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BQ78350-R1: locked FETs

Matteo Rigamonti
Expert 1170 points
Part Number: BQ78350-R1
Other Parts Discussed in Thread: BQ76200, BQ76930, , BQSTUDIO

Good morning,

I've completed my new BMS design with BQ78350-R1, BQ76930 anb BQ76200. I've assembled the battery pack with such BMS, but both CHG and DSG FETs are locked in the off state.

I see flags XCHG and XDSG, as well as TCA and TDA are asserted, but no protections are active.

I've tried also to send the FET_EN commands and to reset the BQ78350-R1, but no effect happens.

In attachment you can find the .srec file and the .gg file.

Can anybody hel me to understand and unlock FETs, please?

Thank you very much

Matteo

BQ78350R1_config.zip

  • 0
    TI__Guru* 81035 points
    Hi Matteo,

    Are you able to connect BQStudio for debug? Can you send a screen capture of the BQStudio Registers and your schematic?

    Thanks,
    Matt
  • 0 in reply to Matt Sunna
    Expert 1170 points

    Hi Matt,

    thank you for your support.

    Here you can find the BQstudio screenshot:

    Here you can find the schematic part including the BQ78350-R1:

    Some explanations concerning labels:

    • "VCC_aux" comes from BQ76930 REGOUT
    • "Alert" is connected to the ALERT pin of BQ76930 and has a 1M pull down resistor
    • "SDA", "SCL" are connected to correspondent pins of BQ76930
    • "Vaux" comes from the PACKDIV output of BQ76200
    • "Ven" is connected to the PMON_EN pin of BQ76200
    • "PWRM" is connected to the CP_EN pin of BQ76200
    • "SAFE" is left floating

    If you need some other specific part of the schemetic, please let me know. If you need the entire schematic, I can send it directly to you, if you can point out how.

    Thank you again for your support and best regards

    Matteo

  • 0 in reply to Matteo Rigamonti
    TI__Guru* 81035 points
    Hi Matteo,

    I will investigate more on Monday with an EVM, but maybe you can try to play with the settings in the FET Options data flash. I noticed there are some non-default options set, so maybe one of these options is behaving in an a way that was not expected.
    Also, I noticed the ExtAverage cell voltage in the BQStudio window is lower than expected based on the cell voltages reported. This is the value measured on the BAT pin. Maybe this needs to be calibrated?

    Best regards,
    Matt
  • 0 in reply to Matt Sunna
    Expert 1170 points

    Hi Matt,

    thank you for your firsta feedback. I've already used the same FET Options configuration in another project without any problem, except the KEY_EN flag (in that project it was at 0). Anyway the KEYIN input properly works (as you can see in the KEYIN flag) and I tried also to deactivate it without success.

    Best regards

    Matteo

  • 0 in reply to Matteo Rigamonti
    TI__Guru* 81035 points
    Hi Matteo,

    I have loaded your settings onto an EVM and I am able to enable the FETs. I am also able to toggle the DSG FET with the KEYIN pin.
    When I first loaded the settings, I did see the exact same issue you described and my register screen looked the same as yours. However, the problem was due to the PRES pin on my EVM - I forgot to install the jumper to pull this pin low. I notice in your register screen that the PRES bit in the Operation Status is low - I think this may be causing the FETs to turn off.

    Matt
  • 0 in reply to Matt Sunna
    Expert 1170 points
    Hi Matt,
    as you can see in the above schematic, on PRES pin there is a 10k pull-down resistor.
    I've also measured the voltage on such a pin and it is rigorously 0.
    On the Technical Reference manual I read "If this feature is not required, then the PRES pin should be tied to VSS". So it is correct to have the PRES flag at 0 in the Operation Status register.
    Am I right?

    Thank you

    Matteo
  • 0 in reply to Matteo Rigamonti
    TI__Guru* 81035 points
    Hi Matteo,

    As an experiment, can you try to replace the 10k resistor with 0 ohms? The PRES pin should be tied directly to VSS. This pin is sampled every 250 ms, so the pull-up current is not always present. I think that might be why you measure 0V on this pin.

    Matt
  • 0 in reply to Matt Sunna
    Expert 1170 points

    Great!

    With a 0 ohms resistor instead of 10k the system seems to be unlocked: TCA, TDA, XCHG, XDSG now are at 0.

    Now I have to understand why CHG and DSG flags in Operation Status registe do not turn on:

    If you have any idea, please let me know.

    Thanks a lot!

    Matteo

  • 0 in reply to Matteo Rigamonti
    TI__Guru* 81035 points
    Hi Matteo,

    I'm not sure. I see a different result (CHG and DSG are both enabled), but maybe there is some other difference we haven't thought of. I will let you know if I have any ideas.

    Matt
  • 0 in reply to Matt Sunna
    Expert 1170 points
    Hello,
    I've further investigated this new issue and I've found it is related to an AFE Override alarm. The protection in case of such an alarm is not activated in the Enabled Protection register, but the BQ78350-R1 keeps CHG and DSG in off state.
    However the AFE Override alarm is not true: it disappears by changing the pull-down resistor on the Alert line from 1M to 500k (also if 1M is still in the suggested range - the BQ76930 datasheet says: "It is highly recommended to place an external 500 kΩ–1 MΩ pull-down resistor from ALERT to VSS").
    So that resistor change has solved my issue related to "locked FETs".
    Thank you and best regards

    Matteo