Abstract: The paper presents the design of an efficient Single Inductor Three Output (SITO) BUCK converter with low cross-regulation impact, tailored for applications requiring multiple power supply voltages. The converter employs a Ripple Based Adaptive Off-Time (RBAOT) loop control scheme, leveraging the fast response characteristics of comparators to enhance system performance. Additionally, a phase-locked loop (PLL)-like circuit is introduced to generate adaptive off-times, which locks the converter's switching frequency through a negative feedback loop, thereby eliminating the frequency instability issues associated with ripple control. To further optimize load response speed and protect the power supply range of subsequent circuits, the paper introduces an undershoot threshold voltage detection technique based on the energy sequential distribution strategy. This technique enables sequential energy allocation to be adjusted, effectively reducing the output voltage undershoot. The designed SITO BUCK converter is validated using the Macronix WH 180 nm process. With a 5 V input voltage, it simultaneously provides three output voltages (1.5 V, 1.8 V and 3.3 V), with each channel capable of delivering currents ranging from 100 mA to 650 mA, and a total load current range of 300 mA to 1 A. Results of post-simulation indicate that the converter's switching frequency remains stable at 2 MHz under various load current transient conditions. The ripple coefficients of all output voltages are within 1.7%. Under the maximum load current transient of 450 mA, the measured load regulation is 0.0715 mV/mA, and the cross-regulation is 0.1728 mV/mA. When the total load current is 450 mA, the circuit achieves a peak efficiency of 84.69%.