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BQ25756: Reverse mode operation below -20C

Daniel Wanninger26
Prodigy 130 points
Part Number: BQ25756
Other Parts Discussed in Thread: TMUX7219M

Tool/software:

I have LiFePO4 pack that I might be using with the BQ25756.  It has low-temp rated cells, which can discharge as low as -45C.  My plan is to use it with the BQ25756 in reverse mode to drive a motor.  

The catch is, the BQ25756 can only be configured for reverse mode discharge as low as -20C (with the BCOLD field).  I can disable temperature sensing with the EN_TS field, but that leaves me unprotected when charging.  I can of course just re-enable it when I'm done with the discharge cycle, but this much firmware control leaves me uncomfortable from a safety perspective.

As an alternative, I could have two sets of pull up / pull down resistors on the TS pins to set two different temperature configurations for charging and discharging.  To switch between them, I would use the STAT1 and STAT2 pins - if both were off, then use the discharge pair.  If either were on, then use the charging pair.

I think this gets me what I need but I wanted to confirm that there aren't any gotchas.  Would this approach work?  Is there a better solution to this problem?

  • 0
    TI__Mastermind 32506 points

    Hello Daniel,

    Thanks for being patient with this. Let me look into this and get back to you.

    Best Regards,
    Ethan Galloway

  • 0
    TI__Genius 9900 points

    Hi Daniel,

    Thanks for your patience. Using the STAT1 and STAT2 pins, would you use those as inputs to a logic gate? Just initially looking at this scenario, this should work, but you might be able to remove the logic gate if you just use the /PG pin. Are you using the /PG pin for anything else?

    My initial thought for this problem is to use the /PG pin to drive an analog multiplexer. The /PG pin should operate based on the conditions in section 8.3.8.1 of the datasheet, but I notice that the device also does not drive /PG low when reverse mode is enabled. This could be used directly as the input to the SEL pin on a multiplexer. I am not the expert on our multiplexers, but using our search tool, it seems that the TMUX7219M has the correct operating conditions that you might require for this application.

    One issue I see with this operation is that the recommended operating temperature of the BQ25756 is only as low as -40deg C. We do not warranty operation of the BQ25756 below -40deg C. Would you be discharging the battery all the way down to -45deg C?

    Best,
    Michael

  • 0 in reply to Michael Bradbourne
    Prodigy 130 points

    My plan was to tie both STAT1 and STAT2 to the gate of an N-channel MOSFET with a pullup.  That MOSFET would, in turn, switch other FETs to enable the right pair of pull resistors.

    I hadn't looked into using the PG pin - I'll investigate that option as well.

    I do not plan to operate this below -40°C.

    Thanks for your help!

  • +1 in reply to Daniel Wanninger26
    TI__Genius 9900 points

    Hi Daniel,

    I think both options will achieve the same result. Let me know if you have any further questions.

    Best,
    Michael