Design of synchronous buck converter: What's "ripple voltage"?
The voltage ripple on the capacitor is produced by the ripple current in the inductor and value of the voltage also depends on the ESR (if ESR is big then the output voltage increase). I recommend you this chapter from Power Electronics Course from Coursera. At page 10/10 you can find the variation of the output voltage waveform with respect to inductor current. 
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