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BQ76952: BQ76952 PACK sampling is inaccurate, the error is about 1V

L
Prodigy 220 points
Part Number: BQ76952

Tool/software:

The actual voltage is 50V, the BAT voltage is sampled correctly, the PACK voltage is 1V lower than the BAT voltage, the PACK pin input voltage is also 50V, and the PACK I2C register is only 49V. What is the reason for this?

  • 0
    Prodigy 220 points

    In addition, the PACK voltage content I read is from registers 0x36 and 0x37

  • 0 in reply to L
    TI__Genius 17775 points

    Hello L,

    Is 49V measured with a multimeter at the PACK pin as well?

    If you haven’t done any calibration yet either, Section 2.4 TOS (Top-of-Stack), PACK, and LD Pin Voltage Calibration in the BQ769x2 Calibration and OTP Programming Guide may also be able to help you with that.

    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    Prodigy 220 points

    The same is true when measuring 49V at the PACK pin with a multimeter.
    I want to know if this error is normal at room temperature (without calibration)?

  • 0 in reply to L
    TI__Genius 17775 points

    Hello L,

    Thank you for checking with a multimeter as well. 

    Do you know if your CHG/DSG FETs are both ON or OFF when PACK voltage is 49V?

    Are there also any protections asserted when this occurs?

    This issue sounds like it could be similar to this previous post: Pack Pin Voltage Less than Stack Voltage.

    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    Prodigy 220 points

    The reference post is not consistent with my problem. It is not a problem. The reference post is that the real PACK pin voltage drops, but mine is not. My PACK voltage cannot sample the correct 50V. My charge and discharge FETs are both closed, and the input voltage to the PACK pin is the real 50V. But the PACK pin sample is 49V, which is 1V lower.
    Because it is normal operation, there is no protection or assertion.

  • 0 in reply to L
    TI__Genius 17775 points

    Hello L,

    Apologies, I think I misunderstood your previous statement. I thought you were stating that when you measured the PACK voltage with a multimeter, it was measuring 49V as well.

    If the PACK voltage is actually measuring 50V with a multimeter but the device is reading 49V for PACK, I would recommend trying to calibrate the PACK voltage to see if that helps with the guide I mentioned before.

    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    Prodigy 220 points

    In fact, we have already calibrated it, but we think the error of the original data is problematic. BAT sampling is accurate, but PACK sampling is inaccurate, which is very strange. Your answer is not the answer I want.

  • 0 in reply to L
    TI__Genius 17775 points

    Hello L,

    Did the calibration solve the 1V offset? It may be that the device was stressed during assembly, was this offset present in only the one part, or on other boards as well? How is the PACK pin connected to the rest of the system?

    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    Prodigy 220 points

    How are the voltage values ​​read out of registers 0x36 and 0x37 calculated? Can the original AD value of the PACK voltage be read?

  • 0 in reply to L
    TI__Genius 17775 points

    Hello L,

    I believe the raw PACK pin voltage measurements are given with the subcommand 0xf081 READ_CAL1 as shown in Table 12-23. Subcommands Table (continued) in the BQ76952 Technical Reference Manual (TRM).

    The raw values are then corrected by first subtracting a stored offset trim value, the gain is applied, and then the PACK offset is subtracted, before the final voltage value is reported. Section 4.10 Voltage Calibration (ADC Measurements) in the TRM explains this more in detail.

    Best Regards,
    Alexis