An apparatus for controlling a power converter operable to receive a cyclically varying input signal includes a discrete-time, on-time generator coupled to the power converter and configured to regulate an output voltage of the power converter and a controller configured to compare the output voltage of the power converter against a first predetermined range to: obtain a comparison result at a zero-crossing of the cyclically varying input signal and select one of a plurality of operation levels of the discrete-time, on-time generator in response to the comparison result. The plurality of operation levels includes a linear, discrete-time operation level and a nonlinear, discrete-time operation level.