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BQ25750: BATFET gate drive power source with no VAC present

Seth J
Prodigy 60 points
Part Number: BQ25750

Tool/software:

On the BQ25750, what powers the BATFET gate driver when no VAC is present? There does not appear to be a path to use battery voltage to turn on the BATFET. 

  • 0
    TI__Mastermind 30615 points

    Hello Seth,

    The BQ25750 uses a charge pump to power BATDRV from BATSRC. Let me know if this answers your question.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 60 points

    Hi Ethan, thanks for the reply.

    It looks like BATSRC is connected between the two back-to-back BATFETs (source pins). Does this mean that if BATFET is turned off there is no way to turn it back on again?

    Section 8.3.2 of the datasheet seems to imply BATFET can turn on if there is no VAC present: "If only battery is present and the voltage is above VSRN_OK threshold, the BATFET turns on and connects battery to system." In this case where no VAC is present and BATFET is off initially, where is BATSRC powered from to enable BATFET to turn on? 

  • 0 in reply to Seth J
    TI__Mastermind 30615 points

    Hello Seth,

    Thanks for the clarification.

    Yes, the BQ25750 can turn-on and turn-off the BATFET without VAC present. Just for reference, we have an FAQ for the power-path switchover.

    Let me check internally to get the power source for BATDRV correct and let me know if you have any other questions.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    TI__Mastermind 30615 points

    Hello Seth,

    Thanks for being patient with this answer.

    The initial turn-on of BATDRV turns-on the BATFETs and provides power to BATSRC. Current is also allowed to flow through the body diodes of the BATFETs while the BATFETs are turning-on.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 60 points

    Hello Ethan,

    What path does current take from the battery to get to the BATDRV pin so the BATFETs can turn on? I'm looking for an ordered list, something like this:
    Battery positive terminal -> ? -> BATDRV pin.
    I'm wondering what goes in the "?" spot. 

    I'll note the body diode for the battery-side BATFET is reverse biased so no current will flow through that BATFET from the battery before that BATFET is turned on. Is that correct?

  • 0 in reply to Seth J
    TI__Mastermind 30615 points

    Hello Seth,

    Seth J said:
    What path does current take from the battery to get to the BATDRV pin so the BATFETs can turn on?

    Power comes from the SRN pin. I'll see if I can get you more information than that.

    Seth J said:
    I'll note the body diode for the battery-side BATFET is reverse biased so no current will flow through that BATFET from the battery before that BATFET is turned on. Is that correct?

    That is correct. The battery-side FET is reverse biased. The system-side FET would be forward biased.

    Best Regards,
    Ethan Galloway

  • +1 in reply to Ethan Galloway
    TI__Mastermind 30615 points

    Hello Seth,

    Here's the order of operations for the BATDRV to turn-on. BATDRV is indeed powered by SRN and BATDRV is regulated with respect to BATSRC. Thanks for being patient with this.

    1. The charge pump circuit either steps-down or steps-up the voltage to regulate VBATDRV 10V above the BATSRC pin.

    2. BATDRV turns-on battery-side FET.

    3. As the voltage on BATSRC increases, the charge pump circuit transfers from step-down operation to step-up operation and keeps regulating VBATDRV-VBATSRC=10V.

    4. BATDRV is now able to turn-on the FETs the rest of the way, but also the BQ25750 monitors the IBAT discharge current. If the IBAT discharge current rises up above 8A, the BQ25750 will turn the BATFETs off to clamp the current to protect the BATFETs.

    The 8A power path protection is discussed more in this FAQ. The 8A protection limit turns-off once the BATFETs fully turn-on.

    Let me know if you have any questions about this.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 60 points

    Hi Ethan,

    The information that BATDRV is powered by the SRN pin did not appear to be covered in the datasheet or the FAQ (the FAQ mentions a sensing SRN threshold above 3V but does not explicitly state that BATDRV is powered entirely from that pin without a VAC source) so I appreciate the effort and time taken to find and provide this answer. 

    Regards,
    -Seth