Research And Design Of A Step-down DC-DC Converter With The Valley Current Mode

Switching power supply integrated circuits have many advantages,such as cost-effective,high integration density,superior performance and simple peripheral circuit etc.And very suitable for application in portable electronic products,have been widely used in communication,electronic computer and consumer electronic products and other fields.In this paper,we mainly research the design,simulation and achieve about a Step-down DC-DC converter.The paper start from theoretical analysis,to design the circuit,and to simulate the circuit performance,design a step-down DC-DC converter with the valley current mode which can be applied to portable electronic products.This paper first introduces the development of switching power supply,as well as the research objective and significance,and then introduces three topology of switching power supply,two kinds of control method,and three modulation types,also detail expound peak and valley current control mode.Make a brief analysis about the step-down DC-DC stability and synchronous rectifier technology,providing a comprehensive theoretical support for the realization of chip analyse and design.The synchronous step-down switching regulator controller analysed and designed in this paper,using a constant on-time,valley current mode control.Build the circuit modules of the chip use the Cadence spectre software,and simulation four key modules and whole circuit,and the simulation results show that the single modules and whole circuit meet the over chip requirements.The chip has a wide input voltage range of 4V to 32 V,allows a voltage as high as 32 V to be stepped down to as low as a 0.6V output,2% to 90% duty cycle,typical value of reference voltage is1.18 V,typical value of on-time Ton is 250 ns.Except the key modules introduced in the paper,the chip also integrates current foldback protection,overvoltage protection,undervoltage protection and soft-start and other protective circuits.Its advanced functions and high accuracy reference are ideal for powering high performance server,ASIC and computer memory systems.The paper made innovations in the following respects in design process:Firstly: The use of a topology with the valley current mode.Compared with the volatge control mode,current control mode can make the chip have a fast transient response speed.It’s most use peak control mode in the converter,valley currentcontrol mode will be high efficiency conversion no matter in the heavy or in the light load full load.Secondly: The use of one-shot timer with a constant on-time,make frequency as a constant,and do not change with output voltage.Turn-off the main switch by detection the minimum inductor current,and adjust the switching period.This control mode can supply minimum turn-on time,contribute to work in high frequency,and produce low duty ratio output voltage by high input voltage.Thirdly: This converter do not required slop compensation circuit.As the constant on-time of the chip and the valley current mode control,produce the same interference in before the cycle and at the end of the cycle,so converter remain stable,that make the error of the previous cycle does not affect the next cycle,and no relationship with duty ratio,so even in the case when the duty ratio is greater than50% this chip even not generate sub-harmonic oscillations,and the slop compensation circuit is not used.