7.3.1 VDDQ SMPS, Light Load Condition

TPS51116 automatically reduces switching frequency at light load condition to maintain high efficiency. This reduction of frequency is achieved smoothly and without increase of V_OUTripple or load regulation. Detail 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 valley touches zero current, which is the boundary between continuous conduction and discontinuous conduction modes. The rectifying MOSFET is turned off when this zero inductor current is detected. As the load current further decreased, the converter runs in discontinuous conduction mode and it takes longer and longer to discharge the output capacitor to the level that requires next ON cycle. The ON-time is kept the same as that in the heavy load condition. In reverse, when the output current increase from light load to heavy load, switching frequency increases to the constant 400 kHz as the inductor current reaches to the continuous conduction. The transition load point to the light load operation I_OUT(LL) (i.e. the threshold between continuous and discontinuous conduction mode) can be calculated in Equation 1:

Equation 1.

where

• f is the PWM switching frequency (400 kHz)

Switching frequency versus output current in the light load condition is a function of L, f, V_IN and V_OUT, but it decreases almost proportional to the output current from the I_OUT(LL) given above. For example, it is 40 kHz at I_OUT(LL)/10 and 4 kHz at I_OUT(LL)/100.