The TPS92515 contains circuitry to ensure proper operation of the internal MOSFET. Typically a capacitor tied to the switchnode (SW pin) and a diode connected to the VCC supply powers the BOOT pin. Each time the diode conducts current, a path is created from the VCC pin to charge the BOOT capacitor. The connection allows the BOOT capacitor to float with the switch-node voltage and internal FET source. Anytime the main switching diode conducts current, the switch-node falls to a diode drop below ground. This creates a path for the boot capacitor to be charged in approximately 150 ns or less. A typical BOOT capacitance of 0.1 µF can maintain the ON-state of the FET for approximately 5 ms. This timing allows conversion duty-cycles of >> 99%. Anytime the BOOT voltage reaches a level that does not allow proper FET turn-on, the high-side FET will be turned off.
Although the internal VCC regulator typically supplies power to the BOOT drive circuitry, that power can be supplied by a suitable external source. Use this configuration to save power dissipation in the device and to lower the junction temperature. Ensure the external source does not exceed 5 V and that it can supply an adequate average current equal to or greater than 3 × 10-9 × fSW.