The TPS54538 is designed to operate in pulse frequency modulation (PFM) mode at light load currents to boost light load efficiency when MODE pin is configured to PFM mode.

When the load current is lower than half of the peak-to-peak inductor current in CCM, the devices operate in discontinuous conduction mode (DCM). In DCM operation, the low-side switch is turned off when the inductor current drops to I_{LS_ZC} to improve efficiency. Both switching losses and conduction losses are reduced in DCM, compared to forced CCM operation at light load.

At even lighter current load, pulse frequency modulation (PFM) mode is activated to maintain high-efficiency operation. When either the minimum high-side switch on time, t_{ON_MIN}, or the minimum peak inductor current I_{PEAK_MIN} is reached, the switching frequency decreases to maintain regulation. In PFM mode, the switching frequency is decreased by the control loop to maintain output voltage regulation when load current reduces. Switching loss is further reduced in PFM operation due to less frequent switching actions. Since the integrated current comparator catches the peak inductor current only, the average load current entering PFM mode varies with the applications and external output LC filters.

In PFM mode, the high-side MOSFET is turned on in a burst of one or more pulses to provide energy to the load. The duration of the burst depends on how long the feedback voltage takes to catch V_REF. The periodicity of these bursts is adjusted to regulate the output, while zero current crossing detection turns off the low-side MOSFET to maximize efficiency. This mode provides high light-load efficiency by reducing the amount of input supply current required to regulate the output voltage at small loads. This trades off very good light-load efficiency for larger output voltage ripple and variable switching frequency.