A digital pulse-frequency-modulated controller is designed by discretizing an analog integral controller for an LLC resonant DC-DC converter using the so called IA-PIM digital redesign method. It regulates the output voltage by adjusting the frequency of the input switching, which in turn changes the gain of the LLC converter. Its performances to sudden changes in the load are evaluated though extensive simulations. These results show that the digital controller can realize the performance of an analog controller at a sampling frequency that can be much lower than those currently prevail, which may be advantageous in terms of reduced computational requirements and power consumption.