VNH5019 motor controller
I think you should do it the other way around.


First decide what maximum voltage the motor must be running on (this could be the rated voltage or lower if you want that).  
Just apply this voltage to the VNH5019 motor Controller board as shown below indicated with motor power, but make sure this applied voltage is within the limits 5.5V to 24V.  
Using PWM you can vary the voltage that the board applies to the motor, ranging from 0V to applied voltage.