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BQ2970: BQ2970 battery reverse connection protection

Yuting Li
Intellectual 1420 points
Part Number: BQ2970

Hi team,

My customer use bq29700 in their project. They tests the battery reverse connection protection. Customer connect 330ohm resistor on BAT pin. When battery is reverse connection, customer measure about the current 300mA current through VSS. Why is the reverse current so high?

Is there a TVS between BAT and VSS? When battery is reversely connected, the TVS will be forward bias and the current flow through bq29700 is [V(bat)-V(TVS forward bias)] / 330ohm. Is the analyze right? Thanks.

  • 0
    TI__Guru 54715 points

    Hi Yuting,

    There will be some voltage drop with reverse voltage connection of the power supply.  Note that it violates abs max.  It is not characterized.   If reversing the cell is a possibility you will need some circuit in the battery to prevent the reverse voltage

  • 0 in reply to WM5295
    TI__Intellectual 1420 points

    Hi WM,

    So BQ2970 don't have the reverse connection protection. I saw someone said it have this function in other E2E thread. So what does it mean in datasheet 10.2.2?

    "An RC filter is required on the BAT for noise, and enables the device to operate during sharp negative
    transients. The 330-Ω resistor also limits the current during a reverse connection on the system."

  • 0 in reply to Yuting Li
    TI__Guru 54715 points

    Hi Yuting,

    It is difficult to tell what the BQ2970 data sheet author intended. The discussion in 10.2.2 you reference is supporting the need for the RC filter.  A resistor will limit current.  Looking at the ABS MAX table in the data sheet https://www.ti.com/document-viewer/BQ2970/datasheet/GUID-74849E6F-B364-4AB3-9C00-A8A6ED1CEB83#TITLE-SLUSBU9SLUSBU9 neither V- above  BAT (system reversal) nor BAT below VSS (cell reversal) is allowed by more than 0.3 V.  Note 1 says it can damage the device, not that it will. So while the 330 ohm will limit current in a reversed condition it is not a recommended condition and could be damaging.