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BQ25756: Reverse mode threshold

John Griffith
Mastermind 21769 points
Part Number: BQ25756
Other Parts Discussed in Thread: BQ25856-Q1,

Tool/software:

Hello,

My customer has a super cap with voltage range of 2.5-4.0V and they are concerned by the datasheet stating that SRN must be greater than 3.0V (section 8.3.6). Two clarification questions:

  • If SRN drops below 3V while reverse mode is active will reverse mode disable? 
  • If so can this limit be adjusted?  

Thanks,
John 

  • 0
    TI__Mastermind 30965 points

    Hello John,

    Thanks for being patient with this question. I'll get back to you later this week on this question.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    TI__Mastermind 21769 points

    Thanks Ethan for following up that you are on this. Engineer followed up on the request again today, and I said we are working on it. Sooner the better, thanks! 

    John

  • 0 in reply to John Griffith
    TI__Mastermind 30965 points

    Hello John,

    Thanks for being patient with this.

    • If SRN drops below 3V while reverse mode is active will reverse mode disable? - Yes
    • If so can this limit be adjusted?  This limit can not be adjusted.

    If the customer needs a higher operating range, they might want to stack capacitors in series to get a higher voltage.

    Also, we have a part called the BQ25856-Q1. This part is just like the BQ25756, but it has auto reverse mode and is automotive qualified. Auto reverse mode will allow the reverse mode to quickly turn ON when the input voltage is removed.

    Let me know if you have any questions about this.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    TI__Mastermind 21769 points

    Hi Ethan,

    Unless I am missing something, it doesn't look like the BQ25856-Q1 solves the issue either. The datasheet says on page 18:

    The internal bias circuits are powered from either VAC or SRN. When VAC rises above VVAC_OK, charging is allowed. When BAT rises above 3 V, reverse mode operation is allowed.

    They are still going to run into the issue with a 2.5V super cap, right? This isn't adjustable either? They don't want to put to super caps in series, these are very large and expensive. 

    John

  • 0 in reply to John Griffith
    TI__Mastermind 30965 points

    Hello John,

    Yes, the BQ25856-Q1 will have the same issues as the BQ25756 regarding the minimum VBAT voltage in reverse mode. The BQ25856-Q1 does have Auto Reverse Mode though and this makes the BQ25856-Q1 work faster in a super capacitor backup system.

    Since we are talking over email, I'm going to close out this E2E thread. If you have any more questions, let me know over the email thread.

    Best Regards,
    Ethan Galloway