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About TPS92640 Cboot capacitance

Ozawa
Intellectual 850 points
Other Parts Discussed in Thread: TPS92640

I have two questions about TPS92640.

◆1. For the capacity of the bootstrap capacitor
 

My Customer using 0.47μF described in the application notes of 20A output.

And,In the data sheet it is described as recommended 100nF.


But customers are thinking to change the circuit constants according to their specifications.

So, I want you to tell me the caluculation of Cboo capacitance.

Customer thinking that following the information necessary for the calculation.

・gate capacitance of MOSFET
・gate-source forward leakage current　of MOSFET
・High-side driver circuit Iq
・Level shifter circuit Iq
・Reverse current of the bootstrap diode
・leakage current of bootstrap

◆2.  bootstrap diode
When the reverse current from the diode is large, I think it has to be taken increase the capacity of the capacitor.

Even as a forward voltage of 1V in FRD has occurred, I believe that can ensure the voltage to drive the gate of a fully MOSFET.
Thus, although Customer believe that it should use the FRD than SBD, how might it be?

  • 0
    TI__Mastermind 25500 points
    I am honestly not sure how you would go about that calculation. I could try to find out if really needed. But what I can tell you is that a 100nF should be fine for any application. The limiting factor is the VCC current limit and 100nF is enough to run any FETs up to the VCC current limit where the device cannot drive any more (due to gate charge and switching frequency). The diode leakage should not be significant nor should the other Iqs. The forward voltage isn't all that important either since it is a fast peak charge and the diode should not be conducting at the next cycle. Using a 0.47uF will reduce the high side gate voltage ripple just a little which may or may not help with the Rds(on) slightly, but the difference is likely minimal. Use 0.47uF if you want to feel safe, but 0.1uF should be fine either way.
  • 0 in reply to Clinton Jensen
    Intellectual 850 points

    Clinton Jensen-san

    Thank you for your reply comment.

    We will answer to customer.

    In addition, we received a new question.

    We will be asked about the resistance RF which are connected in series to the CS terminal of the TPS92640.

    Please tell us about the method of calculating the RF resistance value.

    ◆Customer Speck

    Input Voltage：12V

    Output Voltage：2.5-8.0V

    Output Current：16A fix

  • 0 in reply to Ozawa
    TI__Mastermind 25500 points

    RF is there just to provide a little bit of filtering and transient protection. A 475ohm to 1kohm resistor should be fine for any application.