Research And Design Of Fast-transient-response BUCK Converter With Ripple-based Control

The trend of miniaturization and portability for the electronic equipment demands higher integration and efficiency of power management IC.With the advantages of high efficiency and wide range of voltage regulation,the BUCK converter is gradually replacing the linear power supply and has become the mainstream product in many applications.Fast transient response is one of the key requirements of a BUCK converter.For a buck converter,the conventional control schemes include voltage-mode control and current-mode control.In voltage or current control mode,the speed of transient response is limited by the bandwidth and the compensation loop.Comparatively speaking,the ripple-based constant on-time?RBCOT?controlled BUCK converter is simple in system architecture.Because the ripple on the equivalent series resistor(R_ESR)of output filter capacitor can reflect the change of the load current,the fast load transient response can be realized by directly feeding the ripple to the control system.In addition,the switching frequency of the RBCOT BUCK converter will be reduced with the decrease of output current when the converter operates in discontinuous current mode?DCM?.The advantage of high light-load efficiency can effectively extend the battery life of portable electronic equipment.Therefore,RBCOT BUCK converter has received wide attentions for its fast transient response and high light load efficiency.However,there are still some problems in RBCOT control,such as low accuracy of the output voltage,unfixed working frequency and sub-harmonic oscillation with small R_ESR of output capacitor.This paper mainly studies the method to improve the stability and output accuracy of RBCOT BUCK converter without sacrificing its transient response performance.The research content will include the following parts:1.Introduce the conventional control method of ripple-based control BUCK converters,and analyzes its existing problems:stability issue when the output capacitance is small,output voltage deviation?offset?,and the influence of ripple compensation and output voltage error correction circuit on the load transient response of load.2.In order to improve the output accuracy of RBCOT BUCK converter and maintain its fast load transient response,an output error correction circuit based on error feedback and sample-hold technology is proposed.3.Using the 0.18?m BCD process,the fast-transient-response RBCOT BUCK converter is designed.The circuit simulation of the system was carried out by Cadence Spectre.All performance index met the expected requirement,and the performance of the traditional error correction circuit was compared with the proposed error correction circuit under the same working conditions.The simulation results show that the RBCOT BUCK converter designed in this paper has achieved the stability with small output capacitor R_ESR.The error correction function can be realized in the continuous current mode?CCM?and DCM operating mode of the BUCK converter.When the load current change from 4A to 1A within 5ns,the overshoot is less than100m V and the adjustment time is less than 20?s.