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APW7074 Datasheet(PDF) 14 Page - Anpec Electronics Coropration |
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APW7074 Datasheet(HTML) 14 Page - Anpec Electronics Coropration |
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14 / 20 page  Copyright © ANPEC Electronics Corp. Rev. A.2 - Feb., 2006 APW7074 www.anpec.com.tw 14 Application Information (Cont.) The poles and zero of this transfer functions are: OUT LC C L 2 1 F × × π × = OUT ESR C ESR 2 1 F × × π × = The F LC is the double poles of the LC filter, and FESR is the zero introduced by the ESR of the output capacitor. PHASE L OUTPUT COUT ESR Figure 2. The Output LC Filter FLC FESR -40dB/dec -20dB/dec Frequency(Hz) Figure 3. The LC Filter GAIN and Frequency The PWM modulator is shown in Figure 4. The input is the output of the error amplifier and the output is the PHASE node. The transfer function of the PWM modulator is given by: Figure 4. The PWM Modulator Output of Error Amplifier ΔVOSC PWM Comparator Driver Driver PHASE VIN OSC The compensation network is shown in Figure 5. It provides a close loop transfer function with the highest zero crossover frequency and sufficient phase margin. The transfer function of error amplifier is given by: + + = = sC3 1 R3 R1// sC2 1 R2 // sC1 1 V V GAIN OUT COMP AMP ( ) × + × × × + + × + + × × + × × × + = C3 R3 1 s C2 C1 R2 C2 C1 s s C3 R3 R1 1 s C2 R2 1 s C1 R3 R1 R3 R1 The poles and zeros of the transfer function are: C2 R2 2 1 F Z1 × × π × = ( ) C3 R3 R1 2 1 F Z2 × + × π × = + × × × π × = C2 C1 C2 C1 R2 2 1 F P1 C3 R3 2 1 F P2 × × π × = PWM Compensation The output LC filter of a step down converter introduces a double pole, which contributes with -40dB/decade gain slope and 180 degrees phase shift in the control loop. A compensation network among COMP, FB and V OUT should be added. The compensation network is shown in Fig. 5. The output LC filter consists of the output inductor and output capacitors. The transfer function of the LC filter is given by: OSC IN PWM V V GAIN ∆ = 1 C ESR s C L s C ESR s 1 GAIN OUT OUT 2 OUT LC + × × + × × × × + = |
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