A fully digital synthesizable digital controller (152, 152a) controls a switch-mode DC-DC converter (150, 230, 240, 250, 260) having switching elements (154) and an LC circuit (156, 157) for producing an output voltage (160) that is maintained at a desired level regardless of load changes that can occur on the output. The digital controller (152, 152a) comprises an input stage (164), proportional-integral-derivative (PID) compensator (170), and a digital sigma-delta modulator (172). The input stage (164) produces a difference signal between a reference voltage Vref and a feedback voltage Vfbk, and comprises (i) first and second delta-sigma-delta modulators (178, 180) and a subtractor (182), (ii) a delta-sigma-delta modulator (180) and a subtractor (182); or (iii) a comparator (218). The PID compensator (170) processes the difference signal to compensate for an undesired phase shift and to stabilize the feedback loop. The digital sigma-delta modulator (172) generates a switching element control signal for controlling at least one of the switching elements.