Breaking these down more, the gain of the ADC is really a combination of an analog front end and the ADC itself. VLSB ranges from 2.5mV to 10mV depending on the output voltage programmed into the part. As mentioned before, the gain of the buck stage is the same as analog and as such is described in the s domain for simplicity. It could, however be described in the Z domain which is the only way to describe the digital compensator.