This chip is a bootstrap half bridge driver. From the datasheet:
The low side driver for the bottom MOSFET is powered directly from the chip's power supply.
To provide enough Vgs to turn on the top MOSFET, the high side driver needs a power supply at a higher voltage than the top MOSFET's source. In your case this is done with a bootstrap capacitor (C3 in your schematic).
When the bottom MOSFET turns on (or when the load pulls the output to 0V) the bootstrap cap will charge through the internal diode. Then, to turn on the top MOSFET, first the low side MOSFET is turned off, then the high side driver uses the bootstrap cap as power supply to turn on the top MOSFET. The top MOSFET source then rises to the main supply voltage. Since this cap is bootstrapped to the top MOSFET source, which is its local "ground" for the high side driver, the positive side of this bootstrap supply follows the source and the capacitor provides a supply above VCC to keep the MOSFET on.
However, it's a capacitor, and the chip will draw some current from it, so it will eventually discharge.
When the bootstrap cap is discharged, the top driver simply loses its power supply and stops operating.
Thus, these bootstrap high and low side drivers are not capable of keeping the top MOSFET on continuously. They need to periodically recharge the cap by turning off the top MOSFET, and either turning on the bottom MOSFET or allowing the load to pull SW node to 0V.
Since you don't have a bottom MOSFET, the circuit won't work without a load to pull SW to 0V. With a load present, you can't use 100% duty cycle either, it has to turn off periodically to recharge the cap.
For a high side switch that can remain on continuously, some common choices are :
P channel MOSFET with adequate driver: does not require an extra supply above the main supply voltage.
N channel MOSFET: in this case the extra supply can be generated with a charge pump or an isolated DC DC converter.
