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UCC27211A-Q1: How to calculate boot cap value?

PY (Pengyu) Song
Expert 5860 points
Part Number: UCC27211A-Q1

Hi team,

How to calculate the required boot cap value? In datasheet, we recommend to use cap from 0.022uF to 0.1uF, but how to calculate in a given application? 

Can boot cap > 0.1uF? Can boot cap < 0.022uF? 

"High-side bootstrap supply. The bootstrap diode is on-chip but the external bootstrap capacitor is required. Connect positive side of the bootstrap capacitor to this pin. Typical range of HB bypass capacitor is 0.022 µF to 0.1 µF. The capacitor value is dependant on the gate charge of the highside MOSFET and must also be selected based on speed and ripple criteria."

  • 0
    TI__Genius 13445 points
    Hi PengYu,

    Please refer to this document for boot cap calculation.
    www.ti.com/.../slua887.pdf

    Please let us know if you have further questions or press the green button if this addressed your question.

    Regards,

    -Mamadou
  • 0 in reply to Mamadou Diallo
    TI__Expert 5860 points
    Hi Mamadou,

    For UCC27211A-Q1, can boot cap > 0.1uF? Can boot cap < 0.022uF?
  • 0 in reply to PY (Pengyu) Song
    TI__Mastermind 23015 points
    Hello PengYu,
    I work with Mamadou also supporting the 100V half bridge drivers.
    The 0.022uF to 0.1uF capacitance range is mentioned as typical capacitance range we see in many applications.
    There are certainly cases where we have seen the boot cap needs to be higher than 0.1uF because of driving large MOSFET gate charge. It is not uncommon to see larger than the 0.1uF value. We do not recommend oversizing the boot capacitor beyond what is required if using larger boot capacitor values.
    For the less than 0.022uF. I am not sure why you would want a smaller value when you are using a 4A driver.
    Do you have a case where you need a small boot capacitor value for your application?

    Regards
    Richard Herring
  • 0 in reply to Richard Herring
    TI__Expert 5860 points
    Hi Richard,

    Forgot about the <0.022uF case, as i just want to understand if there is a hard limit.

    regarding "We do not recommend oversizing the boot capacitor beyond what is required if using larger boot capacitor values." - what if a larger boot cap might be needed and is it OK to have a cap >0.1uF? Or we will just not recommend boot cap >0.1uF for any situation?
  • 0 in reply to PY (Pengyu) Song
    TI__Mastermind 23015 points
    Hello PengYu,
    It is OK to have boot capacitors >0.1uF, and we have seen a number of designs with this case, for example 220nF to 470nF.
    I commented that if you calculate you need a 0.15uF capacitor, for example. Placing a much larger capacitor, 1uF for example, is not necessarily better. You would like the boot capacitor to charge as quickly as possible on the initial LO on pulses, and HS switching to ground.

    I hope this helps,
    Richard Herring
  • 0 in reply to Richard Herring
    TI__Mastermind 23015 points

    Hello PengYu,

    I hope the last post answered your questions.

    Regards,

    Richard Herring