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Original question:

BQ25750: Simple Buck Boost charger - no power path FET - no reverse mode

BQ25750: Charger without System Supply or Reverse Mode

Yibo Zhang
Prodigy 10 points
Part Number: BQ25750
Other Parts Discussed in Thread: BQ25756

Tool/software:

I'm designing a drone battery charging system based on the BQ25750.

While the chip supports system power supply and Reverse Mode, my application focuses solely on charging the battery and does not require these two modes.

Therefore I removed BATSRC and BATDRV, and connect SYS to VAC across a 10K resistor.

Could TI experts validate if these modifications could introduce issues, such as risks of incomplete charging, or potential conflicts with default power management logic? 

Thank you in advance for your insights!

BQ25750_Charger_Schematic.pdf

  • 0
    TI__Mastermind 31255 points

    Hello Yibo,

    Thanks for working on this.

    I recommend shorting SYS to directly to VAC and shorting BATSRC to SRN when the BATFETs are not being used.

    I'll review your schematic next week. Friday is a TI holiday.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 10 points

    Got it! I fixed those issues. Please review this one. Thank you for your help, Ethan.

    7713.BQ25750_Charger_Schematic.pdf

  • 0 in reply to Yibo Zhang
    TI__Mastermind 31255 points

    Hello Yibo,

    Your welcome.

    Here are my recommendations:

    • We recommend a minimum of 80µF on both the input and output to keep the charge stable. Can you increase the capacitance here?
    • VAC will need a 1µF capacitor close to the pin.
    • Besides for that the circuit looks good.

    Can you tell me a bit more about your circuit specifications? I'm not quite sure what the purpose of Q11 and Q12 is.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 10 points

    Hi Ethan,

    Thank you so much for you correction and explanation. 

    Q11 and Q12 were copied from the official example design and just left there. I think these FETs are combined with ACDRV, ACUV, and ACOV to control the "System supply" in a certain range. 

    However, I don' t need such system voltage supply. Probably I can do the following?

    • Remove Q11 and Q12.
    • Pull up or short ACUV to VAC 
    • Pull down or short ACOV to GND.
    • Disable  ACDRV. But I'm not sure whether I should pull up, pull down, or left it floating.

    It seems a big part of the system will be removed. I just wondering if I can do so.

    Thank you!

    Best, 

    Yibo

  • 0 in reply to Yibo Zhang
    TI__Mastermind 31255 points

    Hello Yibo,

    Yes, you can make these changes and the system should work fine.

    By the way, if you are not using the ACFETs or BATFETs, you might want to BQ25756.

    Let me know if you have any questions about this.

    Best Regards,
    Ethan Galloway