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bq24630 going beyond 10 amps?

Ben Stelle
Intellectual 620 points
Other Parts Discussed in Thread: BQ24630, CSD18504Q5A

Hello, we have a design using the bq24630 very effectively at 10 amps, using a 4.7uH inductor and a 10mOhm current sensing resistor as the datasheet suggests.  However, our client has now requested that the design be tweaked to allow for charge currents as high as 12Amps.  We had the customer switch the current sensing resistor from 10mOhm to 9mOhm and suddenly the design seems to no longer sustain charge, but rather starts and then stops repeatedly in a cycle.

I couldn't think of a good reason that this might happen, other than maybe now the inductor is too high to develop the necessary current at this slightly higher level.  Both the ISET1 and ISET2 are at 1.98Volts.  We have 2 x 22uF of output capacitance.  Again the design works fine at 10Amps with the 10mOhm resistor, in fact when it failed with the 9mOhm resistor I had them switch back to make sure everything was still working and it still works.  Am I now needing to have them change inductor values maybe to as low as 3.3uH to allow the charge current to build, or have I stumbled blindly into some corner this part just won't work in?

Regards,

Ben

  • 0
    TI__Expert 8690 points
    Hi Ben,

    Yes you can charge higher than 10A. You need to reduce sense resistor to 8mOhm. Also, the limiting factor here is the gate drive of the FET. You want to find a FET that has lower gate charge than the one currently being used, and that has the appropriate current/voltage rating. I recommend looking at CSD18504Q5A power N-FET.

    Thanks,
    Steven
  • 0 in reply to Steven Bangdiwala
    Intellectual 620 points
    Steven, thanks for your reply. We don't quite need 12Amps, so we opted for the 9mOhm resistor. We are also using a www.aosmd.com/.../AON6246.pdf power N-FET. Is this FET good enough? If so we would like to stick with it since we have the board built to it and there is a fair amount of inventory.

    Also, do we need to change our inductor value from 4.7uH? The trend of the table in the datasheet would seem to indicate that we might.

    I am trying to understand what might be stopping the charger from working when we change only the current sense resistor in the charge path. We are not using the power management stuff and have properly bypassed all of that. Again the design as is works well to 10amps.

    Regards,

    Ben
  • 0 in reply to Ben Stelle
    TI__Expert 8690 points
    Ben,

    The only things that contribute to how much charge current you can have is the gate charge and sense resistor. Since you have a lot of FET inventory, you can try reducing the inductor value and see if that works. Also, ISET1 resistors must match the charge current you want. What charge current you are aiming for? Can I see a schematic?

    Thanks,
    Steven