Research On Boost DC-DC Converter Controlled By COT With Fixed Switching Frequency

With the rapid development of integrated circuit industry,more and more kinds of electronic products are derived.Smart phones and laptops have become indispensable devices and tools in our lives.As the key structure of these products,the power source plays a crucial role in the application of products.DC-DC converter can generate different DC voltages to ensure the normal operation of the rest of the circuit modules in the chip.As one of them,Boost DC-DC converters are popular in the market due to its boost characteristics and wide application range.However,with the progress of the times,the requirements of the market are higher and higher.In order to meet the requirements of some new electronic products such as high stability,high reliability and fast transient response,DC-DC converter still faces great challenges.DC-DC converters need to adopt different control modes in different application environments.Voltage control mode and current control mode are two typical control modes,and both theory and practical application have been mature about these two kinds of modes.The constant on/off time control mode appears later than the former,which is popular for its fast transient response speed.Therefore,it has gradually become a research hotspot.However,the constant on-time control（COT）mode has the problem that the working frequency changes with the input and output,which leads to a large electromagnetic interference and limits its application range.In order to improve the above problems,this paper studies the design of Boost DC-DC converter based on fixed switching frequency under CMOS technology.Based on the constant on-time control mode,and improve it.In the on-timer circuit module,the input and output voltage are connected with the on-time after the signal conversion,that is to say,the obtained on-time T_ON is proportional to the input and output voltage,while the frequency f_s is also related to the on-time and the input and output voltage.Therefore,in order to realize the fixed frequency operation of the converter circuit,it is necessary to ensure that the ratio coefficient between the on-time and the input and output voltage is fixed,so the working frequency of the switch tube can not change with the change of the input and output by setting the parameters of the resistance and capacitance.The input voltage range of the system is 2.4～4.2V,the output voltage is stable at 5V,and working frequency is 1MHz,and the load current is 1～2A.The system uses the combination of voltage control loop and current control loop to generate control signals,so as to ensure that the switch tube and rectifier tube work alternately and produce a stable output voltage.In addition to the key circuits module design,such as Error Amplifier,Comparator,On-Timer module,etc.,the system also includes over temperature protection circuit,under voltage protection circuit,minimum off time circuit and other protection modules,so as to ensure that the circuit works in a safe and reliable environment.In order to avoid the generation of surge current and overshoot voltage,a soft-start circuit is also designed in the system.When the circuit starts to work,output voltage is low,and a reference voltage which changes synchronously with the feedback voltage is generated to ensure the stability of the circuit.According to the requirements,completing the design of each circuit module.The simulation platform of Cadence spectre is used,and with the help of the process of SMIC 0.18μm CMOS to complete the simulation verification of each modules and system.The results show that the system can be started normally and the output voltage is stable at 5V.Under different input voltage conditions,the operating frequency is basically constant.In the process of load current jumping from 1A（2A）to 2A（1A）,the frequency hardly changes with the change of load current,and has a fast transient response speed.In the process of input voltage rising or drop from 2.4V to 4.2V and4.2V to 2.4V,the output voltage is stable at about 5μs,the overshoot voltage is about 70m V,and the undershoot voltage is about 30m V;the output voltage ripple is not higher than 10m V under different input voltage and load current,and the performances meet the practical requirements.