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SC4525ASETRT 数据表(PDF) 13 Page - Semtech Corporation |
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SC4525ASETRT 数据表(HTML) 13 Page - Semtech Corporation |
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13 / 19 page  Figure 7. Block diagram of control loops The block diagram in Figure 7 shows the control loops of a buck converter with the SC4525A. The inner loop (current loop) consists of a current sensing resistor (R s=4.mW) and a current amplifier (CA) with gain (G CA=28). The outer loop (voltage loop) consists of an error amplifier (EA), a PWM modulator, and a LC filter. Since the current loop is internally closed, the remaining task for the loop compensation is to design the voltage compensator (C 5, R7, and C8). For a converter with switching frequency F SW, output inductance L , output capacitance CO and loading R, the control (V C) to output (VO) transfer function in Figure 7 is given by: (8) This transfer function has a finite DC gain an ESR zero F Z at a dominant low-frequency pole F P at and double poles at half the switching frequency. + - Vo L1 Co Resr COMP EA REF Vc SW CONTROLLER AND SCHOTTKY DIODE FB PWM MODULATOR Vramp CA R4 R6 C5 R7 C8 Io Rs + - Vo L1 Co Resr COMP EA REF Vc SW CONTROLLER AND SCHOTTKY DIODE FB PWM MODULATOR Vramp CA R4 R6 C5 R7 C8 Io Rs 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 = 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 = Including the voltage divider (R 4 and R6), the control to feedback transfer function is found and plotted in Figure 8 as the converter gain. Since the converter gain has only one dominant pole at low frequency, a simple Type-2 compensation network is sufficient for voltage loop compensation. As shown in Figure 8, the voltage compensator has a low frequency integrator pole, a zero at F Z, and a high frequency pole at F P. The integrator is used to boost the gain at low frequency. The zero is introduced to compensate the excessive phase lag at the loop gain crossover due to the integrator pole (-90deg) and the dominant pole (-90deg). The high frequency pole nulls the ESR zero and attenuates high frequency noise. Figure 8. Bode plots for voltage loop design Therefore, the procedure of the voltage loop design for the SC4525A can be summarized as: () Plot the converter gain, i.e. control to feedback transfer function. (2) Select the open loop crossover frequency, F C, between 0% and 20% of the switching frequency. At F C, find the required compensator gain, A C. In typical applications with ceramic output capacitors, the ESR zero is neglected and the required compensator gain at F C can be estimated by (9) Fp Fsw/2 Fz1 Fp1 Fc Fz 1K 10K 100K 1M 10M -60 -30 0 30 60 FREQUENCY (Hz) CONVE RTER GAIN LOOP GAIN COMPE NSATO R GAIN Fp Fsw/2 Fz1 Fp1 Fc Fz 1K 10K 100K 1M 10M -60 -30 0 30 60 FREQUENCY (Hz) CONVE RTER GAIN LOOP GAIN COMPE NSATO R GAIN 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 = SC4525A 3 Applications Information (Cont.) |
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