An enhanced adaptive frequency locked loop for variable frequency controls

These days, variable frequency current mode controls are widely used in voltage regulators (VRs) for their higher light load efficiency, better small signal property and better transient response than fixed frequency current mode controls. One major issue for variable frequency control is its large switching frequency (fsw) variation with duty cycle. This variable fSW creates unpredictable noise in the system which is difficult to filter out, and hence not preferred by many frequency sensitive applications like mobile applications. While using variable frequency control e.g. constant on-time control (COT) in these applications, people use `Adaptive COT' control which slowly changes the Ton with duty cycle at steady state operation to keep the steady state frequency constant. V<sub>in</sub>/V<sub>ref</sub> Feed-forward method and `Phase Locked Loop (PLL)' method are very widely used to realize Adaptive COT control in the industry. Normally, V_in/V_ref feed-forward method is very simple and fast but naturally suffers inaccuracy issue for its feed-forward approach; while PLL method is very accurate but suffers from complicated additional loop adjustment. Therefore, this paper proposes a new accurate but simple frequency control loop for variable frequency controls. This paper also proposes two additional techniques for performance improvements in proposed method, namely, faster loop settling time by using feed-forward signal and better loop stability with duty cycle variation by adding adaptive loop gain control.