A new current mode constant on time control with ultrafast load transient response

These days inductor current ripple based `current mode constant on time (CMCOT)' control is widely used in voltage regulators (VRs) for its higher light load efficiency, higher bandwidth and simple compensation network. In today's VRs, high load current with very high slew rate is a very common requirement in microprocessor or memory applications. One issue of this ripple based CMCOT control is that, in the heavy load step up transient, inductor current increment becomes limited by on time (T_on) and minimum off time (T_{off_min}) ratio in each cycle, which can create large undershoot at the output. On the other hand, in the load step down case, if load change occurs at the beginning of the fixed Ton, a large overshoot can occur as well. For the multiphase operation case, its limited pulse overlapping capability is an issue at heavy load step up transient. States of art controllers add various nonlinear controls to the system to solve these issues. In this paper, a new charge based current mode control is proposed to improve the transient performance in CMCOT by naturally increasing or decreasing the on time in load step up or step down transient and by naturally overlapping these extended pulses in multiphase operation without adding any nonlinear control. The proposed concept is also extended to variable on-time variable off-time control to improve it transient performance. Simulation and test results of proposed control with CMCOT in standard VR platform are also presented in this paper.