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BQ40Z50: When Emergency Shutdown enabled, measured current changes and cannot be calibrated correctly.

Roman Khazanskii
Prodigy 190 points
Part Number: BQ40Z50

Steps to reproduce:

1) Calibrate current so when no load is applyed it's zero.

2) Go to unsealed mode

3) Go to DA Configuration and set bit EMSHUT_EN

4) Make software reset

5) Observe current in register 0x0A going up by about 70 milliamps.

I attach to srec files that differ only in this one bit; one measures current correctly and the other one does not.

My question is: is this normal? What should I do about it if I want to measure current correctly and use emergency shutdown?

battery_control.zip

  • 0
    TI__Expert 6575 points
    I have tried both srec files on my EVM, and I am not able to duplicate the issue.
    Is it possible to share your schematic to see if there is anything there that could cause an issue?
    Also, for current calibration, typically we apply 2000mA across the sense resistor and calibrate it.
    Regards,
    Swami
  • 0 in reply to Swaminathan Ramanathan75031
    Prodigy 190 points

    Here is the main piece of our schematics. We also used 2000mA for calibration. I2C pull-up resistors are on the different sheet; i hope that doesn't matter much.

    Battery_board _Main.pdf

  • 0 in reply to Swaminathan Ramanathan75031
    Prodigy 190 points

    It turned out that it was my fault. In technical reference (sluubc1c) is said:

    A high-to-low transition on the not-SHUTDN pin is detected with a debounce delay of 1 s for the low level
    threshold. For example, a push button is pressed again.

    So I presumed that it is safe to always push not-SHUTDN pin high and push it low only when emergency shutdown is required. It turns out that if I do this - current measurement starts to lie :)

    There is no information about how long should not-SHUTDN be high before transition to low; for now I push it high for one millisecond just before EMSHUT is needed and keep it floating all other time. Current measurement seems ok.