An N-channel high-side MOSFET needs its gate several volts above its source. When the source rises toward the DC bus, a ground-referenced logic output can no longer maintain that positive VGS. A floating gate-driver supply solves this level-shifting problem.

In a bootstrap circuit, a diode charges the bootstrap capacitor from the driver supply when the switch node is low. When the high-side device is commanded on, the charged capacitor and high-side driver ride upward with the source, supplying gate charge while maintaining the required gate-to-source voltage.

A first capacitor estimate is Cboot ≥ Qtotal/ΔV, where Qtotal includes MOSFET gate charge plus driver and leakage demands over the high-side interval. Add design margin, account for capacitor bias derating and diode drop, then verify the driver's undervoltage threshold over worst-case temperature and timing.

A basic bootstrap cannot hold the high side on forever because the capacitor must periodically recharge and leakage slowly discharges it. Very high duty cycle, long on-time or startup with the switch node already high may require a charge pump, isolated supply or another high-side bias method.