As described in the datasheet,
“The internal high voltage regulator self-supplies the IC from the Drain pin. The IC starts switching and the internal high voltage regulator turns off as soon as the voltage on pin VCC reaches VCCOFF (4.6V, typical). The internal high voltage regulator turns on to charge the external VCC capacitor when the VCC voltage decreases below VCCON (4.45V, typical). So a small capacitor such as several uF capacitor is enough to hold on the voltage of VCC and this can lower the cost by decreasing the value of the capacitor. When the voltage on pin VCC drops below VCCstop (3.3V, typical), the IC stops working, and then the internal high voltage regulator charges the VCC capacitor again.”
Hence, in normal operation the VCC pin can go as low as 3.3 V, below which the IC would turn off. Similarly, the maximum voltage can be as high as 4.9 V before the part turns off the internal regulator. If the output voltage is greater than 5V then adding a diode/resistor from D2 cathode to VCC can increase efficiency by overriding the internal VCC regulator (pg. 16 data sheet).