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BQ76942: BQ76942 can not update data anymore since each cell voltage reaches a threshold

Hai Phan
Prodigy 110 points
Part Number: BQ76942

Tool/software:

I have problem with BQ76942 circuit with application for 9 cells pin, right now VC10, I tied to VC9 for this application and I use voltage divider to create a pack pin by 9x1,5 kOhm resistor. One more thing, I use external cell balancing circuit with N FET as used in https://www.ti.com/lit/an/sluaa81a/sluaa81a.pdf?ts=1740101522109&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FBQ76942 .

The problem is when I started increase the voltage of whole pack pin in certain value, for example each cell reaches 2,3 V. My MCU didn't update cell voltages anymore, and stop by randomly, sometimes it stop at 2300, sometimes 0V and sometimes 65535, and I called it as a fault. I have also checked the Reg18 pin, before this fault, it had around 1,8 V, after this fault, it had around 2,7V. And one more thing, if I increase voltage to reach approximately 3,9V, BQ76942 IC's temperature is really high.

this figure is a screen in STM32 CubeIDE, MCU receive through I2C communication

Thank you

  • 0
    Prodigy 110 points

    I also wonder that, there is any problem if I tied VC10 to VC9 due to 9 cells instead of 10 cells 

  • 0 in reply to Hai Phan
    TI__Genius 14725 points

    Hello Hai,

    Do you mind sharing your schematic? It is very weird to see your REG18 pin jump to 2.7V after a fault.

    We recommend following Section 16.6 Unused Pins from the datasheet when you have unused pins. When shorting cells, we recommend having cell inputs 1, 2, and 10 always be connected to an actual cell, especially if you are using cell balancing. This Connecting Battery Cells FAQ may also be useful to refer too.

    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    Prodigy 110 points

    I don't know exact problem why it happened, however, I use thermal camera, I saw that BJT had very high temperature any time I continually increased input voltage of total pack pin. After I removed it, the cell balancing circuit worked well, can you explain more about that?

  • 0 in reply to Hai Phan
    TI__Genius 14725 points

    Hello Hai,

    The BREG pin drives the base of an external NPN BJT to provide the 5.5V REGIN pin voltage. REG1 and REG2 LDOs then take their input from the REGIN pin. You can also drive the REGIN pin externally. More information can be found on Section 12.3.1. Preregulator Control of the datasheet and/or through this previous post, REGIN and BREG Transistor Explanation.

    Do you mind sharing the voltage of BAT, PACK, BREG, and REGIN before and after you increase the voltage? Waveforms of the current would also be useful to observe. This previous post may also be helpful for an explanation: REGIN and BREG. Are you using the LDOs to drive anything? 

    Best Regards,
    Alexis