Analysis And Design Of Variable On-Time Control Buck Converters

The Constant On-Time?COT?control is characterized by variable frequency modulation,which can greatly improve the response speed,and can achieve high efficiency under light load due to its automatic frequency-down in light-load.Therefore,it is widely used in low-power Buck converter with fast transient response.However,there are several inherent flaws that limit further improvements in performance:the On-Time of the COT-controlled Buck converter is fixed and T_OFF,MIN can not be avoided,so the max Duty cycle is limied and there is transient bottleneck especially in small duty cycle.In addition,the PLL is used to implement the frequency synchronization of the COT controlled Buck,of which the frequency stabilization effect is better than the AOT control mode that controls the T_ON through the input voltage and output voltage.But there is no complete small signal model to describe the structure,and the PLL bandwidth design in the Buck converter has no theoretical guidance.In view of the above situation,the main work and innovations of this paper can be divided into the following points:1.In this paper,a variable on-time?VOT?control mode is proposed,which is based on the control principle of COT and has a structural improvement for transient bottleneck.According to the internal signal classification of the OST module,two VOT control Buck converters,Voltage Enhancement Mode and Current Enhancement Mode,are designed.The essence of both is that the difference of V_REF-V_FB in the transient response is fed back to the OST module to adjust the on-time T_ON.As a result,the on-time of the converter is variable in the transient state,and the degree of freedom of the equivalent duty ratio is improved to achieve transient enhancement.2.A small signal model of T_ON controlled COT control Buck converter is established by the Describe Function?DF?method and then the Voltage Enhancement Mode VOT control Buck converter is modeled.The closed-loop transfer function of the new third loop is calculated through the introduced Von disturbance,and the stability of the VOT control Buck converter is discussed.It is pointed out that the inner loops of the current and the T_ON are naturally stable,and the stability of the outer voltage loop is affected by the two inner loop transfer functions.3.The design criteria for frequency synchronization of the COT controlled Buck with PLL modulation is proposed.The PLL-modulated Buck converter is classified as variable on-time?VOT?control in terms of the modulation signal.Following the same modeling idea,a complete PLL and Buck double-loops system model was constructed to research the interaction effects of the two loops.Through the small signal model,the effect of the bandwidth of the PLL on the stability of the system is theoretically explained,by comparing the relationship between GBW_PLL and GBW_BUCK,2/Toff.Based on the small signal model,the design of PLL's loop bandwidth for Buck converters frequency synchronization get the first theoretically support,and a design window can be found between fast frequency lock,on-chip compensation and system stability,while the impact of PLL's bandwidth on the system phase margin can be quantified.Based on the 0.35?m BCD process,two chips were designed in this paper.The first chip is a Buck converter that can switch in COT/VOT mode,and the second chip is a PLL-modulated Buck converter.The former simulation verified the transient advantage of VOT,effectively achieves fast transient response and reduces output voltage overshoot/undershoot.The latter successfully taped out and verified the double-loop model through testing.The Buck converter with good stability can achieve frequency lock in steady state and fast response in transient state.