The LM5009 integrates an N-channel buck switch and associated floating high-voltage gate driver. The gate driver circuit works in conjunction with an external bootstrap capacitor and an internal high-voltage diode. A 0.022‑µF ceramic capacitor (C4) connected between the BST pin and SW pin provides the voltage to the driver during the on-time.

During each off-time, the SW pin is at approximately –1 V, and the bootstrap capacitor charges from V_CC through the internal diode. The minimum off timer ensures a minimum time for each cycle to recharge the bootstrap capacitor.

An external re-circulating diode (D1) carries the inductor current after the internal buck switch turns off. This diode must be of the ultra-fast or Schottky type to minimize turn-on losses and current overshoot.