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BQ76920: Cell 5 Voltage reads low

Mark Pearson
Genius 12705 points
Part Number: BQ76920


My customer is seeing low cell voltage on Cell 5 when the battery stack voltage drops below 13.5V.

Please see the attaché PPTX file for details of my conversation with the customer, customer schematic and data graph.

Thanks.

BQ76920 Cell 5 Voltage Issue.pptx

  • 0
    TI__Guru 54715 points

    Hi Mark,

    The schematic looks much like the data sheet. The reported voltage behavior is quite unexpected.  Typical behavior meets the data sheet specifications and error has the characteristic shape of figure 6-2 as shown in the example here.

    In the schematic there is a note on REGOUT which appears to read "45 mA MAX", that value is the maximum short circuit current of REGOUT.  REGOUT can only provide about 20 mA.  If they are providing a constant power load with REGOUT the current will increase as the voltage drops.  The increase in current will pull down REGSRC and BAT.  When BAT is pulled sufficiently below VC5, the top cell will not be able to measure properly.

    While connecting BAT and REGSRC to share the power filter components provides a low component count solution, it will not support a high current solution.  For higher REGOUT currents a separate filter like the bq76920 data sheet figure 8-1 simplified schematicwhere REGSRC is fed from battery+ is preferred or required.  See www.ti.com/.../slua749 figure 13.  But the bq76920 REGOUT will not be able to supply 45 mA if that is the intent of the schematic note. 

  • 0 in reply to WM5295
    Prodigy 40 points
    Hello,

    I am the designer of the circuit for whom Mark made this post. The 45mA note is misleading because we only use it to power the host MCU (STM32F0x2). I am certain that we draw less than 20mA on my entire BMS (BQ76920 + STM32 + CAN Transceiver) with no load applied on the battery terminals.

    I will prototype the solution in Figure 13 that you suggested, but not sure if 100 Ohm and 2.2uF is a good combination for both filters. What would you recommend?
  • 0 in reply to Adam Stroud
    TI__Guru 54715 points
    Hi Adam,
    Interesting point, the example above was with an EVM and the gauge disconnected, so no load on the regulator. I should look at that. 20 mA x 100 ohm is 2V, plus the diode forward biased. That could easily be more than half a cell voltage and the part does not have a characterized limit of VC5 - BAT.
    I would not expect a MCU to provide a heavy load either. It may be good to look at what is happening with the BAT-REGSRC node on your board, to see if it is in line with above or if there may be some unexpected droop. Perhaps the load is just more than observed with the EVM.
    The data sheet includes REGSRC with the CCAP capacitor value, 1 uF minimum recommended. A 1 or 2.2 uF may be a good value. You should have good headroom from REGSRC to the REGOUT, the FET outputs will be affected first. Select the resistor which gives an acceptable drop at maximum load. For example if you want to keep 10V on the FET outputs, allowing 2V you would want to keep 12V at REGSRC. If your minimum battery voltage will be 15V you have 3V to drop across a filter resistor at maximum load.
  • 0 in reply to WM5295
    Prodigy 40 points
    Well, the horrible inaccuracy of the cell 5 reading disappeared when I changed my filter resistor to 50ohms, i.e. reduced the IR drop between Cell 5 and REGSRC/BAT. Thanks for walking me through the issue! I'll continue testing and see if this design change causes problems elsewhere.