An external bootstrap diode between GVDD pin and BST pin is required to allow the bootstrap capacitor to be charged from GVDD pin every cycle when the low-side MOSFET turns on. The charging of the capacitor involves high-peak currents, and therefore, transient power dissipation in the bootstrap diode may be significant. The reverse recovery time of the bootstrap diode must be very small to achieve a reduction in reverse recovery losses. Both the diode conduction losses and reverse recovery losses contribute to the total losses in the gate driver and must be considered in calculation the gate driver's power dissipation.

In order to minimize losses associated with the reverse recovery properties of the diode and ground noise bouncing, a fast recovery diode or Schottky diode with low forward voltage drop and low junction capacitance is recommended. Using Schottky diodes reduce the risk associated with charge supplied back to the gate driver supply from the bootstrap capacitor and minimize leakage current. When the SH pin (switch node) is pulled to a higher voltage, the diode must be able reverse bias fast enough to block any charges from the bootstrap capacitor to the GVDD supply. This bootstrap diode should be carefully chosen such that it is capable of handling the peak transient currents during start-up, and such that its voltage rating is higher than the system DC-link voltage with enough margins.

A bootstrap resistor R_BOOT in series with the bootstrap diode is recommended to reduce the inrush current in D_BOOT and limit the ramp up slew rate of voltage of V_BST-SH during each switching cycle, especially when the SH pin has excessive negative transient voltage. R_BOOT recommended value is between 2 Ω and 10 Ω depending on diode selection. A current limiting resistor of 2.2 Ω is selected to limit inrush current of bootstrap diode, and the estimated peak current on the D_Boot is shown in Equation 1.

where

• V_DH is the bootstrap diode forward voltage drop