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By combining the concepts of multiple-sliding-surface and integral-variable-structure controls, one can develop a robust controller for a multiphase 9 V VRM, which comprises four parallel DC-DC synchronous buck convertors operating at 300 kHz. The advantages of the control scheme are its simplicity in design, good dynamic response, robustness, ability to ify the bus-voltage error and the error between the load currents of the convertor modules, and ability to reduce the impact of high-frequency dynamics due to parasitics on the closed-loop control system. Unlike a conventional variable-structure controller (VSC), which achieves superior transient performance by optimising (and hence, by varying) the switching frequency, the novel controller is able to retain the excellent dynamic performance of a conventional VSC and yet maintain a constant-frequency operation of a PWM controller under 'steady-state' condition. The latter is achieved by obtaining a duty-ratio signal; however, unlike a PWM controller, the new controller calculates the duty ratio based on Lyapunov's stability creitrion. Thus, the new controller also permits interleaved operation of the VRM modules.