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This paper presents a practical injection-based method for continuous monitoring of the crossover frequency and phase margin in digitally controlled switched-mode power supplies (SMPS). The proposed approach is derived from Middlebrook's loop-gain measurement technique, adapted to a digital controller implementation. A digital square-wave signal is injected into the feedback loop and the injection signal frequency is adjusted while monitoring loop signals to obtain the system crossover frequency and phase margin online, i.e., during normal closed loop SMPS operation. The approach does not require open loop or steady-state SMPS operation and is capable of convergence in the presence of load transients or other disturbances. A method for designing the stability margin monitor, based on small-signal models derived using an envelope modeling approach, is also presented. Experimental results are given for multiple power stage configurations demonstrating close matches between monitored and expected crossover frequencies and phase margins.