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Fixed off-time control, contrasting with constant on-time control, has been widely used for power supply controllers in industry owing to its fast transient response. However the equivalent series resistance (ESR) of the output capacitor has a great effect on the control performance. In this reported work, the critical ESR of the output capacitor for the stability of the fixed off-time controlled buck converter is derived by quantitative analysis and verified by circuit simulations, which indicates that the converter is in normal operation when the ESR is larger than the critical ESR, otherwise the converter works abnormally.Introduction: Fixed off-time control is a variable-frequency ripple based control. It benefits from fast transient response and simplicity of design, and thus has been widely used for power supply controllers in industry [1]. The same as for constant on-time control, large output capacitor equivalent series resistance (ESR) is usually recommended for the stable operation of switching DC-DC converters with fixed off-time control. When the ceramic capacitor with small ESR is used, the converter is in chaotic oscillation, with large output voltage and inductor current ripples; while when the OSCON capacitor with large ESR is used, the converter operates normally, with much smaller output voltage and inductor current ripples [2,3]. Therefore, with different types of capacitors, the fixed off-time controlled buck converter has different operation states, which shows that the output capacitor ESR has an influence on the control performance. This Letter focuses on the critical ESR of the output capacitor for stability of the fixed-off-time controlled buck converter.