Design Of Ripple Based DC-DC Converter With Fast Transient Response

Ripple-based constant on-time（RB-COT）buck converter has been widely studied and used because of its simple structure,fast transient response and high light-load efficiency.The traditional implementation is to use ceramic capacitor as output capacitor and inject inductor current ripple into the control loop.And the improvement of output regulation accuracy and loop stability is usually achieved by valley sample-hold circuit and low pass filter.However,the transient response will slow down owing to opposite reaction between the output voltage and the sensed inductor current ripple.In addition,the bandwidth of low-pass filter must be designed according to the lowest switching frequency,which limits the dynamic performance at higher switching frequency.This thesis focuses on the research and design of RB-COT control converter,and two Buck converter chips based on 0.18μm BCD process are proposed.The main innovations are as follows:First of all,in order to achieve both stability and fast transient response for full switching frequency range,an RB-COT Buck converter based on switching current filter is proposed.The passive RC low-pass filter in the original architecture is replaced by switching current low-pass filter,which adaptively changes the bandwidth to reach the optimal value at different switching frequencies.Taking the loop stability in the full frequency range（300k Hz-1.0MHz）as the basic design criterion,the transient response of the Buck converter is improved,especially for high switching frequency.For 1.0MHz switching frequency,the undershoot and overshoot are 35.8m V and 93.7m V in case of2A-to-4A and 4A-2A load change.The step up/down transient recovery time are 9.3μs and 10.4μs,which reduced by 67% compared with the passive RC filter.Secondly,a dual path switching current integrating RB-COT buck converter is proposed in this thesis.The deviation voltage between the reference voltage and the feedback voltage is amplified by the switching current integrator and then superimposed on the reference voltage as the control voltage of the loop comparator.The low-frequency gain of the switching current integrator is very high,so the deviation voltage can be regulated to a very small value to improve the output regulation accuracy.Furthermore,the additional fast signal path is constructed to superimpose the deviation voltage directly on the reference voltage,which enhances the ability of the control loop to perceive the change of output voltage.Simultaneously,it reduces output undershoot/overshoot voltage and transient recovery time.Simulation results show that the output voltage load regulation is as low as 0.05%.The undershoot is 10.2m V and the recovery time is 3.2μs in case of 1A-to-3A load change.The overshoot voltage is 16.9m V and the recovery time is 8.9μs in case of 3A-to-1A load change.The simulation results show that the dual path switching current integrating RB-COT Buck converter maintains the advantage of no need for compensation,and output regulation accuracy and the transient response speed have been greatly improved.