7.3.6 N-Channel Buck Switch and Driver

The LM5007 integrates an N-channel buck switch and associated floating high voltage gate driver. This gate driver circuit works in conjunction with an external bootstrap capacitor and an internal high voltage diode. The bootstrap capacitor is charged by V_CC through the internal high voltage diode. A 0.01-µF ceramic capacitor connected between BST and SW is recommended.

During each cycle when the buck switch turns off, the SW voltage is approximately 0 V. When the SW voltage is low, the bootstrap capacitor is charged from V_CC through the internal bootstrap diode. The minimum off-timer, set to 300 ns, ensures that there is a minimum interval every switching cycle to recharge the bootstrap capacitor.

An external recirculating diode from the SW to RTN is necessary to carry the inductor current after the internal buck switch turns off. This external diode must be an ultra-fast switching or Schottky type to reduce turn-on losses and switch current overshoot. The reverse voltage rating of the recirculating diode must be greater than the maximum line input voltage.