The ripple-based constant on-time (RBCOT) buck converter with small equivalent series resistance (ESR) of the output capacitor has the potential issue of instability. The conventional fixed-ramp and adaptive-ramp ripple compensation cannot keep the system stable under different operational parameters (inductor, output capacitor, input voltage, output voltage and on time). In this paper, a fully adaptive-ramp ripple compensation technique is proposed which is based on the finding that there is an inherent proportional relationship between the slope of the critical stability compensation and the slope of the output voltage at the beginning of on-time. This technique makes the converter has robust adaptability and portability. A simple analog circuitry, which can be easily integrated into an integrated circuit, is provided to implement this technique. Experimental results are provided to verify the proposed concept.