Hysteretic comparators are often used to implement sliding-mode controllers or other type of discontinuous regulators for power switching converters. Their design is usually performed analogically yielding robust and fast controllers that can even allow converter operation with high values of duty cycle. A new analogue implementation based on a low-cost microcontroller is described in this study showing the mentioned advantages plus the flexibility of a programmable digital system. The proposed comparator employs some microcontroller peripherals without consuming execution time of the control algorithm and without requiring interruption management. It provides variable hysteresis width to obtain constant switching frequency in sliding-mode strategies and can be used in any type of hard switching converter. The dynamic performance of the method is verified experimentally in a boost converter-based rectifier with power factor correction and output voltage regulation in one single stage. The boost converter operates in sliding mode with constant switching frequency and results in a total harmonic distortion of the input current <1%.