MC34063 step-down circuit

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MC34063 step-down circuit [Copy link]

The MC34063 chip consists of a reference voltage generator with automatic temperature compensation function, a comparator, a duty cycle controllable oscillator, a trigger, and a high current output switch circuit. It has the advantages of complete functions, low price, small size, high efficiency, and only a small number of external components. Its main features are shown in the table:

The basic internal structure and pin functions of the MC34063 chip are shown in the figure below.

The MC34063 chip has an external timing capacitor at pin 3, which can adjust the frequency of the oscillator and determine the on-time of the switch tube. The detection terminal at pin 7 is used to monitor the peak current of the load to control the high and low levels of the trigger terminal. At the same time, the chip realizes the current limiting function by monitoring the voltage difference between pins 6 and 7. When the voltage difference exceeds 300mV, the chip starts the overcurrent protection function, and the oscillator quickly charges the timing capacitor, thereby reducing the on-time of the output switch tube and extending its off time.

The MC34063 step-down conversion switch circuit is shown in the figure below.

The working principle of the step-down circuit is that the chip 5 pin monitors the output voltage through the external precision resistors R30 and R31. The output voltage U _O is _{input into} the inverting input terminal of the comparator after voltage division and the voltage U _{REF . When U REF} is lower than the reference voltage, the comparator outputs a jump voltage, the S pin of the trigger is high level, and when the oscillator charges the capacitor, the R pin is high level, so that the Q end of the trigger is high level, so that the output switch tube is turned on, and then the input voltage U _IN charges the output filter capacitor C23 _to increase the output voltage U _O , so as to achieve the effect of automatically controlling the stability of U _O. When UREF is higher than the reference voltage, the S pin of the trigger is low level, the Q end is low level, so that the drive tube Q ₂ is turned off and the switch tube Q ₁ is turned off.

The output voltage is only related to the resistance values of R30 _and R31 _. The formula for calculating the output voltage is:

The constant 1.25 is the internal reference voltage, which remains constant.