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TPS65911: Resistor in series with bootstrap capacitor??

Josef Rypar47
Prodigy 240 points
Part Number: TPS65911

Hello,

I've found discrepancy in the section 7.2.2.2.5 of the SWCS049S datasheet. See screenshot:

I assume that the turn on/off times can be regulated by placing resistors in series with gates of the MOSFETs. The above statement seems to be incorrect. The bootstrap capacitor serves as a energy storage for high-side driver and shall not be compromised. Please confirm.

Regards, Josef

  • 0
    TI__Genius 10000 points
    Josef,

    I have assigned this thread to the device expert. Can you re-attach the screenshot? It doesn't look like the attachment went through.

    Thanks,
    Nastasha
  • 0 in reply to Nastasha
    TI__Mastermind 48691 points
    Hi Josef,
    You're right both about "the turn on/off times can be regulated by placing resistors in series with gates of the MOSFETs." and "The bootstrap capacitor serves as a energy storage for high-side driver and shall not be compromised."; but also, the statement "a small resistor in series with
    the bootstrap capacitor" is ok if we change to "small value (not size) resistor"; since the on/off time doesn't need slow too much, could be just slowing down a few nano seconds (so, resistor could be a few decade Ohm) can help a lot not only to decrease voltage ringing on the switch node but also reduce EMI.
    I think it's your choice if you like to place small value resistor in series with the gate of the MOSFET.
  • 0 in reply to Phil Yi
    Prodigy 240 points

    Hi Phil,

    In fact the resistor is still in the current path from bootstrap capacitor to the gate, regardless of whether it sits on the capacitor side or MOSFET side. This also means that both the resistor positions affect only high-side transistor. The turn on/off  times of the low-side MOSFET has to be adjusted by gate resistor - there is no other option. Therefore two resistors are still needed.

    Is there any benefit placing the resistor in series with bootstrap capacitor compare to the MOSFET gate terminal? Or are those two locations functionally equivalent?

    Thank you!

    Josef

  • 0 in reply to Josef Rypar47
    TI__Mastermind 48691 points
    Hi Josef,
    I agree with you; my personal practice is also to use the gate resistor for decreasing voltage ringing on the switch node and reducing EMI.
    The resistor is typically used in low-frequency systems to limit Cb charging/discharging peak current and is not necessary in systems running above 1 MHz.