SLUSCE9B June 2017 – March 2020 UCC27712
The bootstrap capacitor should be sized to have more than enough energy to drive the gate of FET Q1 high, and maintain a stable gate drive voltage for the power transistor.
The total charge needed per switching cycle can be estimated with:
This design example targets a boot capacitor ripple voltage of 0.5 V. Therefore, the absolute minimum CBOOT requirement is:
In practice, the value of CBOOT needs to be greater than the calculated value. This allows for capacitance shift from DC bias and temperature, and also skipped cycles that occur during load transients. For this design example 2x 220-nF capacitors were chosen for the bootstrap capacitor.