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APW7098 Datasheet(PDF) 22 Page - Anpec Electronics Coropration |
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APW7098 Datasheet(HTML) 22 Page - Anpec Electronics Coropration |
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Copyright © ANPEC Electronics Corp. Rev. A.6 - Oct., 2009 APW7098 www.anpec.com.tw 22 Application Information (Cont.) PWM Compensation (Cont.) The pole and zero frequencies of the transfer function are: Figure 8. Compensation Network 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 × = The closed loop gain of the converter can be written as: GAIN LC X GAINPWM X GAINAMP Figure 9. shows the asymptotic plot of the closed loop converter gain, and the following guidelines will help to design the compensation network. Using the below guidelines should give a compensation similar to the curve plotted. A stable closed loop has a -20dB/ decade slope and a phase margin greater than 45 degree. 1. Choose a value for R1, usually between 1K and 5K. 2. Select the desired zero crossover frequency F O= (1/5 ~ 1/10) X FSW Use the following equation to calculate R2: 3. Place the first zero F Z1 before the output LC filter double pole frequency F LC. F Z1 = 0.75 X FLC Calculate the C2 by the equation: R1 F F V V R2 LC O IN OSC × × ∆ = 4. Set the pole at the ESR zero frequency F ESR: F P1 = FESR Calculate the C1 by the following equation: 0.75 F R2 2 1 C2 LC × × × π × = 1 F C2 R2 2 C2 C1 ESR − × × × π × = 5. Set the second pole F P2 at the half of the switching frequency and also set the second zero F Z2 at the output LC filter double pole F LC. The compensation gain should not exceed the error amplifier open loop gain, check the compensation gain at F P2 with the capabilities of the error amplifier. F P2 = 0.5 X FSW F Z2 = FLC Combine the two equations will get the following component calculations: F LC Frequency(Hz) 20log (R2/R1) 20log (V IN/ΔVOSC) F Z1 F Z2 F P1 F P2 F ESR PWM & Filter Gain Converter Gain Compensation Gain V REF V OUT V COMP R1 R3 C3 R2 C2 C1 FB 1 F 2 F R1 R3 LC SW − × = SW F R3 1 C3 × × π = Figure 9. Converter Gain and Frequency Output Inductor Selection The duty cycle (D) of a buck converter is the function of the input voltage and output voltage. Once an output volt- age is fixed, it can be written as: |
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