Stability Prediction of Integrated-Circuit Based Constant ON-Time Controlled Buck Converters

The commonly used average model is not applicable to predict the stability performances of constant
on
-time (COT) controlled buck converters. It has been reported that the issues of stability can be addressed by using the control signal to output voltage transfer function of the converters, named
G
_VC
, for such class of converters.
G
_VC
can be mathematically derived using describing-function approach to include the effects of side-band harmonics of the pulsewidth modulation (PWM) modulator. This is essential for modeling the control behavior of the COT converter class. However, the actual
G
_VC
can deviate significantly from the derived ideal model. Therefore, a measured
G
_VC
transfer function is important for real-world stability predictions.
G
_VC
measurement, however, is often not feasible because the points of measurements are physically inaccessible due to integrated control circuit implementation. In this article, a realistic
G
_VC
extraction method is proposed for each of the four commonly used COT schemes. Based on the measured
G
_VC
, a procedure to predict the stability boundary is proposed and verified.