Critical ESR of output capacitor for stability of fixed off-time controlled buck converter

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.