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Constant on-time (COT) controlled multi-phase buck converter has been widely used in high-current applications such as computing devices to achieve high entire-load-range efficiency. However, the literature lacks comprehensive analysis and design guide of the current-balance loop in COT control, resulting in possible low efficiency, per-phase current protection false-trigger, and stability issue. To solve the aforementioned issues, dc inductor current equations and small-signal models are proposed for COT control with the current-balance loop. Current-balance loop gain design guideline is then proposed to achieve accurate dc current balance and stability. Experiment and simulation results verify the analysis and the accuracy of the proposed models.