7.3.7 N-Channel Buck Switch and Driver

The LM5008A 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 0.01-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 0 V and the bootstrap capacitor charges from V_CC through the internal diode. The minimum off-timer, set to 300 ns, ensures a minimum time each cycle to recharge the bootstrap capacitor.

The internal pre-charge switch at the SW pin is turned on for ≊ 150 ns during the minimum off-time period, ensuring sufficient voltage exists across the bootstrap capacitor for the on-time. This feature helps prevent operating problems which can occur during very light-load conditions, involving a long off-time, during which the voltage across the bootstrap capacitor could otherwise reduce below the Gate Drive UVLO threshold. The pre-charge switch also helps prevent start-up problems which can occur if the output voltage is pre-charged prior to turnon. After current limit detection, the pre-charge switch is turned on for the entire duration of the forced off-time.