Adaptive ripple-based constant on-time control with internal ramp compensations for buck converters

This paper presents adaptive ripple-based constant on-time control architecture for buck converter ICs with wide duty ratio applications. An internal ramp compensation with emulating inductor current ripple is implemented to solve the ripple oscillation instability issue when using low-ESR output capacitors in buck converters. Therefore, no external compensation is required to simplify the system applications and to achieve universal applications. However, in order to guarantee the stability margin and the jitter performance for wide duty-ratio applications with different switching frequencies, a large internal ramp is normally applied, but it may not be optimal for different applications. Besides, in order to avoid the voltage droop caused by the internal ramp compensation, a high-pass filter is usually utilized to cancel the DC offset. However, the DC cancellation network may cause phase margin issues when applying too large internal ramp. Hence, the internal ramp must be carefully designed to guarantee the stability margin while maintaining fast load transients and adequate phase margin for wide duty-ratio applications. In this paper, an adaptive ripple tuning algorithm is proposed to fine tune the internal ramp compensation automatically based on the duty ratios, output voltage, and the switching frequency. The small-signal model is derived to provide the design guide, and the simulation and experimental results are given to verify the effectiveness of the proposed adaptive ripple-based control.