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This paper introduces design and implementation of a digital controller for an experimental low-power flyback converter in a solar-powered system with power management. Multiple operating modes are used to maintain high efficiency over wide range of input voltage and load. A current, crossover frequency and phase margin estimation technique to perform load-dependent mode switching and control system stability is proposed and tested. An adaptive tuning digital PID regulator design example is described, with emphasis on practical limitations imposed by the LM3S1138 controller and the program of arithmetic due to sampling and processing. The MIMO loop adaptively adjusts the regulator parameters to minimize the error between the desired and estimated crossover frequency and phase margin. Small-signal models are derived, and the MIMO control loop is designed to achieve stability and performance over a wide range of operating conditions. Experimental results of demonstrating system functionality are presented for a flyback converter.