Driving a high-side N-channel FET
That chip uses bootstrapping and, bootstrapping requires that the load voltage goes high and low quite frequently. If this does not happen then the bootstrap capacitor loses its charge and you will be unable to drive the HO pin above the level of voltage on the source of the upper MOSFET. And, that means you can no longer activate the high-side MOSFET.
If you want a solution you should use an isolating DC-DC converter that can apply a voltage higher than Vin to the upper driver circuit.