The IR2110 has two problems with high side only operation. 1) The Low side needs to have VCC (low side output supply) hooked up, otherwise it inhibits both the high and low outputs. This is a very minor problem - just hook it up but don't use it. 2) The major problem: if you use a bootstrap supply on the high side (C between Vs and Vb, diode from Vcc to Vb to charge up C when buck regulator diode is conducting), how do you start the darn thing? You need to have current flowing in the buck inductor to make it drive the diode into conduction. But without a high side supply, there's no way to guarantee a start up.
(Tangent here: if you are *lucky* then, when you first turn on the power supplies, the Vb supply current flowing from Vcc through the diode and inductor to the load will be enough to get you started, if you hold off the input pwm signals to the IR2110 just the right amount of time. If you wait too long, then the capacitor voltage decays too much from the current drawn by the high side circuitry. If you turn it on too quickly, the IR2110 will ignore you. Also the Vcc supply plus diode drop plus resistor divider effect of Rload and high side static current has to produce enough Vbs voltage to overcome the undervoltage lockout of the highside supply.)
So, if you want to make the IR2110 work reliably, you need a second mosfet that you put across the inductor diode. At start-up, you *make sure* that the high side pwm signal is off , then turn on the second mosfet to pull Vs to ground and charge up the high side power supply (for 10-50us), then turn off the second mosfet, wait 50-500 us and turn on the high side pwm. Any time you shut down the pwm and restart it, you need to again "flash" the second mosfet for 10-50 us. The reason for the short time is that the second mosfet will be discharging the output capacitor though the inductor - no current limit - ouch!
OK, so are you sure you want to use the IR2110? P-channel MOSFETs are *much* easier to use, even with the voltage translation issue.