When the SS/PFM pin is terminated with a 35.7-kΩ or lower resistance, the TPS542A50 operates in pulse-frequency modulation (PFM) for light load conditions to maintain high efficiency.

As the output current decreases from heavy-load conditions, the inductor current also decreases until the valley of the inductor current reaches zero amps, which is the boundary between continuous-conduction mode (CCM) and discontinuous-conduction mode (DCM). The synchronous MOSFET turns off when this zero inductor current is detected. As the load current decreases further, the converter runs in DCM. In DCM operation, the on-time is maintained to a level approximately the same as during CCM and the converter off-time is modulated to maintain the proper output voltage. For the application of 5-V input voltage, it is not recommend to operate in PFM due to the accuracy of the zero comparator which will be reduced because of the low input voltage.