TPSM86638 operates in forced continuous conduction mode (FCCM) in light load conditions and allows the inductor current to become negative. In FCCM, the switching frequency is maintained at a quasi-fixed level over the entire load range, which is designed for applications requiring tight control of the switching frequency and output voltage ripple at the cost of lower efficiency under light load compared with that under Eco-mode. This mode also can help to avoid switching frequency dropping into audible range that can introduce some audible "noise".

TPM86637 is set to 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 a point that the rippled valley touches zero level, which is the boundary between continuous conduction and discontinuous conduction modes. The rectifying 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 in the continuous conduction mode so that longer time is needed to discharge the output capacitor with smaller load current to the level of the reference voltage. This process 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 IOUT(LL) current can be calculated by Equation 1.