The bootstrap capacitor must be sized to maintain the bootstrap voltage above the undervoltage lockout for normal operation. Equation 3 calculates the maximum allowable voltage drop across the bootstrap capacitor:

Equation 3.

=12 V – 0.85 V – 4.45 V = 6.7 V

where

• V_GVDD is the supply voltage of the gate drive
• V_BOOTD is the forward voltage drop of the bootstrap diode
• V_BSTUV is the threshold of the bootstrap undervoltage lockout

In this example the allowed voltage drop across bootstrap capacitor is 6.7 V. It is generally recommended that ripple voltage on both the bootstrap capacitor and GVDD capacitor should be minimized as much as possible. Many of commercial, industrial, and automotive applications use ripple value between 0.5 V to 1 V.

The total charge needed per switching cycle can be estimated with Equation 4:

=54 nC + 115 μA/20 kHz = 54 nC + 5.8 nC = 59.8nC

where

• Q_G is the total MOSFET gate charge
• I_{LBS_TRAN} is the bootstrap pin leakage current
• f_SW is the is the PWM frequency

The minimum bootstrap capacitor can then be estimated as below assuming 1V of ΔV_BSTx:

Equation 5.

= 59.8 nC / 1 V = 59.8 nF

The calculated value of minimum bootstrap capacitor is 59.8 nF. It should be noted that, this value of capacitance is needed at full bias voltage. In practice, the value of the bootstrap capacitor must be greater than calculated value to allow for situations where the power stage may skip pulse due to various transient conditions. It is recommended to use a 100 nF bootstrap capacitor in this example. It is also recommenced to include enough margin and place the bootstrap capacitor as close to the BSTx and SHx pins as possible.

Equation 6.

= 10*100 nF= 1 μF

For this example application, choose a 1-µF C_GVDD capacitor. Choose a capacitor with a voltage rating at least twice the maximum voltage that it will be exposed to because most ceramic capacitors lose significant capacitance when biased. This value also improves the long-term reliability of the system.