Analog Design Review
Buck Converter
A buck converter is a closed-loop system that converts a DC input (Vin) to a lower DC output voltage (Vout). At the functional level, there are four functional blocks in the buck conversion topology: power stage, compensator, PWM generator, and transducer. The summing node indicated in the block diagram below compares the desired output voltage (Vref) and the “real” output voltage. In the analog implementation, this comparison is performed mainly by the op-amp, as a transconductance (Gm) amplifier.
Power Stage
Power stage describes the core buck converter, which typically includes the switches, inductors, output capacitors, diode, and load resistors. This section of the design is not essentially changed from an analog to a digital implementation.
Compensator
The compensator block ensures the stability of the closed-loop system using a feedback amplifier, such as a transconductance (Gm) amplifier. In the digital implementation, the equivalent of the compensator is normally called the controller. In the analog implementation, the compensator is made up of an op-amp network with some resistors and capacitors. The resistor and capacitor, along with the transconductance amplifier, are designed to stabilize the system.
PWM Generator
The PWM generator consists of a comparator and flip-flops. The output of the compensator is an almost constant value which is a function of the output of the transducer. The switches in the power stage must be kept closed for a period of time which controls its amplitude. The inputs of the PWM generator are the sawtooth signal generated by the oscillator and the output of the compensator. The sawtooth, comparator, and flip-flops are used to generate a fixed frequency/variable duty cycle waveform where the duty cycle is directly proportional to the amplitude of the compensator output. This waveform is fed into the power switches via the MOSFET drivers to control their on and off times.
