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BQ25750: Unable to start charging, VBAT_OV_STAT flag

Albert Tournilhac
Prodigy 10 points
Part Number: BQ25750
Other Parts Discussed in Thread: BQSTUDIO

Tool/software:

Hello,

I'm using the BQ25750 for a new design which have an integrated 50V 15A battery charger, but I have some difficulties to make it work correctly.

Do debug it, I made a board only with the BQ25750 part and I have the same issue : 

  • when the input voltage is in the correct windows, I see a voltage at the output but not what I set (30V instead of 50V)
  • if I try to draw some current with an electronic load, the output voltage collapse
  • sometimes, I have the VBAT_OV_STAT flag trigged

EVAL_BQ25750.pdf

Here my schematic, could you review it ? All mosfets are BSC057N08NS3G.

 

I have the same results when I mount only one of the mosfets pairs. I also tried with lower outuput voltage regulation settings or other fsw but same thing.

I'm using bqstudio for debbuging.

I don't need the batfet, maybe there is something here.

Thank you,

  • 0
    TI__Mastermind 19520 points

    Hello Albert,

    Thanks for working with this. I'm looking over your schematic and I have a few suggestions.

    • Make sure to spread the capacitance across the current sense resistance. You can check this FAQ for more info.
    • What are the components marked FBx? I'm not familiar with this schematic symbol.
    • The minimum DCR for the inductor is 1.75mΩ. Can you change to an inductor that has a higher DCR?
    • Since you are charging at 15A, I highly recommend using an external gate drive supply. An external gate drive of 10V will help with the thermals and efficiency in your application

    Also, if you test the charger charging at 4A, do you see anything different.

    Let me know if those changes fix the issues you are seeing

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 10 points

    Hello Ethan,

    Thanks for your support !

    • I'm going to try to add these capacitances.
    • FBx are ferrite beads, BBPY00160808121Y00 at the mosfets gates and 74279202 for FB1
    • I tried with the same inductor of the evaluation board (IHLP6767GZER100M01), I have the same result
    • Ok for the external gate drive supply, for the moment I just tried charging at 2-3A

    I'll come back to you after some tests.

    Is it wrong to drive parallel mosfets with this component ?

    Do I let BATSRC and BATDRV floating?

    And last thing, I just want to limit output current, can I remove all the RAC_SNS function ?

    Best regards,

  • 0 in reply to Albert Tournilhac
    TI__Mastermind 19520 points

    Hello Albert,

    Can you try removing and shorting across the ferrite beads on the MOSFET gates? I wouldn't recommend using ferrite beads on the FET gates.

    Albert Tournilhac said:
    Is it wrong to drive parallel mosfets with this component ?

    This IC can drive FETs in parallel. We do recommend to limit the total switch node capacitance CSW (nF) < 160/VIN. I'd also recommend increasing the gate drive voltage to increase the gate drive current.

    Albert Tournilhac said:
    And last thing, I just want to limit output current, can I remove all the RAC_SNS function ?

    Yes, you can do that. You can short ACN to ACP and tie the pins to VAC to remove this function.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 10 points

    Hello Ethan,

    I added capacitances as explain in your topic but it don't seem to help. Removing the ferrite beads have the same effect.

    Althrought, driving the FETs with a 10V supply can allow me to draw some current ! But it is quite unstable, I need to increase relly slowly the current or I have the VBAT_OV_STAT flag trigged.

    I will remake the design to be as close as possible of the evaluation board and increase the FETs drive tracks width. Maybe there is something here.

    Ethan Galloway said:
    Yes, you can do that. You can short ACN to ACP and tie the pins to VAC to remove this function.

    Ok, did you mean VSYS or VAC is correct ? And do I let BATSRC and BATDRV floating if I don't need it ?

    Thanks for your support.

  • 0 in reply to Albert Tournilhac
    TI__Mastermind 19520 points

    Hello Albert,

    Albert Tournilhac said:
    Ok, did you mean VSYS or VAC is correct ? And do I let BATSRC and BATDRV floating if I don't need it ?

    Thanks for being patient with this. You'll need to short VAC (pin 33), ACN, and ACP together.

    It's fine to leave BATSRC and BATDRV floating.

    Also, there's another thing I found in your schematic, you'll need to increase the BTST capacitors to 100nF instead of 47nF.

    Best Regards,
    Ethan Galloway