UNLESS OTHERWISE NOTED, this document contains PRODUCTION DATA.
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.
To further lower the AC impedance for a wide frequency range, it is recommended to have bypass capacitor with a low capacitance value, in this example a 100 nF, 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.