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APW7065C Datasheet(PDF) 13 Page - Anpec Electronics Coropration |
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APW7065C Datasheet(HTML) 13 Page - Anpec Electronics Coropration |
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13 / 19 page  Copyright © ANPEC Electronics Corp. Rev. A.2 - Mar., 2008 APW7065C www.anpec.com.tw 13 Application Information (Cont.) t SW is the switching interval D is the duty cycle Note that both MOSFETs have conduction loss while the upper MOSFET, including an additional transition loss. The switching internal, t SW, is a function of the reverse transfer capacitance C RSS. The (1+TC) term is to factor in the temperature dependency of the R DS(ON) and can be extracted from the “R DS(ON) vs. Temperature” curve of the power MOSFET. 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- ure 6. The output LC filter consists of the output inductor and output capacitors. The transfer function of the LC filter is given by: MOSFET Selection (Cont.) The poles and zero of this transfer functions are: The F LC is the double poles of the LC filter, and FESR is the zero introduced by the ESR of the output capacitor. Figure 3. The Output LC Filter The PWM modulator is shown in Figure 5. The input is the output of the error amplifier and the output is the PHASE node. The transfer function of the PWM modu- lator is given by: Figure 5. The PWM Modulator Figure 4. The LC Filter GAIN and Frequency 1 C ESR s C L s C ESR s 1 GAIN OUT OUT 2 OUT LC + × × + × × × × + = OUT LC C L 2 1 F × × π × = OUT ESR C ESR 2 1 F × × π × = PHASE L OUTPUT C OUT ESR OSC IN PWM V V GAIN ∆ = The compensation network is shown in Figure 6. 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 F LC F ESR -40dB/dec -20dB/dec Frequency(Hz) Output of Error Amplifier ΔV OSC PWM Comparator Driver Driver PHASE V IN OSC |
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