PIC-003 - Application Note PIC-003 BUCK Converter Control...

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August 08 Tel: 408.830.9742 • Fax: 408.830.9749 • www.aosmd.com 1 BUCK Converter Control Cookbook Zach Zhang, Alpha & Omega Semiconductor, Inc. A Buck converter consists of the power stage and feedback control circuit. The power stage includes power switch and output filter. It converts a higher input voltage to a lower output voltage. The feedback control circuit regulates the output voltage by modulating the power switch duty cycle. Stable operation of switching mode DC/DC converter requires an adequate loop gain and phase margin in frequency domain. This application note provides an overview of the control circuit small-signal modeling, power stage modeling and feedback compensation design. 1. Buck Power Stage Small-Signal Analysis In this application note, the AOZ101X is used as an example to explain Peak Current Mode Control (PCMC) and its small signal analysis. PCMC makes the converter power stage resemble a voltage-controlled-current-source. It simplifies the feedback compensation design since the complex pole pair associated with output LC filter are removed from the loop compensation equation. PCMC also provides other benefits such as voltage feedback forward and inherent cycle-by-cycle current limit. 1.1 Simplified buck converter equivalent circuit Figure 1 below shows a simplified schematic of buck converter and modulator with Peak-Current-Mode-Control scheme. Figure 1: PCMC buck converter block diagram Application Note PIC-003
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August 08 Tel: 408.830.9742 • Fax: 408.830.9749 • www.aosmd.com 2 The output voltage is fed back to error amplifier A1 and is compared against the internal reference voltage, V ref , which is 0.8V for the AOZ101x. Any slight difference between the feedback voltage and reference voltage results in large swing in the error amplifier output voltage. The output voltage of error amplifier A1 is also called control voltage, which actually controls the switch current level and the output voltage. The inductor ripple current is translated into a ramp voltage signal by the current sense amplifier A2, which amplifies the voltage drop across the high side switch. This ramp represents the inductor current plus compensation ramp signal together and is fed back to the PWM comparator, forming an inner current control loop. At the beginning of each switching period, control voltage is higher than the current signal voltage. The output of the PWM comparator is high and the high-side switch is on. The input voltage applies to the output filter inductor, L. The inductor current increases at a constant slew rate decided by input voltage, output voltage, inductance and switching frequency. The inductor current ramp up slew rate is: Δ I L is the peak to peak inductor current ripple. Δ t on is the high-side switch turn on time. The current signal voltage ramps up as the switch current increasing. When the current signal voltage, V
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PIC-003 - Application Note PIC-003 BUCK Converter Control...

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