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BQ24616 / Drive capability of BATDRV driver

S.Satoshi
Guru 21685 points
Other Parts Discussed in Thread: BQ24616

Hi,

My customer plan to use two BATFET in parallel to reduce IR loss in BATFET. Then we need to know the drive capability of BATDRV driver to judge how many FETs can be driven. Can you provide any information about BATDRV driver?

Best Regards,

Sonoki / Japan Disty

  • 0
    TI__Genius 11480 points
    Satoshi,

    Unfortunately, I am not able to speak to the exact details of the BATDRV driver circuit. I would say that some experimentation should be able to find the maximum number of FET's in parallel, though.

    In general, I would guess that it would be able to drive two, given that the Qg was low enough.

    What is the Rds(on) of the FET they have? The BATFET only switches occasionally, so Qg is not as important. The customer could sacrifice Qg to use a single FET with a lower Rds(on). I have seen FET's with Rds(on) as low as 2mOhms.
  • 0 in reply to David Wiest Jr
    Guru 21685 points

    Hi David,

    Thank you for your comment. Regarding FET, let me discuss with SI7617DN that is reference in datasheet.

    I asked about parallel use in previous post, but my customer is asking to use in series, not parallel. They plan to use two FETs in series for fail safe if BATFET will be short fail. Then can you recommend to use two BATFET in series like attached?

    Best Regards,

    Sonoki

  • 0 in reply to S.Satoshi
    TI__Genius 11480 points

    Hi Sanoki,

    I understand, that is a valid concern on their part.

    In terms of the BATDRV capability, it should be okay. It will probably turn on a little slower, but that is not necessarily bad, since ACDRV and BATDRV should always have some deadtime in between. I do have  some comments/concerns:

    • There will be ~1.4V drop (and the corrosponding efficiency hit) in the time between battery supplementing the system and the  BATFETS turning on. Can their system  handle that voltage drop without crashing?
    • When the BATFET is off, the gate, source, and  drain node will all be equal to ACN (no current  is  flowing, so the source of the bottom FET will be pulled to gate). I am not  sure how the top FET will handle this, but my guess is that there is a chance  it  results in a failed FET.
    • I am also concerned that  when BATDRV is ACN-6, the top FET will not  turn on all the way, resulting in efficiency loss/voltage drop. This may be acceptable, but  something  to think about.

    In general, I would say  that if you have a board you can  test with you can certainly try, it shouldn't be catastrophic if it  fails. I don't have much confidence in this arrangement working, though. Maybe it is  possible  to use the BATDRV signal as a "logic" signal that triggers an external gate drive for the second FET, or a similar  arrangement.

  • 0 in reply to David Wiest Jr
    Guru 21685 points
    Hi,

    Now my customer is also considering to use three BATFET in parallel. They plan to use TPCA8120 from Toshiba and it has Ciss=7420pF that is quad of Si7617DN (Used in Fig.18 in datasheet). Can you please confirm if bq24616 can drive TPCA8120 x3 without any issue?

    [TPCA8120]
    www.mouser.com/.../TPCA8120_datasheet_en_20140214-738321.pdf


    Best Regards,
    Sonoki
  • 0 in reply to S.Satoshi
    TI__Genius 11480 points

    Hi Sonoki,

    The BATFET in the EVM has a Qg of ~40nC. Using a back of the napkin calculation, if the quad FET has 4 ~8nC in parallel, then that is ~32nC. I think it will be okay. Since they are a quad package I would think that process variation shouldn't be a huge issue, which could cause some FETs to turn on and others off. Even then, I don't think it would be catastrophic.