Research On Design Technology Of Buck DC-DC Converters With Constant On-Time Control

With the rapid development of semiconductor integrated circuits,besides the major markets including automobiles,communications,computing,consumer and industry,power management chips also have broad applications in emerging industries such as mobile intelligence,the Internet of Things,cloud computing,and big data.The switch power supply is widely used because of its high efficiency,low heat,stable performance,small size,and light weight.As the key element of switching power supply,the converter has varieties of control techniques that consist of voltage mode,current mode,hysteric mode,constant on-time mode,constant off-time mode,etc.In the present mobile application market,the fast transient response is one of the most critical design parameters to meet while minimizing the output capacitance and cost.In battery-powered small electronic equipment,especially portable equipment and wearable electronic equipment,the light load efficiency is another important requirement for extending the battery run-time.For the advantages such as the fast transient response and the high efficiency at light load,the converter with constant on-time control is favored in the battery-powered mobile application market and small electronic equipment.Keeping up with market development,this dissertation focuses on the analysis of and the research on the issue that relates to the stability,modeling,and light load efficiency of the buck DC-DC converter with constant on-time control(COT).And then the solutions and the corresponding specific circuits are proposed.The main innovations of this dissertation are as follows.1.The stability of the buck DC-DC converter with constant on-time control is analyzed from the two aspects: loop stability and jitter performance.The stability criterion that the sub-harmonic oscillation can be avoided for constant on-time control is obtained.For constant on-time voltage-mode control,the uncertainty that when the main power MOSFET is turned on,which is affected by the noise coupled on the feedback pin,causes the jitter.Through the analysis and calculation,it can be concluded that for the jitter performance,it is not the magnitude of the output voltage ripple matters,but the downslope of the FB voltage.Therefore,the jitter performance can be improved by adding ramp compensation signal of a certain slope on the feedback pin.For the constanton-time current-mode control,a similar analysis is adopted and a compensation method is proposed in this dissertation.Through the addition of the ramp compensation signal on the COMP pin,the jitter of the converter caused by the uncertainty that when the main power MOSFET is turned on is reduced.2.Several popular modeling methods for current-mode controlled converters are studied: the state-space averaging method,the Ridley model and the modeling based on the description function.The modeling approach based on the describing function method,which is suitable for constant on-time control,is utilized to model the constant on-time current-mode controlled buck DC-DC converter with ramp compensation.The inductor,the switches,and the PWM modulator are treated as a single entity and modeled based on the describing function method.The model can achieve accurate results and predict the system response very well for constant on-time current-mode control.The model is used to analyze the influence of ramp compensation on the circuit and to determine the range of the slope of the ramp compensation signal.A simple equivalent circuit representation is given for easy understanding and simulation of constant on-time current-mode control.3.Based on the research on improving jitter performance and the determination of the slope of the ramp compensation signal,a ramp compensation circuit for constant on-time current-mode-controlled buck DC-DC converter is proposed.The circuit structure is simple,the number of devices is small,and it is easy to realize.By using the control signal in the same phase with the switching signal of the control converter,the slope signal with small change of slope is obtained by charging/discharging the capacitor and amplifying the amplifier.Then it is added on the COMP pin to improve the jitter performance of the converter.4.An adaptive current-threshold determination method is proposed for load variation and a corresponding current-threshold detection circuit suitable for the converter with constant on-time control is designed.While reducing the output voltage ripple at light load,it is also designed to reduce the conduction loss associated with the power MOSFETs,capacitor esr,and inductor DCR,which is caused by inductor current ripple.Then the efficiency is raised and the demand of mobile and portable devices for battery run-time can be satisfied.Because of its modulation principle,the constant on-time controlled converter itself operates in pulse frequency modulation mode when at light load,and has higher efficiencythan pulse width modulation mode does.However,there is a compromise between the switching loss and the conduction loss.Through the calculation and analysis of switching frequency,power loss and efficiency,the optimal peak current of the inductor corresponding to maximum efficiency is obtained and an adaptive current-threshold detection circuit is proposed.By monitoring the relationship between the peak current of the inductor and the load current,the former is reduced when the load current is less than a certain threshold,then the output voltage ripple is reduced and the efficiency is raised.