SLUSDO2C June 2020 – February 2021 UCC21540-Q1
A VDDA capacitor, also referred to as a bootstrap capacitor in bootstrap power supply configurations, allows for gate drive current transients up to 4-A, the source peak current, and needs to maintain a stable gate drive voltage for the power transistor.
The total charge needed per switching cycle can be estimated with
Therefore, the absolute minimum CBoot requirement is:
In practice, the value of CBoot is greater than the calculated value. This allows for the capacitance shift caused by the DC bias voltage and for situations where the power stage would otherwise skip pulses due to load transients. Therefore, it is recommended to include a margin in the CBoot value and place it as close to the VDD and VSS pins as possible. A 50-V 1-µF capacitor is chosen in this example.
Care should be taken when selecting the bootstrap capacitor to ensure that the VDD to VSS voltage does not drop below the recommended minimum operating level listed in section 6.3. The value of the bootstrap capacitor should be sized such that it can supply the initial charge to switch the power device, and then continuously supply the gate driver quiescent current for the duration of the high-side on-time.
If the high-side supply voltage drops below the UVLO falling threshold, the high-side gate driver output will turn off and switch the power device off. Uncontrolled hard-switching of power devices can cause high di/dt and high dv/dt transients on the output of the driver and may result in permanent damage to the device.
To further lower the AC impedance for a wide frequency range, it is recommended to have bypass capacitor placed very close to VDDx - VSSx pins with a low ESL/ESR. In this example a 100 nF, X7R ceramic capacitor, is placed in parallel with CBoot to optimize the transient performance.
Too large CBOOT is not good. CBOOT may not be charged within the first few cycles and VBOOT could stay below UVLO. As a result, the high-side FET does not follow input signal command. Also during initial CBOOT charging cycles, the bootstrap diode has highest reverse recovery current and losses.