The bootstrap resistor is used to (1) limit the peak current during gate driver startup and (2) control the slew rate (dv/dt) at BOOT. The peak current through the bootstrap diode, and through the BST switch if utilized, can become excessively high during the initial charging period. Furthermore, excessive slew rates at BOOT can cause a slight overshoot of the BP5H voltage during startup. To mitigate these issues a bootstrap resistor of at least 2Ω is recommended.

While the bootstrap resistor does alleviate peak current and slew rate issues, this resistor in conjunction with the bootstrap capacitor introduces a time constant τ:

where:

• R_BOOT is the value of the bootstrap resistor in ohms
• C_BOOT is the value of bootstrap capacitor in Farads
• D is the duty cycle of the switching converter

The time required to charge and refresh the charge of the bootstrap capacitor needs to be checked against the time constant. Lastly, the resistor can experience high power dissipation during the initial charging period. Select a resistor that can handle the energy during this charging period:

where:

• C_BOOT is the value of bootstrap capacitor in Farads
• V_BOOT is the final voltage of the bootstrap capacitor