| Nowadays, portable electronic products are widely used in people’s daily life,work and study instead of bulky electronic products. These portable electronicproducts require a high-performance power management chip to ensure proper work.Switching power supply owing to its small size, high efficiency, wide voltage rangelittle noise, etc, are widely used in various electronic products.Aiming at the growing market demand for power management chip, the essaystudies the voltage-mode PFM Boost DC-DC converter. The paper researches a fewfunctional modules of switching power supply, including the reference voltage,comparator circuit, frequency adjustable ring oscillator, start-up circuit, overcurrentprotection circuit, and completes design and simulation of the overall circuit.This dissertation realizes boost DC-DC conversion based on PFM, the outputvoltage is sampled by control circuit, when the output voltage varies, a variablefrequency is generated and acted on the power switch, the power swith turned-onand turned-off time is different, thus the output voltage maintains stability. In orderto reduce the system losses when the load is light, PSM is introduced, the pulse isskipped several signals when the load is light, the power switch remains off duringthis time, the system stays idle state.Because of the output voltage is higher and more stable than the input voltageand of the booster DC-DC converter, the circuit uses the output voltage as thepower supply voltage of each module, the paper designs a start-up circuit based onthe principle of capacitance bootstrap to solve the system low output voltage cannotsatisfy the the each module normal work at the beginning. The essay designs thesecond-order curvature compensation bandgap reference circuit to provide thereference voltage to the circuit voltage comparator and the hysteresis comparator. Inorder to achieve pulse frequency modulation, a ring oscillator that can producedifferent frequencies is designed, by controlling the size of the tail current of thedelay unit. A comparator produces a control signal according to the size of samplingsignal of the output voltage and the reference voltage, making its produce differenteffects on oscillator frequency, the comparator is composed of two stage operationalamplifier and output stage. In order to control the oscillator skip a certain periodwhen the output voltage rises to a certain value, the hysteresis comparator isdesigned.Design of a voltage-mode PFM boost DC-DC converter is based on SMIC0.18μm process, when the input voltage is1.8V, the load current is100mA, the output voltage can be stabilized at3V, the output voltage ripple is less than50mV,When the input voltage changes from0.9V to2V, the output voltage can bemaintained constantly, The maximum operating frequency can reach400kHz. Atlight load, the maximum efficiency can reach90%, the efficiency is80%even inheavy load. The allowable working temperature is from-30oC to100oC. |
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