Buck Converter with Gate Driver
The TC4427 by itself can not generate a gate drive voltage greater than its own supply voltage.  That follows from the functional block diagram on p.2 of the datasheet.

It would make the question clearer, if you add the gate driver to the schematic on the O.P. and show its supply rails.

For the time being, I'll assume that the TC4427 is powered from the same +17.4V that goes to the input of the buck.  In that case, the source of the N-channel MOSFET will not be any higher than 17.4 V - Vth.

Typically, buck converters have a bootstrap gate driver1 and an N-channel MOSFET, or a P-channel MOSFET.

1 Bootstrap gate driver (see also here) is not the only option for N-challel MOSFETS.  Other options: gate driver transformer, additional supply rail for gate driving with elevated voltage.