Design Of A Fixed Frequency COT Controlled BUCK Converter

In recent years,with the rapid development of science and technology,a variety of new electronic devices have emerged,and human society has been inseparable from these electronic devices.Electronic devices are inseparable from power supply,and power management chips have attracted much attention.Switching power supplies represented by BUCK converters have the widest range of applications due to the advantages of low power consumption,high efficiency,small size,and high integration.Among them,the BUCK converter based on COT control mode has both high light-load efficiency and fast transient response speed,and has received extensive attention from designers;however,because its operating frequency is not fixed,the design of the filter circuit is more complicated,and its use scope is also limited.Therefore,it is very meaningful to realize the constant frequency COT controlled BUCK converter.Based on the traditional current-mode COT control mode,a constant frequency COT controlled BUCK converter is designed in this thesis.Its input voltage range is 4V?17V,output voltage range is 0.8V?5.5V,working temperature range is-55??125?,load current range is 0A?3A,typical working frequency is 1.5MHz.The converter adjusts the on-time generation circuit through the internal phase-locked loop to achieve a constant operating frequency,solving the problem of electromagnetic interference,and avoiding complicated filter circuit design.Under light load conditions,the converter can be put into DCM mode,thereby reducing the switching loss at light load.In addition,the converter also has functions such as soft-start,over-temperature protection,over-voltage protection,under-voltage lockout,short-circuit protection,output voltage tracking and external clock frequency synchronization.This thesis first introduces the basic principles of BUCK converters,and then designs the converters at the system level.The COT-controlled BUCK converters are modeled and analyzed using the descriptive function method,and the corresponding loop compensation circuit is designed.The system stability was verified by Simplis modeling and simulation.The simulation results show that the system loop phase margin is about 62 deg,and the GBW is about 169 kHz;when the load current jumps between 0A and 3A,the undershoot of the output voltage is about 50 mV,and the overshoot is about 80 mV,and it can quickly restore stability.There is no oscillation phenomenon,and the system loop is stable.Based on the0.18?m BCD process,the system's overall circuit design and circuit layout design were carried out in the Cadence software platform,and simulation verification was carried out.The simulation results show that the chip can be soft-started and powered up normally,the operating frequency in steady state is 1.5MHz,it has a fast transient response speed,the conversion efficiency meets the reference index value,and the short-circuit protection function is normal.When the input voltage changes from 0V to 17 V,the linear adjustment rate is about 0.2%;when the load current changes from 0A to 3A,the load adjustment rate is about 0.13%.Finally,the chip was tested,and the test results showed that the chip functions are normal,and the highest conversion efficiency can reach 96%.In terms of transient response speed and conversion efficiency,compared with foreign chips of the same type,the results show that the performance of the chip designed in this thesis is better than the same type of foreign chips.