 |
|
| If you can't view the Datasheet, Please click here to try to view without PDF Reader . |
|
|
|
| If you can't view the Datasheet, Please click here to try to view without PDF Reader . |
|
|
| Datasheet File OCR Text: |
| power management 1 www.semtech.com SC4508A buck or buck-boost (inverting) current mode controller description revision: july 25, 2007 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 ? hiccup mode after 32 cycle-by-cycle ocp ? 4mm x 4mm mlpq-12 lead free package. this product is fully weee and rohs compliant ? 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 SC4508A is a low voltage current mode switching regulator controller that drives a p-channel power mosfet with programmable switching frequency. it can be configured in either buck or buck boost (inverting) con- verters. 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 di- vider. the switching frequency is programmable up to 1.5mhz, allowing small inductor and capacitor values to minimize pcb space. the operating current level is pro- grammable via an external sense resistor. accessible ref- erence voltage allows users to make output voltage as low as they want. gnd comp ss/en osc fb+ fb- cs out v+ vref SC4508A vin vout vout vref cs out v+ fb+ fb- comp ss/en osc SC4508A gnd vin
2 ? 2007 semtech corp. www.semtech.com power management SC4508A electrical characteristics absolute maximum rating 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. 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 e / s s ) 1 ( 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 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 e / s 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 e / s 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 3 . 25 . 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 f 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 2 . 0 -7 . 0v e g a t l o v t e s f f o 2v m t n e r r u c s a i b t u p n i 0 0 10 0 3a 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 unless specified: v dd = 12v, v fb+ = v ref , v fb- = 0, out = open, c vdd = 1uf, c ss/en = 0.1uf, c osc = 330pf. t a = t j = -40c to 125c note: (1) voltage from internal circuitry could be higher than 3.2v. see application information, soft-start section. 3 ? 2007 semtech corp. www.semtech.com power management SC4508A 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 r o t a r a p m o c m w p e l c y c y t u d m u m i x a mz h k 0 0 1 , f n 5 . 1 = c7 9% z h m 5 . 1 , f p 0 0 1 = c5 9% e m i t n o m u m i n i m 0 0 2s n n o i t a s n e p m o c e p o l s 3 6s t / v m 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 5 2 9 4 . 05 . 05 7 0 5 . 0v e g a t l o v t u p t u ot a v , c 5 2 = d d v 5 =6 9 4 . 05 . 04 0 5 . 0v 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 =50 1v 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 =24v m n w o d t u h s / e l b a n e / t r a t s t f o s t n e r r u c e g r a h cv n e / s s v 9 . 0 >0 2a v n e / s s v 9 . 0 <0 1 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 c i g o l e l b a n e 2v e g a t l o v c i g o l n d h s 5 3 . 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 t n e r r u c r e v o e v i t u c e s n o c o t u a e r o f e b s e l c y c k c o l c n w o d t u h s t r a t s e r 2 3 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 g 8m h o ) l ( e c n a t s i s e r - n o e v i r d e t a g 8m h 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 v 5 =5 1m 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 v 5 =5 1m h o e m i t e s i rc t u o f p 0 0 0 1 =0 2s n e m i t l l a fc t u o f p 0 0 0 1 =0 2s n unless specified: v dd = 12v, v fb+ = v ref , v fb- = 0, out = open, c vdd = 1uf, c ss/en = 0.1uf, c osc = 330pf. t a = t j = -40c to 125c 4 ? 2007 semtech corp. www.semtech.com power management SC4508A 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. r e b m u n t r a pe g a k c a p ) 1 ( t r t l m a 8 0 5 4 c s ) 2 ( 2 1 - q p l m b v e k c u b a 8 0 5 4 c sd r a o b n o i t a u l a v e b v e t s o o b - k c u b a 8 0 5 4 c sd r a o b n o i t a u l a v e ordering information pin configuration top view (mlpq - 12, 4mm x 4mm) 1 out 2 cs 3 ss/en 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 e / 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 e / 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 . 0 . 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 n o i t a s n e p 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 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 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 ? 2007 semtech corp. www.semtech.com power management SC4508A block diagram marking information top view yyww = date code (example: 0012) 6 ? 2007 semtech corp. www.semtech.com power management SC4508A application information the SC4508A 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 amplified current signal for operations when duty- cycle is larger than 50%. in over load or output shortage condition, if the sensed current signal reaches typical 100mv, the switch is turned off immediately in the same cycle. if the sensed current signal continues up to 32 cycles, 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 1.4v during which the switch is held off. with the ?hiccup? mode over current protection, the thermal stress is reduced in the faulty conditions. frequency setting the switching frequency in the SC4508A is user- programmable. the advantages of using constant frequency operation are simple passive component selection and ease of feedback compensation. before setting the operating frequency, the following trade-offs should be considered. 1) passive component size 2) circuitry efficiency 3) emi condition 4) minimum switch on time and 5) maximum duty ratio for a given output power, the sizes of the passive components are inversely proportional to the switching frequency, whereas mosfet?s/diodes switching losses are proportional to the operating frequency. other issues such as heat dissipation, packaging and the cost issues are also to be considered. the frequency bands for signal transmission should be avoided because of em interference. the free-running frequency of the internal oscillator can be programmed with an external capacitor from the osc pin to the ground. the SC4508A controller is capable of operating up to 1.5 mhz. it is necessary to consider the operating duty-ratio before deciding the switching frequency. minimum switch on time consideration in the SC4508A the falling edge of the clock turns on the mosfet. the inductor current and the sensed voltage ramp up. after the sensed voltage crosses a threshold determined by the error amplifier output, the mosfet is turned off. the propagation delay time from the turn-on of the controlling mosfet to its turn-off is the minimum switch on time. the SC4508A has a minimum on time of about 180ns at room temperature. this is the shortest on interval of the controlling pfet. the controller either does not turn on the mosfet at all or turns it on for at least 80ns. for a buck converter, the operating duty cycle is v o /v in . so the required on time for the mosfet is v o /(v in fs). if the frequency is set such that the required pulse width is less than 180ns, then the converter will start skipping cycles. due to minimum on time limitation, simultaneously operating at very high switching frequency and very short duty cycle is not practical. if the voltage conversion ratio v o /v in and hence the required duty cycle is higher, the switching frequency can be increased to reduce the sizes of passive components. there will not be enough modulation headroom if the on time is simply made equal to the minimum on time of the SC4508A. for ease of control, we recommend the required pulse width to be at least 1.5 times the minimum on time. current sense and current limit the SC4508A senses peak inductor current by a current sense resistor. the sensed voltage is referenced to vdd and the typical current limit threshold is 100mv. the current sense resistor can be calculated by the following equation assuming the current limit is 20% above peak incuctor current: rs = ) pk ( i % 120 mv 100 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 7 ? 2007 semtech corp. www.semtech.com power management SC4508A application information (cont.) 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 error amplifier the error amplifier in the SC4508A is a transconductance error amplifier, which is easily configured as a type two compensator by connecting compensation network from the comp pin to agnd. the output voltage of the error amplifier is compared to the sensed current signal (amplified by gain 8) plus internal 500mv ramp to generate duty cycle. both the non-inverting and the inverting inputs of the error amplifier are brought out as device pins so that a converter can be configured as either buck or buck-boost converter. soft-start and overload protection the undervoltage lockout circuit discharges the ss/en capacitors. after v dd rises above 2.5v, the ss/en capacitors are slowly charged by internal 10ua current source. as the ss/en capacitor continues to be charged, the vref and the comp voltage follows. the converter gradually delivers increasing power to the output. the inductor current follows the comp voltage envelope until the output goes into regulation. after the ss/en capacitor is charged above 1.4v (high enough for the error amplifier to provide full load current), the overload detection circuit is activated. if the cs pin senses 32 consective switching cycles of over current, the SC4508A will shut down and hold off the mosfet while discharging the soft-start capacitor. the ss/en capacitor is discharged with an internal 12ma current sink. the overload latch is reset when the ss/en capacitor is discharged below 0.5v. the ss/en capacitor is then recharged with the 10a current source and the converter undergoes soft-start. if overload persists, the SC4508A will undergo repetitive auto-shutdown-restart hiccup mode. in normal operation, the vref voltage follows the ss/ en voltage from 0 to 0.5v as the ss/en voltage ramps up from 1.4v to 1.9v. after the ss/en voltage rises above 1.9v, it settles at final value depending upon vdd. if vdd higher than 7v, it is clamped around 6.8v. in the worst case, the clamped voltage is about 10v. therefore, the external soft-start capacitor should be rated at least 16v. the ss/en pin can also be used as the enable input. the mosfet will be turned off if the ss/en pin is pulled below 0.5v. converter specifications buck or buck-boost converter design includes the following specifications: input voltage range: ] v , v [ v max , in min , in in input voltage ripple (peak-to-peak): d v in output voltage: v o output voltage accuracy: e output voltage ripple (peak-to-peak): d v o nominal output (load) current: i o maximum output current limit: i o,max output (load) current transient slew rate: di o (a/s) circuit efficiency: h selection criteria and design procedures for the following are described. 1) output inductor ( l ) type and value 2) output capacitor ( c o ) type and value 3) input capacitor ( c in ) type and value 4) power mosfets 5) current sensing and limiting circuit 6) voltage sensing circuit 7) loop compensation network inductor (l) and ripple current the output inductor selection/design is based on the output dc and transient requirements. both output current and voltage ripples are reduced with larger inductors but it takes longer to change the inductor current during load transients. conversely smaller inductors results in lower dc copper losses but the ac 8 ? 2007 semtech corp. www.semtech.com power management SC4508A core losses (flux swing) and the winding ac resistance losses are higher. a compromise is to choose the inductance such that peak-to-peak inductor ripple- current is 20% to 30% of the rated output load current or the inductor dc current. assuming that the inductor current ripple (peak-to-peak) value is l i ? = *idc, the inductance value will then be: ) 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 the peak current in the inductor becomes (1+ /2)*idc and the rms current is: 12 1 i i 2 dc rms , l + = the idc is the inductor average or dc current. the following are to be considered when choosing inductors: a) inductor core material: for high efficiency applications above 350khz, ferrite, kool-mu and polypermalloy materials should be used. low-cost powdered iron cores can be used for cost sensitive-applications below 350khz but with attendant higher core losses. b) select inductance value: sometimes the calculated inductance value is not available off-the-shelf. the designer can choose the adjacent (larger) standard inductance value. the inductance varies with temperature and dc current. it is a good engineering practice to re-evaluate the resultant current ripple at the rated dc output current. c) current rating: the saturation current of the inductor should be at least 1.5 times of the peak inductor current under all conditions. output capacitor (c o ) and v out ripple the output capacitor provides output current filtering in steady state and serves as a reservoir during load transient. the output capacitor can be modeled as an ideal capacitor in series with its parasitic esr ( r esr ) and esl ( l esl ) (figure 1). co lesl resr figure 1. an equivalent circuit of c o if the current through the branch is i b (t), the voltage across the terminals will then be: ) t ( i r dt ) t ( di l dt ) t ( i c 1 v ) t ( v b esr t 0 b esl b o o o + + + = this basic equation illustrates the effect of esr, esl and c o on the output voltage. the first term is the dc voltage across c o at time t=0 . the second term is the voltage variation caused by the charge balance between the load and the converter output. the third term is voltage ripple due to esl and the fourth term is the voltage ripple due to esr. the total output voltage ripple is then a vector sum of the last three terms. since the inductor current is a triangular waveform in buck configuration with peak-to-peak value * i o , the ripple-voltage caused by inductor current ripples is: s o o c f c 8 i v ? the ripple-voltage due to esl is: d i f l v o s esl esl = ? and the esr ripple-voltage is: o esr esr i r v = ? aluminum capacitors (e.g. electrolytic, solid os-con, poscap, tantalum) have high capacitances and low esl?s. the esr has the dominant effect on the output ripple voltage. it is therefore very important to minimize the esr. application information (cont.) 9 ? 2007 semtech corp. www.semtech.com power management SC4508A when determining the esr value, both the steady state ripple-voltage and the dynamic load transient need to be considered. to keep the steady state output ripple-voltage < ? v o , the esr should satisfy: o o 1 esr i v r ? < to limit the dynamic output voltage overshoot/ undershoot within (say 3%) of the steady state output voltage) from no load to full load, the esr value should satisfy: o o 2 esr i v r < then, the required esr value of the output capacitors should be: r esr = min{r esr1 ,r esr2 } the voltage rating of aluminum capacitors should be at least 1.5 v o . the rms current ripple rating should also be greater than: 3 2 i o usually it is necessary to have several capacitors of the same type in parallel to satisfy the esr requirement. the voltage ripple cause by the capacitor charge/ discharge should be an order of magnitude smaller than the voltage ripple caused by the esr. to guarantee this, the capacitance should satisfy: esr s o r f 2 10 c > buck-boost converter has higher ripple current than buck in output. the rms value is the most important factor to consider. it has to be less than the output capacitor ripple current rating. the buck-boost output capacitor rms current is: in d o o cap _ rms v v v i i + d esr esr i r v ? = ? where, v d and i d are rectifier forward voltage and current. in many applications, several low esr ceramic capacitors are added in parallel with the aluminum capacitors in order to further reduce esr and improve high frequency decoupling. because the values of capacitance and esr are usually different in ceramic and aluminum capacitors, the following remarks are made to clarify some practical issues. remark 1: high frequency ceramic capacitors may not carry most of the ripple current. it also depends on the capacitor value. only when the capacitor value is set properly, the effect of ceramic capacitor low esr starts to be significant. for example, if a 10 f, 4m ? ceramic capacitor is connected in parallel with 2x1500 f, 90m ? electrolytic capacitors, the ripple current in the ceramic capacitor is only about 42% of the current in the electrolytic capacitors at the ripple frequency. if a 100 f, 2m ? ceramic capacitor is used, the ripple current in the ceramic capacitor will be about 4.2 times of that in the electrolytic capacitors. when two 100 f, 2m ? ceramic capacitors are used, the current ratio increases to 8.3. in this case most of the ripple current flows in the ceramic decoupling capacitor. the esr of the ceramic capacitors will then determine the output ripple-voltage. remark 2: the total equivalent capacitance of the filter bank is not simply the sum of all the paralleled capacitors. the total equivalent esr is not simply the parallel combination of all the individual esrs either. instead they should be calculated using the following formulae. ) c c ( c c ) c r c r ( ) c c ( c c ) r r ( : ) ( c b 1 a 1 b 1 a 1 2 b 1 2 b 1 a 1 2 a 1 2 b 1 a 1 2 b 1 2 a 1 2 2 b 1 a 1 eq + + + + + + = 2 b 1 a 1 2 b 1 2 a 1 2 2 b 1 a 1 2 a 1 a 1 2 b 1 b 1 2 b 1 2 a 1 2 b 1 a 1 b 1 a 1 eq ) c c ( c c ) r r ( ) c r c r ( c c ) r r ( r r : ) ( r + + + + + + = where r 1a and c 1a are the esr and capacitance of electrolytic capacitors, and r 1b and c 1b are the esr and capacitance of the ceramic capacitors respectively. (figure 2) c1a r1a c1b r1b ceq req figure 2. equivalent rc branch. application information (cont.) 10 ? 2007 semtech corp. www.semtech.com power management SC4508A req and ceq are both functions of frequency. for rigorous design, the equivalent esr should be evaluated at the ripple frequency for voltage ripple calculation when both ceramic and electrolytic capacitors are used. if r 1a = r 1b = r 1 and c 1a = c 1b = c 1 , then r eq and c eq will be frequency- independent and r eq = 1/2 r 1 and c eq = 2c 1 input capacitor (c in ) the input supply to the converter usually comes from a pre-regulator. since the input supply is not ideal, input capacitors are needed to filter the current pulses at the switching frequency. a simple buck converter is shown in figure 3. figure 3. a simple model for the converter input in figure 3 the dc input voltage source has an internal impedance r in and the input capacitor c in has an esr of r esr . mosfet and input capacitor current waveforms, esr voltage ripple and input voltage ripple are shown in figure 4. figure 4. typical waveforms at converter input. it can be seen that the current in the input capacitor pulses with high di/dt. capacitors with low esl should be used. it is also important to place the input capacitor close to the mosfets on the pc board to reduce trace inductances around the pulse current loop. the rms value of the capacitor current is approximately ] ) d 1 ( d ) d 1 )( 12 1 [( d i i 2 2 2 o cin ? + ? + = the power dissipated in the input capacitors is then: p cin = i cin 2 r esr for reliable operation , the maximum power dissipation in the capacitors should not result in more than 10 o c of temperature rise. many manufacturers specify the maximum allowable ripple current (rms) rating of the capacitor at a given ripple frequency and ambient temperature. the input capacitance should be high enough to handle the ripple current. for higher power applications, multiple capacitors are placed in parallel to increase the ripple current handling capability. sometimes meeting tight input voltage ripple specifications may require the use of larger input capacitance. at full load, the peak-to-peak input voltage ripple due to the esr is: dc esr esr i ) 2 1 ( r v + = ? the peak-to-peak input voltage ripple due to the capacitor is: s in dc c f c di v ? from these two expressions, c in can be found to meet the input voltage ripple specification. power mosfets selection main considerations in selecting the mosfets are power dissipation, cost and packaging. switching losses and conduction losses of the mosfets are directly related to the total gate charge ( c g ) and channel on-resistance ( r ds(on) ). in order to judge the performance of mosfets, the product of the total gate charge and on-resistance is used as a figure of merit (fom). transistors with the same fom follow the same curve in figure 5. application information (cont.) 11 ? 2007 semtech corp. www.semtech.com power management SC4508A 0 5 10 15 20 0 20 40 fom:100*10^{-12} fom:200*10^{-12} fom:500*10^{-12} on-resistance (mohm) gate charge (nc) 50 1 cg 100 rds , () cg 200 rds , () cg 500 rds , () 20 1 rds figure 5. figure of merit curves. the closer the curve is to the origin, the lower is the fom. this means lower switching loss or lower conduction loss or both. it may be difficult to find mosfets with both low c g and low r ds(on . usually a trade-off between r ds(on and c g has to be made. mosfet selection also depends on applications. in many applications, either switching loss or conduction loss dominates for a particular mosfet. for buck and buck- boost converters with high input to output voltage ratios, the mosfet is hard switched but conducts with very low duty cycle. for such applications, mosfets with low c g should be used. mosfet power dissipation consists of: a) conduction loss due to the channel resistance r ds(on) , b) switching loss due to the switch rise time t r and fall time t f , and c) the gate loss due to the gate resistance r g . the rms value of the mosfet switch current is calculated as: ) 1 ( d i i 12 dc qrms 2 + = the conduction losses are then p tc = i qrms 2 r ds(on) idc is average inductor current. in buck converter, it is also load current. in buck-boost, it is load current divided by 1-d. r ds(on) varies with temperature and gate-source voltage. curves showing r ds(on) variations can be found in manufacturers? data sheet. from the fds6675 datasheet, r ds(on) is less than 14m ? when v gs is greater than 10v. however r ds(on) increases by 30% as the junction temperature increases from 25 o c to 110 o c. the switching losses can be estimated using the simple formula: s in dc 2 f r 2 1 ts f v i ) 1 )( t t ( p + + = where t r is the rise time and t f is the fall time of the switching process. different manufacturers have different definitions and test conditions for t r and t f . to clarify these, we sketch the typical mosfet switching characteristics under clamped inductive mode in figure 6. figure 6. mosfet switching characteristics in figure 6, q gs1 is the gate charge needed to bring the gate-to-source voltage v gs to the threshold voltage v gs_th , q gs2 is the additional gate charge required for the switch current to reach its full-scale value i ds . and q gd is the charge needed to charge gate-to-drain (miller) capacitance when v ds is falling. switching losses occur during the time interval [ t 1 , t 3 ]. defining t r = t 3 -t 1 and t r can be approximated as: gsp cc gt gd 2 gs r v v r ) q q ( t ? + = where r gt is the total resistance from the driver supply rail to the gate of the mosfet. it includes the gate driver internal impedance r gi , external resistance r ge and the gate resistance r g within the mosfet i.e. r gt = r gi +r ge +r g v gsp is the miller plateau voltage shown in figure 11. similarly an approximate expression for t f is: application information (cont.) 12 ? 2007 semtech corp. www.semtech.com power management SC4508A gsp gt gd 2 gs f v r ) q q ( t + = only a portion of the total losses (p g = q g v cc f s ) is dissipated in the mosfet package. here qg is the total gate charge specified in the datasheet. the power dissipated within the mosfet package is: s cc g gt g tg f v q r r p = the total power loss of the top switch is then: p t = p tc +p ts +p tg if the input supply of the power converter varies over a wide range, then it will be necessary to weigh the relative importance of conduction and switching losses. this is because conduction losses are inversely proportional to the input voltage. switching loss however increases with the input voltage. the total power loss of mosfet should be calculated and compared for high-line and low-line cases. the worst case is then used for thermal design. 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 on the peak reverse voltage, the peak current and average power dissipation. the following could be used to determine the diode reversed voltage: 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. once the power losses (p loss ) for the mosfet and freewheeling diode are known, thermal and package design at component and system level should be done to verify that the maximum die junction temperature (t j,max , usually 125 o c) is not exceeded under the worst- case condition. the equivalent thermal impedance from junction to ambient ( ja ) should satisfy: loss max , a max , j ja p t t ? ja depends on the die to substrate bonding, packaging material, the thermal contact surface, thermal compound property, the available effective heat sink area and the air flow condition (free or forced convection). actual temperature measurement of the prototype should be carried out to verify the thermal design. overload protection and hiccup during start-up, the capacitor from the ss/en pin to ground functions as a soft-start capacitor. after the converter starts and enters regulation, the same capacitor operates as an overload shutoff timing capacitor. as the load current increases, the cycle-by- cycle current-limit comparator will first limit the inductor current. if the over-current persists for more than 32 consecutive switching cycles, the controller will shut off the mosfets. meanwhile an internal 12ma current source discharges the soft-start capacitor c ss connected to the ss/en pin. when the capacitor is discharged to 0.5v, a 10 a current source recharges the ss/en capacitor to 0.9v and driver stage is enabled. then a 20ua current source continues to charge the soft-start capacitor. as the soft-start capacitor reaches 1.4v, vref will start to follow the soft- start capacitor voltage until vref=0.5v. if overload persists, the controller will shut down the converter when the soft-start capacitor voltage exceeds 1.4v. the converter will repeatedly start and shut off until it is no longer overloaded. this hiccup mode of overload protection is a form of foldback current limiting. the following calculations estimate the average inductor current when the converter output is shorted to the ground. a) the time taken to charge the capacitor from 0.5v to 0.9v ua 10 v ) 5 . 0 9 . 0 ( c t ss 1 ssr ? = if c ss = 0.1 f, t ssr1 is calculated as 4ms. b) the time to charge the capacitor from 0.9v to 1.4v (driver is enabled but output duty cycle is 0) application information (cont.) 13 ? 2007 semtech corp. www.semtech.com power management SC4508A a 20 v ) 9 . 0 4 . 1 ( c t ss 2 ssr ? = when c ss = 0.1 f, t ssr2 is calculated as 2.5ms. note that during this period, the converter does not start switching until ss/en reaches 1.4v. c) the effective start-up time is: khz 200 32 t sse = assuming inductor current hitting current limit for 32 cycles after ss/en reaches 1.4v and fs=200khz. the average inductor current is then: 2 ssr 1 ssr sse lim leff t t t i i + = i leff 0.025 i lim and is independent of the soft start capacitor value. the converter will not overheat in hiccup. setting the output voltage the non-inverting input of the error amplifier is brought out as a device pin (pin 9) to which the user can connect pin 4 or an external voltage reference. a simple voltage divider (r o1 at top and r o2 at bottom) sets the converter output voltage. in buck converter, the voltage feedback gain (h=0.5/v o ) is related to the divider resistors value as: o1 o2 r h 1 h r ? = once either r o1 or r o2 is chosen, the other can be calculated for the desired output voltage v o . since the number of standard resistance values is limited, the calculated resistance may not be available as a standard value resistor. as a result, there will be a set error in the converter output voltage. this non-random error is caused by the feedback voltage divider ratio. it cannot be corrected by the feedback loop. the following table lists a few standard resistor combinations for realizing some commonly used output voltages. ) v ( o v 6 . 0 9 . 0 2 . 15 . 1 8 . 15 . 23 . 3 h / ) h - 1 ( 2 . 0 8 . 0 4 . 12 6 . 24 6 . 5 ) m h o ( 1 o r 0 0 2 6 0 8 k 4 . 1k 2 k 1 6 . 2k 2 0 . 4k 2 6 . 5 ) m h o ( 2 o r k 1 k 1 k 1k 1 k 1k 1k 1 only the voltages in boldface can be precisely set with standard 1% resistors. from this table, one may also observe that when the value 5 . 0 5 . 0 v h h 1 o ? = ? and its multiples fall into the standard resistor value chart (1%, 5% or so), it is possible to use standard value resistors to exactly set up the required output voltage value. in buck-boost converter, output voltage is set by o1 o o2 r v 0.5 r = the input bias current of the error amplifier also causes an error in setting the output voltage. the inverting input bias currents of error amplifiers is -100na. since the non-inverting input is biased to 0.5v in buck converter, the percentage error in the second output voltage will be ?100% (0.1 a) r o1 r o2 /[0.5 (r o1 +r o2 ) ]. to keep this error below 0.2%, r o1 //r o2 < 10k ? . loop compensation SC4508A is a current-mode controller. current-mode control is a dual-loop control system in which the inductor peak current is loosely controlled by the inner current- loop. the higher gain outer loop regulates the output voltage. since the current loop makes the inductor appear as a current source, the complex high-q poles of the output lc networks is split into a dominant pole determined by the output capacitor and the load resistance and a high frequency pole. this pole-splitting property of current-mode control greatly simplifies loop compensation. application information (cont.) 14 ? 2007 semtech corp. www.semtech.com power management SC4508A the inner current-loop is unstable (sub-harmonic oscillation) unless the inductor current up-slope is steeper than the inductor current down-slope. for stable operation above 50% duty-cycle, a compensation ramp is added to the sensed-current. in the SC4508A the compensation ramp is made switching frequency dependent. the slope of the compensation ramp is: mv f * 500 s s e = the slope of the internal compensation ramp is above the minimal slope requirement for current loop stability and is sufficient for all the applications. with the inner current loop stable, the output voltage is then regulated with the outer voltage feedback loop. a simplified equivalent circuit model of the buck converter with current mode control is shown in figure 7. k figure 7. a simple model of buck converter with current mode control. the transconductance error amplifier (in the SC4508A) has a gain (g m ) of 100a/v. the target of the compensation design is to select the compensation network consisting of c 2 , c 3 and r 2 , along with the feedback resistors r o1 , r o2 and the current sensing gain, such that the converter output voltage is regulated with satisfactory dynamic performance. with the output voltage v o known, the feedback gain h and the feedback resistor values are determined using the equations given in the ?output voltage setting? section with: o v 5 . 0 h = for the rated output current i o , the current sensing gain k is fixed as: s r 8 1 k ? = from figure 7, the transfer function from the voltage error amplifier output (v c ) to the converter output (v o )is: 1 p 1 z o vc c o s s 1 s s 1 kr ) s ( g : ) s ( v ) s ( v + + = = where, the single dominant pole is: o oesr o 1 p c ) r r ( 1 s + = and the zero due to the output capacitor esr is: o oesr 1 z c r 1 s = the dominant pole moves as output load varies. the controller transfer function (from the converter output (v o ) to the voltage error amplifier output (v c ) is: 2 p 2 z 3 2 m c s s 1 s s 1 ) c c ( s g ) s ( g + + + = where 2 2 2 z c r 1 s = and 3 2 3 2 2 2 p c c c c r 1 s + = the loop transfer function is then: t(s)=g vc (s)g c (s)h to simplify design, we assume that c 3 < 16 ? 2007 semtech corp. www.semtech.com power management SC4508A where, the single dominant pole is: o o 1 p c r d 1 s + = figure 9. a simple model of buck-boost converter with current mode control. and the zero due to the output capacitor esr is: o oesr 1 z c r 1 s = and the rhp zero associated to the topoly is: l d r ) d 1 ( s o 2 zrhp ? ? ? = the dominant pole moves as input voltage and output load varies. the controller transfer function (from the converter output (v o ) to the voltage error amplifier output (v c ) is: 2 p 2 z 3 2 m c s s 1 s s 1 ) c c ( s g ) s ( g + + + = where 2 2 2 z c r 1 s = and 3 2 3 2 2 2 p c c c c r 1 s + = the loop transfer function is then t(s)=g vc (s)g c (s)h to simplify design, we assume that c 3 < 18 ? 2007 semtech corp. www.semtech.com power management SC4508A 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 1 314 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 0 . 8 1 4 116 rk 0 . 2 5 117 rk 0 1 6 111 ua 8 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 18.0k c1 47uf/16v c8 470uf/6.3v l1 10uh tp1 tp7 comp 10 nc 5 ss/en 3 osc 11 fb+ 9 fb- 8 vref 4 out 1 cs 2 vdd 12 agnd 6 pgnd 7 u1 SC4508A u1 tp6 r6 2.0k 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=470khz 5v/1 a 19 ? 2007 semtech corp. www.semtech.com power management SC4508A 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 0 111 lh u 3 3 1 111 r6 0 . 0 2 112 r0 0 1 3 114 rk 0 2 4 115 rk 9 9 . 4 5 116 r9 9 4 6 117 rk 0 1 9 111 ua 8 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 0.06 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 4.99k c7 0.1uf comp 10 nc 5 ss/en 3 osc 11 fb+ 9 fb- 8 vref 4 out 1 cs 2 vdd 12 agnd 6 pgnd 7 u1 SC4508A u1 r2 100 r6 499 r7 10k tp5 c3 1nf tp4 r4 20k c9 100uf/6.3v tp10 f s=250khz tp9 12v gnd -5v/0.5 a 20 ? 2007 semtech corp. www.semtech.com power management SC4508A typical characteristics 50ms/div a: output voltage, 5v/div b: ss/en pin voltage, 5v/div c: inductor current, 2a/div d: vref pin voltage, 0.5v/div buck converter sartup buck converter sartup buck converter sartup buck converter sartup buck converter sartup 500ns/div a: out pin voltage, 10v/div b: phase node voltage, 10v/div c: inductor current, 200ma/div buck-boost converter startup buck-boost converter startup buck-boost converter startup buck-boost converter startup buck-boost converter startup buck ccm operation buck ccm operation buck ccm operation buck ccm operation buck ccm operation buck-boost dcm operation buck-boost dcm operation buck-boost dcm operation buck-boost dcm operation buck-boost dcm operation buck overcurrent protection buck overcurrent protection buck overcurrent protection buck overcurrent protection buck overcurrent protection buck-boost overcurrent protection buck-boost overcurrent protection buck-boost overcurrent protection buck-boost overcurrent protection buck-boost overcurrent protection 5ms/div a: output voltage, 5v/div b: ss/en pin voltage, 5v/div c: inductor current, 2a/div d: vref pin voltage, 0.5v/div 500ns/div a: out pin voltage, 10v/div b: phase node voltage, 10v/div c: inductor current, 1a/div 10ms/div a: out pin voltage, 10v/div b: ss/en pin voltage, 1v/div c: inductor current, 2a/div 10ms/div a: out pin voltage, 10v/div b: ss/en pin voltage, 1v/div c: inductor current, 2a/div 21 ? 2007 semtech corp. www.semtech.com power management SC4508A 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 |
Price & Availability of SC4508A
 |
|
|
|
|
All Rights Reserved © IC-ON-LINE 2003 - 2022 |
| [Add Bookmark] [Contact Us] [Link exchange] [Privacy policy] |
|
Mirror Sites : [www.datasheet.hk]
[www.maxim4u.com] [www.ic-on-line.cn]
[www.ic-on-line.com] [www.ic-on-line.net]
[www.alldatasheet.com.cn]
[www.gdcy.com]
[www.gdcy.net] |