High Frequency Small Signal Model for Inverse Charge Constant On-Time (IQCOT) Control

Abstract

These days, constant on-time current mode (COTCM) control schemes are very popular in the multiphase voltage regulator (VR) controllers because of its higher light load efficiency and better small signal characteristics. The issue of these ripple based COTCM control in multiphase operation is that when the inductor current ripple becomes very small at certain duty cycles due to ‘ripple cancellation effect’, control becomes very noise sensitive. Recently, a novel non-ripple based ‘Inverse Charge Constant On-Time (IQCOT)’ control has been proposed which can operate seamlessly at ripple cancellation point in multiphase operation. This new control improves the transient response in single and multiphase operations too. However, a high frequency small signal model for this new IQCOT control is required for its high bandwidth control loop design. In this paper, a high frequency accurate small signal model for IQCOT control has been derived using describing function method. Form the derived model it is found that the quality factor (Q) of one double pole set varies with duty cycle change which makes the high bandwidth design very challenging for a wide duty cycle range. To overcome this challenge, an auto-tuning method for Q-value control is also proposed in this paper. The derived high frequency model and auto-tuning method are also verified by small signal bode plot simulation and test results.