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BQ25792: Dual supply configuration & Fuel gauge recommendation for 4s

Akin Raj
Prodigy 120 points
Part Number: BQ25792
Other Parts Discussed in Thread: TPS25751, , BQ25798

Tool/software:

Hi,

I am using the BQ25792RQMR IC for battery charging and the TPS25751 as the PD controller in my project. We have dual power inputs:

  • USB Type-C (VAC1), which is detected and controlled by the battery charger.

  • Pogo pin connection (VAC2), which provides a secondary power source.

Among these, only the Type-C input supports fast charging. We are using a 4S battery configuration, with the charging voltage set to 16.8V and charging current configured to 3A via register settings.

Questions:

  1. Behavior with Limited Input on VAC2:
    If the supply is switched to VAC2 (Pogo Pins), where the maximum input available is 5V at 2A, will the battery charger automatically adjust the charging voltage and current?

    • Does the input voltage dip in an attempt to maintain the default 16.8V/3A charging setting?

    • Or is there an internal mechanism in the BQ25792 that dynamically adjusts the charge parameters based on the available input power?

  2. Configuration Adjustment for VAC2 Detection:

    • How can I modify the charging current and voltage settings dynamically when VAC2 is detected to avoid potential power dips?

    • Does the BQ25792 provide a built-in configuration method to handle different input sources efficiently?

Additionally, could you recommend a suitable fuel gauge IC for a 4S battery configuration?

Thanks in advance!

  • 0
    TI__Guru**** 166626 points

    Hi Akin,

    Regarding 1, the charger voltage (VREG) does not change with input source change.  The charge current automatically reduces to prioritize ISYS since 5V*2A=10W <<16.8V*3A.  The charger's VINDPM results to 5V-0.7V at switchover.  After that, the VINDPM feature prevents the input voltage from dropping below 4.3V.  There will be a momentarily dip in input voltage < UVLO (converter off), forcing V((SYS)=V(BAT), at changeover from VAC1 to VAC2 because both MUX FET pairs must be briefly turned off to prevent reverse current.

    Regarding 2, the host can I2C write different current and voltage settings at anytime.  However, the charger handles all this so no writes should be necessary. 

    Please note that BQ25792 is not recommended for applications with user removable batteries.  Please change to pin 2 pin BQ25798 for that case.  To protect against overvoltage at 4S battery hot plug, please add either the capacitor on BATP if shipFET is installed OR a TVS diode on BAT if not shipFET. 

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 120 points

    Hi Jeff,

    Sorry for the delayed response.

    When we connect a 5V 2A input, as you mentioned, there's a noticeable voltage dip since the charging parameters are set to 16.8V @ 3A. Could you please advise on how we can prevent this voltage drop?

    Also, in our application, the battery will only be replaced once every 6 months or even up to a year. Given this usage pattern, would the BQ25792 still be suitable, or should we consider switching to the BQ25798, as you previously recommended?

    Additionally, could you suggest a suitable fuel gauge IC that supports a 4S battery configuration for this use case?

    Looking forward to your guidance.

  • 0 in reply to Akin Raj
    TI__Guru**** 166626 points

    HI Akin,

    The voltage dip is likely due to highly resistive connection (cables, connector, etc) from the power supply to the chargers VBUS pin.  With only 5V*2A=10W input, the charge current will be less than 10W/16.8V = 595mA.

    Since the charger will not be operated without a battery, BQ25792 is likely okay for your application.  Just know that if charging is enabled (CE pin and bit and not TS fault) when the battery is removed (or if it discharges completely resulting in protector FET opening), the OVP on BAT and SYS causes the SYS output to shut down for 30ms.  You can prevent this by increasing the BAT pin capacitance to 200uF.

    I will forward this email to the fuel gauge team for help with selecting a gauge.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 120 points

    Hi Jeff,

    Thank you for confirming the details. It's clear now that even if the charger IC is connected to a 5V 2A supply, the actual charging current will be limited to less than 595 mA, regardless of the charging current and voltage settings (e.g., 3 A at 16.8 V).

    Also, I appreciate the clarification regarding the BAT pin — adding a 200 µF capacitor to prevent the 30 ms SYS shutdown in the absence of a battery is noted and very helpful.

    Looking forward to the fuel gauge recommendation from the team.
    Best regards,
    Akin