MC33060 buck chopper circuit

Figure 4 MC33060 step-down chopper circuit MC33060 serves as the main control chip to control the on and off of the switch tube. From its internal structure and function, it can be seen that there is a +5V reference voltage inside MC33060, which is usually used as the inverter of
　　the two comparators. Reference voltage, the comparators on pins 1 and 2 in the design are used as output voltage feedback, and the comparators on pins 13 and 14 are used to detect whether the current of the switch tube is overcurrent. In the circuit, pin 2 is connected to the reference voltage through an inverting circuit, and the buck output feedback is connected to pin 1 of MC33060 through a non-inverting circuit. When the circuit is in working condition, the voltages of pin 1 and pin 2 will be compared with each other, and the output waveform pulse width will be adjusted according to the difference between the two to achieve the purpose of controlling and stabilizing the output.
　　The overcurrent protection in the circuit uses a 0.1 ohm power resistor with a rated power of 1W as the sampling resistor. At the current overflow point, the voltage on the sampling resistor is 0.1V. Pin 14 is used as the sampling point, so the reference voltage of pin 13 is divided by Vref The voltage is set to 0.15V, leaving a certain margin compared to 0.1V. When the sampling voltage is higher than the set value, MC33060 will automatically protect and turn off the PWM output. The protection point is also related to the control signal of pin 3. Based on the functional analysis of this pin, the integral feedback circuit is selected so that when the buck circuit is no-load or full load, the voltage of the Comp pin is always within the normal range (0.5V-3.5V) within. The frequency of the
　　output PWM waveform is determined by the capacitor value of pin 5 and the resistance value of pin 6. The buck circuit uses a waveform frequency of 25KHz. Select a capacitor with a CT value of 1nF and an ordinary resistor with a RT value of 47K to meet the design requirements.