Analysis of the ramp compensation approaches to improve stability for buck converters with constant on-time control

This study presents new sub-harmonic analysis for constant on-time control with ramp compensation. Ramp compensation of constant on-time control can be fulfilled by either directly feeding back a sensed inductor current signal as an internal ramp or using an external compensation ramp with a fixed slope during the off time and a fixed level during the on time. Based on the inductor current information and the charge variations of the output capacitor, the sub-harmonic mechanism is analysed in detail for constant on-time control with ramp compensation. Also, as well as explaining the benefits of using sensed inductor current signal, the effects of the sensed inductor current signal and the external compensation ramp are quantitatively compared. It is revealed that the external compensation ramp requires a lower ramp to avoid sub-harmonic oscillation. Based on the detailed theoretical analysis, experimental results demonstrate the stability criterion for constant on-time control with ramp compensation.