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BQ77PL900: DSG MOS closure problem

Gabba
Prodigy 85 points
Other Parts Discussed in Thread: BQ77PL900

Hi,
When I connect the battery pack it does not close the DSG MOS. When I attach a charger for the "first switch on" the bq77pl900 closes DSG MOS, but when I switch off the charger, bq77pl900 does not hold the MOS closed in standalone mode.
I tested as below:
- With a uC connected to bq77pl900 by I2C bus,  I try to read status register and the bq77pl900 does not report any fault, and the cells voltage are in normal condition.
- If the bq77pl900 is host mode, the BMS closes the DSG MOS, even with charger ON or OFF. 
- I try to change the bq77pl900 IC but the problem persists.
When I tested, I noticed some strange behavior of the bq77pl900.
When uC operates, the BQ closes MOS DSG. Note that uC has a separate supply. 
When uC is turned off, the BQ NOT HOLD closed MOS DSG.
Can you help me?? The behaviour of the bq77pl900 I observed is not described on data sheet!
DG

  • 0
    TI__Guru 54715 points

    Check to see if your uC is setting host mode (HOST bit in STATE_CONTROL register address 0x02) at the wrong time or to the wrong polarity. If the host mode bit is set, the FET control is determined by the combination of the devices safety status and the host FET control bits CHG and DSG in the OUTPUT_CONTROL register (address 0x01). I am sometimes startled when the FETs turn off when I enter host mode because I forgot to set these bits.

    When you first wake the part, FET control will be done by the part itself.  Once you enter host mode, the uC will control the FETs unless the safety system overrides the setting. If you turn off the uC, the part retains its register settings.  If you want the part to return to controlling the FETs by itself, clear the HOST bit before turning off the uC. Note that the registers can be read and written without being in host mode.

  • 0 in reply to WM5295
    Prodigy 85 points

    Thanks for your fast reply.

    However, also removing the uC, lefting only the bq77pl900 in stand alone mode,it does not hold closed the MOS.

    For this reason I changed the IC on the board with another new, but without any results. I do not think that the problem is due to other circuitry because the layout is the same of the demoboard.

    thanks again,

     

    DG

  • 0 in reply to Gabba
    TI__Guru 54715 points

    That is certainly odd. It seems you can see proper behavior on the demoboard but not the target system. Somehow you need to identify the difference between the 2 and relate it to a repairable defect.

    From the part behavior, I would expect anything that would turn off the FETs to be reported in the status register. Other things to look at might be the XALERT status which should be normal=no fault (high), XRST should be not reset (high). Both regulators should be on and the VLOG should be at the selected voltage and stable. Be sure your I2C status is read from the part and not from some other device or short.

    If it is a new board design, be certain the schematic matches the demoboard portion. Also be sure the board matches the schematic and the assembly & component values match the design also. The differences between the demoboard and your board of course will be the source of the board, components, production date of the '900 and all testing.  Comparison measurements may help. It can be uncomfortable to risk a working system, but you might consider swapping the '900s to see if the problem follows the board design or the device.