8.2.2.2 Bootstrap Diode Selection

The bootstrap diode blocks the high voltage from the gate drive circuitry when the switch node swings high, with the rated blocking voltage equal to the maximum V_ds of the GaN FET. For low or moderate frequency operation ultra-fast recovery diodes (<50 ns) are recommended. The internal low voltage switch in the LMG1210 acts to reduce the reverse recovery. For high-frequency operation a Schottky diode is recommended. To minimize switching losses and improve performance, it is important to select a diode with low capacitance.

For extreme cases, where the low-side FET on time is less than 20 ns, TI recommends using a small GaN FET as synchronous bootstrap instead of a diode. In this case, TI recommends leaving the BST pin floating or connected to V_DD, and to connect the source of the synchronous bootstrap directly to V_DD.