Okay, so you're doing 80V in, up to 80V out, and 5A... is that maximum continuous current or peak current? Try to find the other current.
A brushless DC motor requires a much more complicated drive scheme - multiple sets of fets to switch the multiple windings on and off at the appropriate moments while monitoring the speed and position of the shaft in it's rotation. You vary the speed by changing the timing and duty cycle of these fets - not by adjusting the supply voltage. If you're working with a brushless DC motor, you need to do some more basic research to understand what one is, and the big picture about how to drive them.
An adjustable DC voltage is suitable for controlling the speed of a normal DC motor, but not a brushless one... (also - often you can adjust speed of a normal DC motor by just PWMing the power to it, without bothering with a buck converter...)
Also - you misread my question - I asked what you were using to control the mosfet (ie, the thing that is driving the gate of the MOSFET and setting the duty cycle and all that), not what the buck converter would be powering ((though, per above, what you said was indeed very important since it calls your whole project into question!)
My general impression is that you have no freaking clue what you're doing - so some more general information would be a good idea so that we can give you some more general advice. Maybe a link to the motor you're using? Description of what you're doing?