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BQ24259: power the system without battery

Howard Zou
Genius 12365 points
Part Number: BQ24259

Hi, my customer is using BQ24259 and the schematic is as below.

When they detach the battery, connect USB_VBUS to 5V DC source(capable to provide current larger than 10A) and turn on the system, Vsys will be 4.3V at first, and then drop to 2.2V. The voltage at USB_VBUS is still 5V during the drop of Vsys.

But the current consumed by Vsys is only 1.8A at most. So I wonder why the charger can not support such load on Vsys.

  • 0
    TI__Genius 16175 points

    Hi

    I think the problem may cause by input current limit.

    Can you connect PSEL pin from VSYS to ground?

    PSEL: Power source selection input. High indicates a USB host source and Low indicates an adapter source.

    From your design, input current limit should be limited in 500mA.

    Thanks

  • 0 in reply to Paul Yeh
    TI__Genius 12365 points

    Paul,

    In datasheet it says that the device will always run input current limit detection. So I guess it's not caused by PSEL.

    8.3.1.3.3 Input Current Limit Detection

    After the PG is LOW (bq24259) or REG08[2] goes HIGH, the charger device always runs input current limit detection when a DC source plugs in unless the charger is in HIZ during host mode.

    The bq24259 sets input current limit through PSEL and OTG pins. After the input current limit detection is done, the detection result is reported in VBUS_STAT registers (REG08[7:6]) and input current limit is updated in IINLIM register (REG00[2:0]). In addition, host can write to REG00[2:0] to change the input current limit.

  • 0 in reply to Paul Yeh
    TI__Genius 12365 points
    Can I float PSEL? Is it high or low when floating.
    Or can you think of any method to solve this problem without redesign the schematic and layout?
  • 0 in reply to Howard Zou
    TI__Mastermind 25800 points
    Hey Howard,

    Keep in mind that on startup, because you have no battery present, ILIM would be in tri-state, meaning it can be read as HIGH or LOW.

    I would also check that you are not in VINDPM either due to large line resistance between VBUS and your input power supply.

    Please read the register state from the charger to verify this (REG08[3]) to determine whether you are in DPM or not.


    Regards,
    Joel H