The external bootstrap capacitor that is required for the driver is connected between BOOT and ASW. The bootstrap capacitor voltage serves as the input to the high-side linear regulator BP5H that provides the gate drive voltage for the high-side GaN FET. A general guideline for bootstrap capacitor selection is that its value should be at least 10× greater than the gate capacitance of the high-side GaN FET that is being driven:

where:

• C_g is the gate capacitance for the high-side GaN FET

A more detailed calculation of the minimum bootstrap capacitance needed can be found using Equation 2:

where:

• Q_g is the total gate charge for the high-side GaN FET
• I_QBG is the BOOT to AGND quiescent current
• D_MAX is the maximum duty cycle
• I_QHS is the high-side quiescent current
• f_SW is the switching frequency

and ∆V_BOOT is the maximum allowable drop on BOOT for proper operation:

where:

• VIN is the gate driver input voltage
• n is the number of external bootstrap diodes placed in series
• V_F is the forward voltage drop of the bootstrap diode
• V_{BOOT_UVLO} is the falling undervoltage lockout threshold of BOOT (6.65 V typical)

Selection of a bootstrap capacitor with low ESR and ESL is recommended. The voltage rating of the bootstrap capacitor should have sufficient margin above the maximum expected bootstrap voltage.