Analysis and design of a fully-integrated current sharing scheme for multi-phase adaptive on -time modulated switching regulators

Multi-phase, interleaved dc-dc converters employing current mode control with adaptive on-time modulation are ideally suited for low-voltage, high current processor power supply applications because of their fast load transient response and minimal external components having simple feedback loop compensation, while maintaining almost constant switching frequency. Like other multi-phase converters, to achieve optimal thermal performance and utilize all the benefits of interleaving, accurate current balancing among all the phases is necessary and paramount. This paper introduces a novel current-sharing scheme for multi-phase, current-mode, adaptive on-time modulated, switching dc-dc converters where the on-time for each phase is adjusted based current sharing error between phase currents. Careful analysis of the share-loop transfer function reveals that a low gain but high bandwidth feedback loop can be designed to achieve accurate and fast current sharing without interfering with voltage and/or current loop of the power supply. As a result, the proposed scheme provides a low cost solution because it does not require any external phase compensation element and additional pin in the controller IC. Experimental results from two-phase prototype step-down converter utilizing the proposed current-sharing scheme with input voltage range of 7-28 V and output voltage of 0.3 -1.5 V with a maximum load current of 44 A are presented in this paper. During steady-state operation, current sharing error between two phases is less than 5% for half-to-full load current. Furthermore, accurate current balancing is maintained during transient events, such as phase add or drop, and load-step insertion or release.