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BQ24780S: Capacitors on EVM

Diego Lewis
Intellectual 1540 points
Part Number: BQ24780S

Hi Team,

What is the purpose of C13, C23, C25,and C27 on the BQ24780s-EVM?  Only C13 is shown in the datasheet, and C23 is nostuff’d on the eval card, and my customer is trying to understand which are needed and why.

Is there a reason that the RC constant on the ACFETs and BATTFET are so large?

Regards, Diego

  • 0
    TI__Expert 3320 points

    Diego,

       Thanks for asking.

       C13 is large because we want to slow down the turn on transition of ACFET to reduce inrush current between adaptor and VBUS capacitor.

       C23 is not populated because HS buck FET contributes a lot of hard switching loss, therefore we don't want to slow down switching by populating this cap. We have the place holder here is because in case for some bad layout or unexpected gate oscillation happens we may need this cap just in case and suffering efficiency.

       C25 for Buck LS FET is working under synchronize rectifier, therefore switching speed is not critical for switching loss, therefore it is better to have a 150pF cap to help reduce gate voltage oscillation.

       C27 (BATFET) will not switch in high frequency therefore it is better to have a 10nF cap to help reduce slow  down BATFET transition process to prevent inrush current between system and Battery.

       Hoping my answers help your understanding. 

    Regards

    Shishuo