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BQ25792: VBUS Only Operation

Brian Sampoh
Prodigy 50 points
Part Number: BQ25792
Other Parts Discussed in Thread: BQ25798

Tool/software:

Hello,

I would like to understand how the BQ25792 behaves when it operates with a detached battery (e.g. user detach or battery fault protection detach). Will undervoltage protection kick in and shut down SYS due to a floating BAT pin?

And if so, is there a simple way to mitigate this behavior (I assume spoofing battery capacitance by adding a huge polymer)? In other words, is a battery (capacity on BAT) mandatory for the BQ25792 to operate correctly? If I remember correctly, the BQ25798 features a proper no-battery detection circuit (which one main reason why I question the robstness of BQ25792 in this regard).

Best regards,
Brian

  • 0
    TI__Expert 6545 points

    Hi,

    Today is a TI US holiday. I will look into this and get back to you tomorrow.

    Best regards,
    Michael Bradbourne

  • 0 in reply to Michael Bradbourne
    TI__Expert 6545 points

    Hi Brian,

    The BQ25792 usually behaves with an overvoltage protection, which will cause SYS to shut down.

    Both BQ25792 and BQ25798 have BATOVP events when battery is removed and charge is still enabled (by CE bit or pin and no TS fault due to missing thermistor).  Unlike the BQ25792, the BQ25798's converter does not shutdown for 30ms due to the BATOVP event.  This shutdown results in SYS voltage collapse if there is a load on SYS.

    The only other choice is to prevent the OVP event by adding at least 200uF capacitor to BAT/BATP.  You might be able to reduce that capacitance slightly for your specific VBUS and BATREG but most likely not below 100uF.  This is after derating for temperature and applied voltage.10x10uF ceramic caps will not be enough due to their derating issues.

    Let me know if you have any further questions.

    Best regards,
    Michael Bradbourne