Output voltage ripple is a key parameter for buck converter, which can make great impact on system application and should be paid more attention on. The principle for generation of output voltage ripple for buck converter in CCM operation is well illustrated in many text books and application notes. However, when focusing on the output voltage ripple in DCM operation, not many detailed description on it. This application note can introduce a method for estimating output voltage ripple in DCM operation for buck converter with D-CAP, D-CAP2 and D-CAP3 mode. Results of bench tests can be given based on TPS56837. Error analysis is also included to illustrate what kind of factors can affect test results. In the conclusion of this paper, a comparison between output voltage ripple in DCM operation and CCM operation in D-CAP buck converters can be made.