Research And Design Of A Constant Off-time Controlled Buck-Boost Converter

The Buck-Boost converter is capable of generating negative voltage,so it is often used in dual power systems,especially,it has been used to provide drive power in OLED displays in recent years.Due to the characteristics of self-luminescence,thin thickness,large view angle,and flexible displays,OLED displays have a huge potential of market.In order to meet the developing trend of market,an output voltage adjustable Buck-Boost converter system circuits are designed in this thesis.The Buck-Boost converter designed in this thesis adopts the constant off-time mode and combines with the peak current mode,which not only improves the response speed,realizes the dynamic adjustment of the current and simplifies the loop compensation,but also eliminates the risk of producing sub-harmonic shock easily of the traditional peak current mode.The system adds a digital interface,through which the rising edge is counted by inputting pulse signal,and the output voltage programmed controlling can be achieved.In addition,multiplexing part of the digital circuits can realize soft starting controlled by digitization.It has not only optimized the controlling mode,but also expanded the application of the Buck-Boost converter.In this thesis,the describing function method is used to model and analyze the constant off-time control mode,deduce the transfer function of the system.And then the transfer function is simulated to determine the stability of the system.In the design of the overall block diagram of the system,firstly,the key module circuits in the system loop are analyzed;Secondly,the module circuits are designed and the verification results are simulated.Finally,the characteristics of the whole system loop are simulated and verified,and the simulation results are analyzed.And all the results meet the targets of original design well.In this thesis,0.18?m CDMOS high-voltage process is used to the design of the Buck-Boost converter,it achieves the adjusting of the output voltage from-1.4V to-5.4V;when the output voltage is-2.5V,ripple voltage is 2.6mV;when the load current is more than100 mA,the efficiency of it is about 85%.