| With the development of the power electronics industry,more and more electronic products cover all aspects of social life,and the power management chips which provide power for them are constantly expanding new application areas.Different industries and applications have their own emphasis on the performance requirements of switching power supplies.Industrial electronics and automotive electronics have very strict requirements for reliability,while portable electronic devices continue to require lower power consumption.Constant on-time(COT)control has the feature of automatic frequency reduction under light load,which can reduce the power consumption of portable electronic devices working in standby and shutdown,and has excellent response speed to load and output.It is suitable for low-power and high-speed applications.The basic RBCOT structure is the simplest,but has the problem of output DC offset.The DC value of the output voltage cannot be well defined,and sub-harmonic oscillation may occur.On the basis of RBCOT,V~2COT introduces a new voltage loop.The small signal differential voltage between the output feedback signal and the reference voltage is amplified by the error amplifier EA and then transfers to the loop comparator,thus eliminating the DC offset of the output voltage and greatly improving output accuracy.Under different input and output voltages,COT fixes the on-time and adjusts the off-time,so the switching frequency cannot be fixed,which amplifies the EMI problem.Therefore,this article uses ACOT control to generate an adaptive on-time that is inversely proportional to the input and proportional to the output,and generates constant switching frequency.Considering power consumption,output accuracy,response speed and EMI,the Buck converter designed in this article finally selects the V~2ACOT architecture.This paper uses the description function method and the average method to model the loop of the V~2ACOT architecture respectively,and derives the loop models under the two methods.By comparing the different assumptions established by the two methods,the applicable occasions of the two models are obtained.The conclusions of the two models are synthesized,in order to select the appropriate ripple compensation and EA parameters.In order to achieve low static power consumption design,the sources of static power consumption of Buck converters in the no-load and non-switching state are summarized.The sub-threshold design reduces the bias current requirements of the sub-circuits,and the dynamic bias design makes the loop comparator enter sleep mode under light load to further reduce static power consumption.Finally,the sub-circuit design and system simulation verification are completed based on the DB 0.18μm BCD process.The switching frequency is 2.2MHz,the maximum load current is 2A,the input range is 2.4-5.5V,and the output range is 0.6-4V.In the no-load static state,the quiescent current of the Buck converter is about 4μA,which achieves the design goal of ultra-low power consumption,while also meeting the needs of high output accuracy and high response speed. |
