A New Fast Adaptive On-Time Control for Transient Response Improvement in Constant On-Time Control

These days, constant on-time current mode (COTCM) control scheme is widely used in the voltage regulator (VR) controllers because it has a higher light-load efficiency and a higher BW design capability while maintaining a simpler compensation requirement. One issue plaguing the COTCM control is its slow transient response which is caused by its fixed T_ON operation. During the heavy-load step-up transient, the duty cycle becomes saturated and the inductor current increment becomes limited by T_ON and the minimum off time (T_{OFF_MIN}) ratio, which can create a large undershoot at load step up. On the other hand, in the load step-down case, if the load step down occurs at the beginning of T_ON, a large overshoot can be created at the output. To solve this issue, this paper presents a method designed to increase the T_ON at load step up and then very quickly, decrease at load step down in order to reduce the undershoot and overshoot at output or otherwise save the output capacitor. In this proposed method, the increase or decrease of T_ON is proportional to the output change which eliminates the chance of any overcorrection or ring-back problem unlike the methods presented in prior forums. This feature enables the control to work seamlessly in a high-frequency load repetitive case in VR applications. Since this T_ON change occurs only in the transient period when duty cycle is saturated, it does not affect the small-signal property of the COT control. Moreover, the proposed methods are very much compatible with the state-of-the-art single and multiphase COT control structures. Simulation and test results in both single and multiphase operations are also presented in the paper to verify the proposed concept.