Design Of Digital Constant-on-Time Buck Converter

In recent years,the market for portable devices such as mobile phones,tablets and smart watches has been growing.As the performance of various portable devices is constantly promoted,higher requirements are put forward for the power supply.Compared with analog controlled power converter,digital controlled power converter is easier to be embedded into SoC chips,so as to meet the requirements of high integration.In the design and use process,digital control DC-DC’s debugging and monitoring is more flexible,and can significantly reduce the number of discrete components,improving the reliability of the system.Compared with fixed frequency PWM control,the constant-on-time control with variable operating frequency can achieve faster response speed and higher light load efficiency,so it has been widely used,but the research on digital constant-on-time control is relatively less.This paper designs a digital COT controll Buck converter.In the design of digital power converter,ADC is the bridge connecting analog sampling and digital control,so ADC architecture selection and circuit design are very important.In order to solve the contradiction between quantization accuracy and sampling range,this paper proposes a DAC to set reference voltage,combined with PGA and differential ADC.The PGA is designed by a full differential operational amplifier with common mode feedback.The differential ADC adopts an asynchronous SAR structure with a precision of 7.8125 mV,a maximum sampling rate of 10 Msps,and a power consumption as low as 9mW.The ADC designed in this paper is based on 0.18 CMOS process and integrated into a digital DC-DC controller chip with good measurement.The digital COT controller designed in this dissertation adopts digital filter to realize current sampling,and adjusts the accuracy of current sampling by frequency of clock,which can effectively reduce the sampling rate of ADC and thus reduces the power consumption.After approximating the filter parameters,the complex decimal and multiplication operations are converted into shift operations to reduce the bit width required by the number part and the power consumption of the corresponding part.By time multiplexing sampling and calculation of input and output voltages,the response of line and load transient are greatly improved.The Buck converter was simulated andverified at the input voltage of 3.3V,output voltage of 1.8V,and switching frequency of1 MHz.The recover time of line transient was nearly negligible,and the load transient response time was reached 44μs/37μs.The line and load regulation of proposed digital COT controlled converter is 0.28% and 0.8%,respectively.The output voltage set accuracy was reached 12 mV,which is 0.6% of output voltage of 1.8V.