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[FAQ] Comparison between DCM output voltage ripple and CCM output voltage ripple in D-CAP buck converter

There are lots of questions asking why DCM output voltage ripple is higher than CCM ripple. This FAQ would give explanation based on D-CAP, DCAP2 and DCAP3 buck converters.

The prerequisite is that on-time of DCM operation and CCM operation is same if Vin and Vout are fixed. Then on-time of low-side FET is same in DCM operation and CCM operation. So time period for one pulse is same in CCM operation and DCM operation, which means Tsw= T1+T2+T3=T11+T12+T13 in following figure:

Output ripple is positive related with electric charged delta_Q, which are marked triangle in above figure. Formula for calculate delta_Q1 and delta_Q2 are shown below:

Since D-CAP converter enters DCM operation when ILoad is lower than 0.5delta_IL(pk-pk value of inductor current), so delta_IL-ILoad1>0.5delta_IL, and delta_IL-ILoad1>ILoad2.

Since ILoad1>ILoad2,  the rising time and falling time of ILoad2 is higher than that of ILoad1, leading to following comparison results:

So delta_Q1>delta_Q2, meaning the output voltage ripple in DCM operation is higher than that in CCM operation of D-CAP buck converter.

For more information about DCM output voltage ripple in D-CAP buck converter, please refer to following application note:Understanding Output Voltage Ripple in DCM Operation of D-CAP Buck Converters