.90 .004 0.10 0.65 bsc .026 bsc 0.30 .012 .020 .016 0.40 0.50 a aaa c a2 seating plane a1 a bxn bbb c a b b e c lxn e/2 e e .088 .094 .098 2.23 2.38 2.48 n 2 1 aaa bbb nom inches b e l n e d c a1 a2 dim a min dimensions millimeters max min max nom .114 .118 .122 2.90 3.00 3.10 d c x p z y h c k g .030 .146 (.116) .102 .087 .067 0.75 3.70 (2.95) 2.20 2.58 1.70 0.40 0.65 .026 .016 this land pattern is for reference purposes only. consult your manufacturing group to ensure your company's manufacturing guidelines are met. notes: 1. inches dimensions p z x y c g h k dim millimeters

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