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BQ77915: current state comparator does not operate

Part Number: BQ77915
Other Parts Discussed in Thread: BQ77PL900, BQ76940, BQ77904

In our recent project we use bq7791506 . We are experienced on BMS control IC's of Texas Instruments.

Previously we used different ICs for different projects such as  bq77PL900, bq77904, bq76940.

In our recent project bq7791506 holds every specification given in datasheet (SLUSCU01) except current state comporator.

Technical details on this situation:

- Rsense= 0,25mOhm

-There is no problem about current sense and current protection cases. (SRP-SRN connections are OK)

- During charge:

  i. battery pack goes into UVP when any cell voltage is under 2,5V (it is OK)

  ii. When charger is started charge  Dicharge FETs are not turned ON. Current flow through body diode of discharge FETs.

     This situation does not change with charge current. we applied charge currents 2A, 5A, 10A, 15A, 20A, 25A, 60A.

      According to datasheet Vstate-c=1,875mV that means current above 7,5A should turn on discharge mosfet.

   iii. Discharge MOSFET is turned on only cell voltages reach 2,9V and above.

-During discharge:

 i.While 100A dishcarge is drawn from pack charge FETS are turned off because of OTC temperature. this causes more power dissipation on FETs and because of increasing temperature BMS turns off the discharge.


- In both situaition (charge and discharge) our FETs or PCB does not fail. It goes in protection and then recovery ( 2,9V for UVD and cooling for over temperature)

-But obviously Current State Comparator does not work in both cases. This is a clear contradiction with datasheet.


We will be glad if you can inform us about this situaiton.

Best regards

Serdar Özbek

  • Hi Serdar,

    Check the installation of your ROCD on the OCDP pin.  If the value is set for an "upper device" it will not need current for cell balancing, but it also will not use the state comparator and so can't do body diode protection.  Current protection still operates.  If the resistor connection is broken it looks like an upper device.

  • Hi,
    thanks for your advice but i think this is not related with our problem. 

    1- OCDP pin programs Over Current Delay Time. We do not have any problem with Over Current Protections.

    2-Our desgin is 4s. There is no upper or lower device.

    3-our problem is shortly:
      Assume that Discharge FET is Turned OFF for any protection reason . If charger is connected and charge current begin to flow ( Vsense>Vstate_D)  Discharge FET is turned ON. But in real it is not turned ON and current flows through body diode of discharge FET. 

    Vice Versa

    Assume that Charge FET is Turned OFF for any protection reason such as OVC. If load is connected and current is drawn from battery and current is high enough Vsense>Vstate_C , Charge FET should ben turned ON. Unless current passes through body diode of Charge FET . 

    In our boards (not in one board we produce 100 boards up to now) this scenario does not work . 

  • Hi Serdar,

    Correct, you would not want to set the part as an upper device, but if the resistance value seen by the OCDP pin is too high it will think it is installed as an upper device, turn off the state comparator and show the behavior you describe.  Since it is systemic across your 100 boards, look for a common fault, possibilities might seem to be:

    • Wrong value for ROCD
    • Missing solder on a resistor pad or OCDP pin of the IC
    • Missing trace.

    Of course it could be a bad IC, but more likely to be an occasional damaged unit rather than all.  Since you have a prior design which sounds like it worked properly, you might swap a suspected bad part with one from a working design board. See if the issue stays with the board or the IC.  If the parts are bad please return through the supply chain.

    Please let us know what you find.

  • hi,

    thanks for your comment. 
    I understood that ROCD effects  Current State Comparator. 
    We did not notice this relation in datasheet. 
    I will test this tomorrow and will inform about result.