While the MODE pin is pulled low via R_MODE, TPS53355 automatically reduces the switching frequency at light load conditions to maintain high efficiency. Detailed operation is described as follows. As the output current decreases from heavy load condition, the inductor current is also reduced and eventually comes to the point that its rippled valley touches zero level, which is the boundary between continuous conduction and discontinuous conduction modes. The synchronous MOSFET is turned off when this zero inductor current is detected. As the load current further decreases, the converter runs into discontinuous conduction mode (DCM). 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. The transition point to the light-load operation I_OUT(LL) (i.e., the threshold between continuous and discontinuous conduction mode) can be calculated as shown in Equation 5.

Equation 5.

where

• ƒ_SW is the PWM switching frequency

Switching frequency versus output current in the light load condition is a function of L, V_IN and V_OUT, but it decreases almost proportionally to the output current from the I_OUT(LL) given in Equation 5. For example, it is 60 kHz at I_OUT(LL)/5 if the frequency setting is 300 kHz.