Research And Design Of A Adjustable Frequency Buck DC-DC With The Constant On-Time Control

With the continuous popularization of portable electronic products,performance requirements for electronic products are also getting higher and higher.As the source of energy for electronic products,and the performance of power directly determines the merits of electronic products.Under the circumstances of continuous breakthrough of integrated circuit technology and the pursuit of green environmental protection,power management chips continue to develop in the direction of high efficiency,high integration,low voltage and high current,and low electromagnetic interference.As a kind of power management chip,DC-DC regulator is favored by people for its advantages of high conversion efficiency,wide range of input and output voltage,high integration,and strong carrying capacity,and has broad space for development.This thesis takes the DC-DC regulator as the research object,analyzes the advantages and disadvantages of the circuit architecture using different modulation modes and control methods,and focuses on the current mode constant on time control mode.This mode has the advantages of simple structure and fast transient response,but its operating frequency changes with the change of the input and output voltage of the system,which makes it more difficult to deal with the electromagnetic interference.Aiming at the shortcomings of the architecture,this thesis optimizes the design of the architecture under the premise of ensuring the stability of the system: For the problem that the operating frequency varies with the input and output voltage of the system,an adaptive technique is used to design the on-time timer,and the on-time is set as a function of the input and output voltage to eliminate the influence of the input and output voltage on the operating frequency in this mode.By using pseudo phase locked loop technology,the timer circuit is designed to combine the timer and phase-locked loop to make the operating frequency of the system track the internal oscillator frequency and further lock the working frequency.The operating frequency of the system is set by a variable resistance,so that the working frequency of the system can be adjusted and the application range of the chip is expanded.By setting the optional external synchronization function,the system can not only work at the frequency required by the user,but also realize the two-phase or even multi-phase parallel connection in the heavy load application environment.By adopting the technologies of synchronous rectification,reduce the power consumption of the system.Both the switching power transistor and the synchronous rectifier are NMOS transistors and integrated into the chip,which reduces the layout area and improves the chip integration.Based on the above analysis,a constant-on-time controlled variable frequency BUCK DC-DC regulator was designed.In addition to the soft-start function,this regulator integrates a series of protection circuits such as over-temperature and short-circuit protection.The input voltage range is 4V to 15 V and the output voltage range is 0.6V to 5.5V.The chip can provide 4A of output current.The typical operating frequency is 1MHz and the synchronous frequency range is ±30%.The system is verified by Spectre simulation tool based on 0.35?m BCD process on Cadence platform,and the simulation results show that the system meets the requirements.