Power over ethernet design
The simplified schematics show only where the power is coming from and going to (so you get an idea of the basic operation), but omit a few parts that are required for correct operation (e.g. capacitors that reduce voltage swing in certain corner cases).

You will need to implement the reference design unless you know what you can omit or alter in your application. Take a lot of care keeping loops small in the switching converter, as the reference design says.