SLVSFG5D September 2020 – March 2022 DRV8300
PRODUCTION DATA
The bootstrap capacitor must be sized to maintain the bootstrap voltage above the undervoltage lockout for normal operation. Equation 2 calculates the maximum allowable voltage drop across the bootstrap capacitor:
=12 V – 0.85 V – 4.5 V = 6.65 V
where
In this example the allowed voltage drop across bootstrap capacitor is 6.65 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 3:
=48 nC + 220 μA/20 kHz = 50 nC + 11 nC = 59 nC
where
The minimum bootstrap capacitor an then be estimated as below assuming 1V ΔVBSTx:
= 59 nC / 1 V = 59 nF
The calculated value of minimum bootstrap capacitor is 59 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.
= 10*100 nF= 1 μF
For this example application choose 1 µF CGVDD 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.