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BQ25756: Reverse Mode Current

Hans Hepach
Prodigy 10 points
Part Number: BQ25756

Tool/software:

Hi,

I copied the EVM design as closely as possible as a first step towards a custom design, but unfortunately it doesn't behave as expected. I want to supply about 10W of power in reverse mode, which doesn't work as I want it to.

What works:

  • charging batteries, mostly 1S LiPo
  • reverse mode @ different output voltages with battery connected
  • reverse mode with DC power supply connected
  • reverse mode with 5V output and 2A with battery voltage (DC power supply) > 8V

What doesn't work:

  • reverse mode with 5V output and 2A with 1S LiPo battery connected
  • reverse mode with 12V output and 1.5A with 1S LiPo battery connected

Basically, the reverse mode works fine if the battery voltage is higher than the output voltage. If the battery voltage is below the output voltage, the correct voltage is produced but the board is unable to provide enough power. and the output voltage immediately drops. Monitoring the registers of the BQ25756, I see the "Device entered input voltage regulation" message as well as the 'DRV_SUP Pin Voltage is out of Valid Range'. I tried supplying the gate voltages externally, but that only gets rid of the last error message and the problem of no power delivery remains.

Any pointers as to where I could start troubleshooting would be very much appreciated.

Best regards

  • 0
    TI__Prodigy 330 points

    Hi Hans,

    The DRV_SUP pin must maintain a voltage above the DRV_UVP value of 3.7V. Additionally, with REGN used as the gate driver supply, it must also maintain a value above the UVP threshold. Using the external supply helps fix this issue at low input voltages as expected.

    When in reverse mode with the battery connected, what is your LiPo battery voltage? By default, the reverse mode system UVP (SYSREV_UV) is 80% of the reverse voltage regulation. You can adjust this register setting to a default 3.3 V to help correct the problem.

    Best Regards,
    Austin 

  • 0 in reply to Austin Allen
    Prodigy 10 points

    Hi Austin,

    thanks for the speedy reply. The problem persists even when I use external DC (benchtop) power supplies for the 'battery' and the DRV_SUP pin. So I do get the correct voltage in reverse mode only when not drawing any current. In this case, 'battery' voltage is 4V and DRV_SUP is 5V.

    Best,
    Hans

  • 0 in reply to Hans Hepach
    TI__Prodigy 330 points

    Hi Hans,

    With a VBAT of 4V and a VAC_REV of 5V, I think you may be too close to the 80% of VAC_REV threshold for SYSREV_UV which is causing the device to enter input voltage regulation. What happens if you try setting SYSREV_UV = 1 (REG0x1E) to change the UV threshold to 3.3V?

    Best Regards,

    Austin

  • 0 in reply to Austin Allen
    Prodigy 10 points

    Hi Austin,
    sorry it took a while to test it. Changing SYSREV_UV = 1 doesn't change anything, unfortunately. I tried setting the reverse mode voltage to 5 V and 12 V with and without SYSREV_UV, but the result is always an undervoltage fault as the board does not supply sufficient power to keep the reverse mode voltage at its intended value.

    As far as I understand, the VAC_REV threshold is dependent on the actual reverse mode voltage and not the battery voltage. I think, the undervoltage fault is not the root of the problem but rather a symptom, but I can't figure out what causes it.

    Best,
    Hans

  • 0 in reply to Hans Hepach
    TI__Prodigy 330 points

    Hi Hans,

    Thank you for the updated information. To better understand how the device is operating, may I see a schematic of your design and a full register reading?

    Best Regards,

    Austin