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SC4524B Datasheet(PDF) 11 Page - Semtech Corporation |
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SC4524B Datasheet(HTML) 11 Page - Semtech Corporation |
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11 / 18 page  Minimum Off Time Limitation The PWM latch in Figure 2 is reset every cycle by the clock. The clock also turns off the power transistor to refresh the bootstrap capacitor. This minimum off time limits the attainable duty cycle of the regulator at a given switching frequency. The measured minimum off time is 100ns typically. If the required duty cycle is higher than the attainable maximum, then the output voltage will not be able to reach its set value in continuous-conduction mode. Inductor Selection The inductor ripple current for a non-synchronous step- down converter in continuous-conduction mode is (3) where F SW is the switching frequency and L1 is the inductance. An inductor ripple current between 20% to 50% of the maximum load current gives a good compromise among efficiency, cost and size. Re-arranging Equation (3) and assuming 35% inductor ripple current, the inductor is given by (4) If the input voltage varies over a wide range, then choose L 1 based on the nominal input voltage. Always verify converter operation at the input voltage extremes. The peak current limit of SC4524B power transistor is at least 2.6A. The maximum deliverable load current for the SC4524B is 2.6A minus one half of the inductor ripple current. Input Decoupling Capacitor The input capacitor should be chosen to handle the RMS ripple current of a buck converter. This value is given by (5) CESAT D IN D O V V V V V D − + + = − = 1 V 0 . 1 V R R O 6 4 1 SW D O L L F ) D 1 ( ) V V ( I ⋅ − ⋅ + = D SW O D O 1 F I % 20 ) D 1 ( ) V V ( L ⋅ ⋅ − ⋅ + = ) D 1 ( D I I O CIN _ RMS − ⋅ ⋅ = ⋅ ⋅ + ⋅ D = D O SW L O C F 8 1 ESR I V SW IN O IN F V 4 I C ⋅ D ⋅ > , R G R G S CA PWM ⋅ ≈ ) / s Q / s 1 () / s 1 ( ) C R s 1 ( G V V 2 n 2 n p O ESR PWM c o ω + ω + ω + + = 7 1 Z 5 R F 2 1 C π = 7 1 P 8 R F 2 1 C π = , C R 1 O p ≈ ω , C R 1 O ESR Z = ω k 3 . 22 10 28 . 0 10 R 3 7 20 9 . 15 = ⋅ = − nF 45 . 0 10 1 . 22 10 16 2 1 C 3 3 5 = ⋅ ⋅ ⋅ ⋅ π = pF 12 10 1 . 22 10 600 2 1 C 3 3 8 = ⋅ ⋅ ⋅ ⋅ π = ⋅ π ⋅ ⋅ − = O FB O C S CA C V V C F 2 1 R G 1 log 20 A dB 9 . 15 3 . 3 0 . 1 10 22 10 80 2 1 10 1 . 6 28 1 log 20 A 6 3 3 C = ⋅ ⋅ ⋅ ⋅ ⋅ π ⋅ ⋅ ⋅ ⋅ − = − − m 7 g 10 R 20 C A = CESAT D IN D O V V V V V D − + + = − = 1 V 0 . 1 V R R O 6 4 1 SW D O L L F ) D 1 ( ) V V ( I ⋅ − ⋅ + = D SW O D O 1 F I % 20 ) D 1 ( ) V V ( L ⋅ ⋅ − ⋅ + = ) D 1 ( D I I O CIN _ RMS − ⋅ ⋅ = ⋅ ⋅ + ⋅ D = D O SW L O C F 8 1 ESR I V SW IN O IN F V 4 I C ⋅ D ⋅ > , R G R G S CA PWM ⋅ ≈ ) / s Q / s 1 () / s 1 ( ) C R s 1 ( G V V 2 n 2 n p O ESR PWM c o ω + ω + ω + + = 7 1 Z 5 R F 2 1 C π = 7 1 P 8 R F 2 1 C π = , C R 1 O p ≈ ω , C R 1 O ESR Z = ω k 3 . 22 10 28 . 0 10 R 3 7 20 9 . 15 = ⋅ = − nF 45 . 0 10 1 . 22 10 16 2 1 C 3 3 5 = ⋅ ⋅ ⋅ ⋅ π = pF 12 10 1 . 22 10 600 2 1 C 3 3 8 = ⋅ ⋅ ⋅ ⋅ π = ⋅ π ⋅ ⋅ − = O FB O C S CA C V V C F 2 1 R G 1 log 20 A dB 9 . 15 3 . 3 0 . 1 10 22 10 80 2 1 10 1 . 6 28 1 log 20 A 6 3 3 C = ⋅ ⋅ ⋅ ⋅ ⋅ π ⋅ ⋅ ⋅ ⋅ − = − − m 7 g 10 R 20 C A = CESAT D IN D O V V V V V D − + + = − = 1 V 0 . 1 V R R O 6 4 1 SW D O L L F ) D 1 ( ) V V ( I ⋅ − ⋅ + = D ) D 1 ( D I I O CIN _ RMS − ⋅ ⋅ = ⋅ ⋅ + ⋅ D = D O SW L O C F 8 1 ESR I V SW IN O IN F V 4 I C ⋅ D ⋅ > , R G R G S CA PWM ⋅ ≈ ) / s Q / s 1 () / s 1 ( ) C R s 1 ( G V V 2 n 2 n p O ESR PWM c o ω + ω + ω + + = 7 1 Z 5 R F 2 1 C π = 7 1 P 8 R F 2 1 C π = , C R 1 O p ≈ ω , C R 1 O ESR Z = ω k 3 . 22 10 28 . 0 10 R 3 7 20 9 . 15 = ⋅ = − nF 45 . 0 10 1 . 22 10 16 2 1 C 3 3 5 = ⋅ ⋅ ⋅ ⋅ π = pF 12 10 1 . 22 10 600 2 1 C 3 3 8 = ⋅ ⋅ ⋅ ⋅ π = ⋅ π ⋅ ⋅ − = O FB O C S CA C V V C F 2 1 R G 1 log 20 A dB 9 . 15 3 . 3 0 . 1 10 22 10 80 2 1 10 1 . 6 28 1 log 20 A 6 3 3 C = ⋅ ⋅ ⋅ ⋅ ⋅ π ⋅ ⋅ ⋅ ⋅ − = − − m 7 g 10 R 20 C A = SW O D O 1 F I % 35 ) D 1 ( ) V V ( L ⋅ ⋅ − ⋅ + = CESAT D IN D O V V V V V D − + + = − = 1 V 0 . 1 V R R O 6 4 1 SW D O L L F ) D 1 ( ) V V ( I ⋅ − ⋅ + = D ) D 1 ( D I I O CIN _ RMS − ⋅ ⋅ = ⋅ ⋅ + ⋅ D = D O SW L O C F 8 1 ESR I V SW IN O IN F V 4 I C ⋅ D ⋅ > , R G R G S CA PWM ⋅ ≈ ) / s Q / s 1 () / s 1 ( ) C R s 1 ( G V V 2 n 2 n p O ESR PWM c o ω + ω + ω + + = 7 1 Z 5 R F 2 1 C π = 7 1 P 8 R F 2 1 C π = , C R 1 O p ≈ ω , C R 1 O ESR Z = ω k 3 . 22 10 28 . 0 10 R 3 7 20 9 . 15 = ⋅ = − nF 45 . 0 10 1 . 22 10 16 2 1 C 3 3 5 = ⋅ ⋅ ⋅ ⋅ π = pF 12 10 1 . 22 10 600 2 1 C 3 3 8 = ⋅ ⋅ ⋅ ⋅ π = ⋅ π ⋅ ⋅ − = O FB O C S CA C V V C F 2 1 R G 1 log 20 A dB 9 . 15 3 . 3 0 . 1 10 22 10 80 2 1 10 1 . 6 28 1 log 20 A 6 3 3 C = ⋅ ⋅ ⋅ ⋅ ⋅ π ⋅ ⋅ ⋅ ⋅ − = − − m 7 g 10 R 20 C A = SW O D O 1 F I % 35 ) D 1 ( ) V V ( L ⋅ ⋅ − ⋅ + = CESAT D IN D O V V V V V D − + + = − = 1 V 0 . 1 V R R O 6 4 1 SW D O L L F ) D 1 ( ) V V ( I ⋅ − ⋅ + = D SW O D O 1 F I % 20 ) D 1 ( ) V V ( L ⋅ ⋅ − ⋅ + = ) D 1 ( D I I O CIN _ RMS − ⋅ ⋅ = ⋅ ⋅ + ⋅ D = D O SW L O C F 8 1 ESR I V SW IN O IN F V 4 I C ⋅ D ⋅ > , R G R G S CA PWM ⋅ ≈ ) / s Q / s 1 () / s 1 ( ) C R s 1 ( G V V 2 n 2 n p O ESR PWM c o ω + ω + ω + + = 7 1 Z 5 R F 2 1 C π = 7 1 P 8 R F 2 1 C π = , C R 1 O p ≈ ω , C R 1 O ESR Z = ω k 3 . 22 10 28 . 0 10 R 3 7 20 9 . 15 = ⋅ = − nF 45 . 0 10 1 . 22 10 16 2 1 C 3 3 5 = ⋅ ⋅ ⋅ ⋅ π = pF 12 10 1 . 22 10 600 2 1 C 3 3 8 = ⋅ ⋅ ⋅ ⋅ π = ⋅ π ⋅ ⋅ − = O FB O C S CA C V V C F 2 1 R G 1 log 20 A dB 9 . 15 3 . 3 0 . 1 10 22 10 80 2 1 10 1 . 6 28 1 log 20 A 6 3 3 C = ⋅ ⋅ ⋅ ⋅ ⋅ π ⋅ ⋅ ⋅ ⋅ − = − − m 7 g 10 R 20 C A = CESAT D IN D O V V V V V D − + + = − = 1 V 0 . 1 V R R O 6 4 1 SW D O L L F ) D 1 ( ) V V ( I ⋅ − ⋅ + = D SW O D O 1 F I % 20 ) D 1 ( ) V V ( L ⋅ ⋅ − ⋅ + = ) D 1 ( D I I O CIN _ RMS − ⋅ ⋅ = ⋅ ⋅ + ⋅ D = D O SW L O C F 8 1 ESR I V SW IN O IN F V 4 I C ⋅ D ⋅ > , R G R G S CA PWM ⋅ ≈ ) / s Q / s 1 () / s 1 ( ) C R s 1 ( G V V 2 n 2 n p O ESR PWM c o ω + ω + ω + + = 7 1 Z 5 R F 2 1 C π = 7 1 P 8 R F 2 1 C π = , C R 1 O p ≈ ω , C R 1 O ESR Z = ω k 3 . 22 10 28 . 0 10 R 3 7 20 9 . 15 = ⋅ = − nF 45 . 0 10 1 . 22 10 16 2 1 C 3 3 5 = ⋅ ⋅ ⋅ ⋅ π = pF 12 10 1 . 22 10 600 2 1 C 3 3 8 = ⋅ ⋅ ⋅ ⋅ π = ⋅ π ⋅ ⋅ − = O FB O C S CA C V V C F 2 1 R G 1 log 20 A dB 9 . 15 3 . 3 0 . 1 10 22 10 80 2 1 10 1 . 6 28 1 log 20 A 6 3 3 C = ⋅ ⋅ ⋅ ⋅ ⋅ π ⋅ ⋅ ⋅ ⋅ − = − − m 7 g 10 R 20 C A = The input capacitance must also be high enough to keep input ripple voltage within specification. This is important in reducing the conductive EMI from the regulator. The input capacitance can be estimated from (6) where DV IN is the allowable input ripple voltage. Multi-layer ceramic capacitors, which have very low ESR (a few mW) and can easily handle high RMS ripple current, are the ideal choice for input filtering. A single 4.7µF X5R ceramic capacitor is adequate for 500kHz or higher switching frequency applications, and 10µF is adequate for 200kHz to 500kHz switching frequency. For high voltage applications, a small ceramic (1µF or 2.2µF) can be placed in parallel with a low ESR electrolytic capacitor to satisfy both the ESR and bulk capacitance requirements. Output Capacitor The output ripple voltage DV O of a buck converter can be expressed as (7) where C O is the output capacitance. Since the inductor ripple current DI L increases as D decreases (Equation (3)), the output ripple voltage is therefore the highest when V IN is at its maximum. A 10µF to 47µF X5R ceramic capacitor is found adequate for output filtering in most applications. Ripple current in the output capacitor is not a concern because the inductor current of a buck converter directly feeds C O, resulting in very low ripple current. Avoid using Z5U and Y5V ceramic capacitors for output filtering because these types of capacitors have high temperature and high voltage coefficients. Freewheeling Diode Use of Schottky barrier diodes as freewheeling rectifiers reduces diode reverse recovery input current spikes, easing high-side current sensing in the SC4524B. These CESAT D IN D O V V V V V D − + + = − = 1 V 0 . 1 V R R O 6 4 1 SW D O L L F ) D 1 ( ) V V ( I ⋅ − ⋅ + = D SW O D O 1 F I % 20 ) D 1 ( ) V V ( L ⋅ ⋅ − ⋅ + = ) D 1 ( D I I O CIN _ RMS − ⋅ ⋅ = ⋅ ⋅ + ⋅ D = D O SW L O C F 8 1 ESR I V SW IN O IN F V 4 I C ⋅ D ⋅ > , R G R G S CA PWM ⋅ ≈ ) / s Q / s 1 () / s 1 ( ) C R s 1 ( G V V 2 n 2 n p O ESR PWM c o ω + ω + ω + + = 7 1 Z 5 R F 2 1 C π = 7 1 P 8 R F 2 1 C π = , C R 1 O p ≈ ω , C R 1 O ESR Z = ω k 3 . 22 10 28 . 0 10 R 3 7 20 9 . 15 = ⋅ = − nF 45 . 0 10 1 . 22 10 16 2 1 C 3 3 5 = ⋅ ⋅ ⋅ ⋅ π = pF 12 10 1 . 22 10 600 2 1 C 3 3 8 = ⋅ ⋅ ⋅ ⋅ π = ⋅ π ⋅ ⋅ − = O FB O C S CA C V V C F 2 1 R G 1 log 20 A dB 9 . 15 3 . 3 0 . 1 10 22 10 80 2 1 10 1 . 6 28 1 log 20 A 6 3 3 C = ⋅ ⋅ ⋅ ⋅ ⋅ π ⋅ ⋅ ⋅ ⋅ − = − − m 7 g 10 R 20 C A = CESAT D IN D O V V V V V D − + + = − = 1 V 0 . 1 V R R O 6 4 1 SW D O L L F ) D 1 ( ) V V ( I ⋅ − ⋅ + = D SW O D O 1 F I % 20 ) D 1 ( ) V V ( L ⋅ ⋅ − ⋅ + = ) D 1 ( D I I O CIN _ RMS − ⋅ ⋅ = ⋅ ⋅ + ⋅ D = D O SW L O C F 8 1 ESR I V SW IN O IN F V 4 I C ⋅ D ⋅ > , R G R G S CA PWM ⋅ ≈ ) / s Q / s 1 () / s 1 ( ) C R s 1 ( G V V 2 n 2 n p O ESR PWM c o ω + ω + ω + + = 7 1 Z 5 R F 2 1 C π = 7 1 P 8 R F 2 1 C π = , C R 1 O p ≈ ω , C R 1 O ESR Z = ω k 3 . 22 10 28 . 0 10 R 3 7 20 9 . 15 = ⋅ = − nF 45 . 0 10 1 . 22 10 16 2 1 C 3 3 5 = ⋅ ⋅ ⋅ ⋅ π = pF 12 10 1 . 22 10 600 2 1 C 3 3 8 = ⋅ ⋅ ⋅ ⋅ π = ⋅ π ⋅ ⋅ − = O FB O C S CA C V V C F 2 1 R G 1 log 20 A dB 9 . 15 3 . 3 0 . 1 10 22 10 80 2 1 10 1 . 6 28 1 log 20 A 6 3 3 C = ⋅ ⋅ ⋅ ⋅ ⋅ π ⋅ ⋅ ⋅ ⋅ − = − − m 7 g 10 R 20 C A = CESAT D IN D O V V V V V D − + + = − = 1 V 0 . 1 V R R O 6 4 1 SW D O L L F ) D 1 ( ) V V ( I ⋅ − ⋅ + = D SW O D O 1 F I % 20 ) D 1 ( ) V V ( L ⋅ ⋅ − ⋅ + = ) D 1 ( D I I O CIN _ RMS − ⋅ ⋅ = ⋅ ⋅ + ⋅ D = D O SW L O C F 8 1 ESR I V SW IN O IN F V 4 I C ⋅ D ⋅ > , R G R G S CA PWM ⋅ ≈ ) / s Q / s 1 () / s 1 ( ) C R s 1 ( G V V 2 n 2 n p O ESR PWM c o ω + ω + ω + + = 7 1 Z 5 R F 2 1 C π = 7 1 P 8 R F 2 1 C π = , C R 1 O p ≈ ω , C R 1 O ESR Z = ω k 3 . 22 10 28 . 0 10 R 3 7 20 9 . 15 = ⋅ = − nF 45 . 0 10 1 . 22 10 16 2 1 C 3 3 5 = ⋅ ⋅ ⋅ ⋅ π = pF 12 10 1 . 22 10 600 2 1 C 3 3 8 = ⋅ ⋅ ⋅ ⋅ π = ⋅ π ⋅ ⋅ − = O FB O C S CA C V V C F 2 1 R G 1 log 20 A dB 9 . 15 3 . 3 0 . 1 10 22 10 80 2 1 10 1 . 6 28 1 log 20 A 6 3 3 C = ⋅ ⋅ ⋅ ⋅ ⋅ π ⋅ ⋅ ⋅ ⋅ − = − − m 7 g 10 R 20 C A = CESAT D IN D O V V V V V D − + + = − = 1 V 0 . 1 V R R O 6 4 1 SW D O L L F ) D 1 ( ) V V ( I ⋅ − ⋅ + = D SW O D O 1 F I % 20 ) D 1 ( ) V V ( L ⋅ ⋅ − ⋅ + = ) D 1 ( D I I O CIN _ RMS − ⋅ ⋅ = ⋅ ⋅ + ⋅ D = D O SW L O C F 8 1 ESR I V SW IN O IN F V 4 I C ⋅ D ⋅ > , R G R G S CA PWM ⋅ ≈ ) / s Q / s 1 () / s 1 ( ) C R s 1 ( G V V 2 n 2 n p O ESR PWM c o ω + ω + ω + + = 7 1 Z 5 R F 2 1 C π = 7 1 P 8 R F 2 1 C π = , C R 1 O p ≈ ω , C R 1 O ESR Z = ω k 3 . 22 10 28 . 0 10 R 3 7 20 9 . 15 = ⋅ = − nF 45 . 0 10 1 . 22 10 16 2 1 C 3 3 5 = ⋅ ⋅ ⋅ ⋅ π = pF 12 10 1 . 22 10 600 2 1 C 3 3 8 = ⋅ ⋅ ⋅ ⋅ π = ⋅ π ⋅ ⋅ − = O FB O C S CA C V V C F 2 1 R G 1 log 20 A dB 9 . 15 3 . 3 0 . 1 10 22 10 80 2 1 10 1 . 6 28 1 log 20 A 6 3 3 C = ⋅ ⋅ ⋅ ⋅ ⋅ π ⋅ ⋅ ⋅ ⋅ − = − − m 7 g 10 R 20 C A = SC4524B 11 Applications Information (Cont.) |
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