The BST pin is the input supply for the high-side internal circuits. The BST pin and corresponding high-side circuits are referenced to the SW pin. The BST pin is powered by the low-side AUX Supply pin through the internal bootstrap diode function. The bootstrap function is inactive when the low-side GaN FET is off and the BST pin must rely on an external BST-to-SW capacitor for the BST power source.

Designing the BST-to-SW capacitance is a trade-off between high-side charge-up time and hold-up time. The BST-to-SW external capacitance is recommended to be a ceramic capacitor that is at least 10 nF over operating conditions.