Waveform of a buck converter
On the wiki page you linked it shows this circuit: -



Note that they have a capacitor in parallel with a load resistor and you have something called "DE".

The capacitor is important because it smooths out the fluctuations in the output voltage and, as a first approximation, the output voltage can be assumed to be constant. As a second approximation, you will see a saw-tooth ripple superimposed on the constant voltage. A more accurate representation is a little curved at the edges.

There is nothing in your question that explains what "DE" is so hopefully you can take what I've said and interpret it in the knowledge of what "DE" is.