Buck converter not working
I think, that's not the right kind of MOSFET and driver.  In the present design, you've got an N-channel MOSFET and a driver MAX627.  You are saying that it's "high side compatible".  But there is nothing in a datasheet that indicates that that driver is for for a high-side N-channel MOSFET.

Combinations that should work:


Keep the driver that you currently have, but use a P-channel MOSFET.
Keep the N-channel MOSFET, but use some high-side gate driving technique (bootstrap gate driver, perhaps).