The DCM stability issue of voltage regulators using a current-mode constant on-time controller control

In recent years, constant on-time (COT) controllers for voltage regulators have received much attention because of their characteristics of high conversion efficiency under light-load conditions. Among the varieties of COT controllers, the current-mode version in combination with a DC output voltage offset correction (OC) circuit is considered in this paper. In the past, discontinuous-conduction mode (DCM) operation has been considered trivial and there was no stability problem under such a condition. But it was observed, in practice, instability can occur under such a condition. And it is critically important to have a rigorous stability analysis because the converter is usually in DCM operation under light-load condition. Describing function approach was usually used to obtain the stability behavior of the converter in which the duty cycle modulator is ripple-based, but it is infeasible under DCM operation. In this paper, a different approach, a time-domain analysis, is used to establish the stability criterion. The criterion is verified and proven to be useful for minimizing the controller chip area of the RC filter in the OC circuit.