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This letter presents an online adaptive tuning technique for digitally controlled switched-mode power supplies (SMPS). The approach is based on continuous monitoring of the system crossover frequency and phase margin, followed by a multi-input-multi-output (MIMO) control loop that continuously and concurrently tunes the compensator parameters to meet crossover frequency and phase margin targets. Continuous stability margin monitoring is achieved by injecting a small digital square-wave signal between the digital compensator and the digital pulsewidth modulator. The MIMO loop adaptively adjusts the compensator parameters to minimize the error between the desired and measured 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. Using modest hardware resources, the proposed approach enables adaptive tuning during normal SMPS operation. Experimental results demonstrating system functionality are presented for a synchronous buck SMPS.