A boost PFM controlled DC/DC converter

DC/DC converters are required for power management systems of various battery-powered electronic products, such as laptops, mobile phones, digital cameras, PDAs, etc. Therefore, DC/DC converters are increasingly widely used. There are also various ways to implement and control them, but output accuracy, conversion efficiency, start-up voltage, etc. are the core issues in DC/DC converters. This article introduces a boost PFM control DC/DC converter with simple structure, complete functions, high output accuracy and low power consumption.

Step-up DC/DC converter structure

a. Functional block diagram and working principle

From the structure and working principle of the traditional boost DC/DC converter, it can be seen that its core problem is the control of the switching transistor M by the driving circuit. This paper proposes a boost PFM controlled DC/DC converter, which uses a built-in MOSFET as the switching tube, including a reference voltage source, an error comparator, an operational amplifier, a PFM control circuit, a MOSFET current limiting protection circuit, an enable control, a voltage sampling and other unit circuits, as shown in Figure 1.

Figure 1 Functional block diagram

The basic working principle is: when the external input voltage VOUT≥0.9V, the converter starts to work normally, and a reference voltage is generated inside the circuit. This voltage and the voltage fed back by the external actual output are amplified by the error, and the output value controls the PFM circuit. When the output voltage is lower than the rated value, the error amplifier CM outputs a high level, the PFM control circuit works normally, and a pulse signal is generated to control the high-power MOS tube M to be continuously turned on and off, so that the output voltage rises. As the output voltage continues to rise and exceeds the rated value, the output of the error amplifier jumps and becomes a low level, controlling the PFM circuit to stop oscillating and keeping the output voltage constant. When a load is provided to the outside and the output voltage is lower than the rated value, the output of the error amplifier jumps again and returns to a high level, controlling the PFM circuit to resume normal operation, generating a pulse signal to control M to be continuously turned on and off, and repeating the starting process, thereby achieving that the output voltage is kept at the rated value when the DC/DC converter is working stably. Among them, by controlling the PFM circuit using the op amp OM output, the duty cycle factor can be automatically switched according to the load size (the duty cycle is 58% at light load and 76% at high load), which reduces the dynamic power consumption of the converter under light load conditions and improves the conversion efficiency.

b. Implementation of PFM control circuit

The PFM control circuit is composed of an oscillator controlled by multiple enable terminals, as shown in Figure 2. Among them, EN1 is the output of the error amplifier CM, and EN2 is the output of the operational amplifier OM. Protect is the output of the overcurrent protection. When the M current exceeds the preset value, protect controls the oscillator to stop oscillating and disconnect M to prevent damage.

Figure 2 Oscillator circuit diagram

c. PFM control circuit simulation results

When simulating the PFM control circuit, the rated value of the output voltage VOUT takes a typical output value of 3.3V. The simulation result is shown in FIG3 .

It can be seen from the simulation results that the implementation of this PFM control circuit can effectively reduce the loss of the converter and improve the operating efficiency under light load.

(a) Light load (b) High load

Figure 3 Pulse waveform of PFM control circuit under different loads

Application Circuit and Simulation Results

a. Application circuit

This converter has very few peripheral components, only one diode, one inductor, and two capacitors. The application circuit is shown in Figure 4.

Under normal working conditions, when the input voltage varies between 0.9V and 2.4V, the output voltage can be maintained at any value between 1.8V and 6.5V, achieving boost DC/DC conversion.

Figure 4 Application circuit diagram

b. Simulation results

This DC/DC converter can work normally at a low voltage of 0.9V; the typical input/output values ​​are: VIN=2.4V, VOUT=3.3V, and the simulation results are shown in Figures 5 and 6.

(a) (b)

Figure 5 Simulation results of output voltage VOUT at different input voltages (VIN)

(a) Light load (b) High load

Figure 6 VOUT output ripple diagram at different loads

From the simulation results, it can be seen that when the load changes, the output voltage fluctuation range of the DC/DC converter is ±8mV, and the output voltage accuracy reaches ±0.3%; while the input voltage of the traditional boost PFM control DC/DC converter is generally greater than 1.2V to work stably, the output voltage fluctuation range is generally ±50mV, and the output voltage accuracy is about ±2%. It can be seen that the boost PFM control DC/DC converter designed in this paper is a low-voltage, stable working DC/DC converter with very low output ripple and high output voltage accuracy.

Conclusion

The boost PFM controlled DC/DC converter designed in this paper has high conversion efficiency, low output ripple and high output voltage accuracy in a wide range when working stably. It also has a simple structure and low power consumption. It is a DC/DC converter with great use value.