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This paper explores the subharmonic oscillation mechanism for constant on-time control, which has been widely used for power supply controllers in industry. As well as analyzing the stability criterion, this research reveals that the circuit propagation delay (Δt) can significantly increase the demand of ESR· Co value for stable operation in practical applications. A thorough study is provided based on the inductor current information and the charge variations of the output capacitor. Thus, critical condition for subharmonic oscillation is derived, and the effect of circuit propagation delay is also quantified. Based on detailed analysis, a prototype with constant on-time control and added inductor current ramp is built. Experimental results verify the theoretical analysis.