7.4.3 Pulse Skip Mode (PSM, TPS562231)

The TPS562231 is designed with Advanced Eco-mode™ to maintain high light load efficiency. As the output current decreases from heavy load condition, the inductor current is also reduced and eventually comes to point that its rippled valley touches zero level, which is the boundary between continuous conduction mode (CCM) and discontinuous conduction mode (DCM). The low-side MOSFET is turned off when the zero inductor current is detected. As the load current further decreases the converter runs into discontinuous conduction mode. The on-time is kept almost the same as it was in the continuous conduction mode so that it takes longer time to discharge the output capacitor with smaller load current to the level of the reference voltage. This makes the switching frequency lower, proportional to the load current, and keeps the light load efficiency high. The transition point to the light load operation current I_{OUT_LL} can be calculated in Equation 1.

Equation 1.

As the load current continues to decrease, the switching frequency also decreases. The on-time starts to decrease once the switching frequency is lower than 250 kHz. The on-time can be about 22% reduced at most for extremely light load condition. This function is employed to achieve smaller ripple at extremely light load condition.