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BQ77PL900: High voltage measured on FET gate pins, chip getting excessively hot & using excessive power, unable to control FETs through I2C.

Marcelo Goluboff
Prodigy 20 points
Part Number: BQ77PL900

Tool/software:

Hi,

I recently designed a battery management board making use of the BQ77PL900 as an AFE & battery management chip. However, I am having some issues with the FETs as well as temperature and power consumption.

The schematic and BOM are attached below. The issues are as follows:

  • PMS is 27.42 volts, PACK is 27.97 volts (at 30v to BAT+)
  • D5 forward voltage is 0.02v
  • Voltage drop over R14 is nothing, R12 same thing
  • R14 voltage is 29.37, R12 voltage is 27.97, PACK is 27.98
  • Both FET gates are at 29.97 and 27.97 volts respectively.
  • The chip is getting hot to the touch with no load on the battery and no code being run on the host microcontroller.
  • The FETs do not seem to react to any sort of voltage condition, or manual control through I2C. I am able to communicate with the BQ77PL900 through I2C and alter the data in registers, but the FETs do not react to these changes.

I have been struggling with this issue for a while now, and I would love some guidance in the right direction to solving this issue.

bom (1).xlsx

8738.schematic.pdf

  • 0
    TI__Mastermind 40140 points

    Hello Marcelo,

    The schematic seems okay. One thing I notice is that the R18 resistor is 1-kOhm and C9 is 10-uF. Have you tried changing these to 100-kOhm and 2.2-uF?

    Are you able to communicate with the GUI of this device?

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Prodigy 20 points

    Hi,

    I changed R18 out for a 100k ohm resistor and C9 for a 2.2uF capacitor. The chip draws less current now but still heats up.

    I do not know what you mean by GUI. I have been able to communicate via I2C to the chip. When I read register 0x00, I receive 0x1C, no matter what I do. (this is strange, as 0x1C means it's both overvoltage and undervoltage...)

    Furthermore, no matter how much I alter registers 0x01 and 0x02, I cannot toggle the state of the MOSFETs.

  • 0 in reply to Marcelo Goluboff
    TI__Mastermind 40140 points

    Hello Marcelo,

    What is the voltage on each of the cell input pins of the IC? Could there be a lose connection to the board/pin?

    If the protection is triggered, it may keep the FETs off.

    Best Regards,

    Luis Hernandez Salomon