<p>When the equivalent series resistance (ESR) of the output capacitor in a ripple-based constant on-time (RBCOT) buck converter is smaller than the critical ESR, sub-harmonic oscillation occurs. Through time-domain analysis and stability analysis in the <i>s</i>-domain of the RBCOT buck converter, this study reveals a key principle: there is an inherent proportional relationship between the derivative of the output voltage at the beginning of the on-time and the slope of the external ramp compensation at the critical stability point. Based on this principle, a novel ramp compensation scheme that is easy to integrate into an IC is proposed. The proposed scheme does not require additional IC peripheral components, and fully adapts to various operating parameters to maintain system stability. Conventional schemes in the literature can only adapt to changes in the input voltage, output voltage, and switching period. In contrast, the scheme proposed in this paper can additionally adapt to changes in the output inductor and output capacitor, offering stronger compatibility, portability, and adaptability. Simulation and experimental results validate the correctness and feasibility of the proposed scheme.</p>