8.3.5.2.7 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, makes sure that a minimum time each cycle to recharge the bootstrap capacitor.

The internal precharge 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 decrease to less than the threshold for the gate drive UVLO. The precharge switch also helps prevent start-up problems which can occur if the output voltage is precharged prior to turnon. After current limit detection, the precharge switch is turned on for the entire duration of the forced off-time.