Low-cost implementation of DC-DC circuit conversion based on MC34063

introduction

In power supply circuits, DC-DC conversion is widely used due to temperature rise, efficiency and other factors. This article introduces a low-cost DC-DC conversion implementation solution, which can achieve buck, boost and voltage reversal applications. Its circuit is simple, low cost, high efficiency and low temperature rise. These circuits are widely used.

The core component of the circuit is MC34063, which is a monolithic bipolar linear integrated circuit dedicated to the control part of the DC-DC converter. It contains a temperature-compensated bandgap reference source, a duty cycle controlled oscillator driver and a large current output switch, which can output a switching current of 1.5A. It can use the least external components to form a switching boost converter, a buck converter and a power inverter.

The package of MC34063 is a plastic-encapsulated dual-row 8-lead straight-plug type. The internal circuit principle block diagram is shown in Figure 1.

1 Working Principle

Due to the built-in high-current power switch, the MC34063 can control the switching current up to 1.5A. The internal circuit includes a reference voltage source, oscillator, converter, logic control circuit and switching transistor.

The reference voltage source is a temperature compensated bandgap reference source. The oscillation frequency of the oscillator is determined by the external timing capacitor at pin 3. The switching transistor is turned ON by the reverse input of the comparator and the logic control circuit connected to the oscillator, and is turned OFF by the next pulse synchronized with the oscillator output.

2 Circuit Principle

The circuit shown in the internal block diagram of Figure 1 is explained as follows:

The oscillator continuously charges and discharges the timing capacitor connected to the CT pin (pin 3) through a constant current source to generate an oscillation waveform. The charging and discharging currents are constant, so the oscillation frequency depends only on the capacity of the external timing capacitor. The C input of the AND gate is high when the oscillator is charging externally, and the D input is high when the input level of the comparator is lower than the threshold level. When both the C and D inputs become high, the trigger is set to a high level and the output switch is turned on. Conversely, when the oscillator is discharging, the C input is low, the trigger is reset, and the output switch is turned off.

The current limit SI detection terminal (pin 5) completes its function by detecting the voltage drop on the resistor connected between V+ and pin 5. When the voltage drop on the resistor is detected to be close to or greater than 300mV, the current limit circuit starts to work, and the timing capacitor is quickly charged through the CT pin (pin 3) to reduce the charging time and the conduction time of the output switch tube, resulting in a longer off time of the output switch tube.

3 Typical Applications

Figure 2 shows a step-down DC-DC conversion application.

The output voltage value can be adjusted by changing the resistance values ​​of R4 and R5. The output voltage conforms to the following formula: Vout = (1 + R4/R5) × 1.25V

The current limiting resistor in the circuit is 0.15Ω, so the input current is limited to 0.3V/0.15Ω=2A. Changing the current limiting resistor can change the current limiting value.

Figure 3 shows a step-up DC-DC conversion application.

The output voltage value is also adjusted by changing the resistance values ​​of R4 and R5. The output voltage conforms to the following formula: Vout = (1 + R4/R5) × 1.25V

The current limiting resistor in the circuit is 0.3Ω, so the input current is limited to 0.3V/0.3Ω=1A.

Figure 4 shows an inverting DC-DC conversion application.

The output voltage value is also adjusted by changing the resistance values ​​of R2 and R3. The output voltage conforms to the following formula: Vout = (1 + R3 / R2) × 1.25V

The current limiting resistor in the circuit is 0.3Ω, so the input current is limited to 0.3V/0.3Ω=1A.