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BQ76952: CHG FET turns on and off on its own in autonomous mode

Mrinal Mani
Prodigy 215 points
Part Number: BQ76952


Tool/software:

I have developed a custom board with high side FET and low side current sensing with BQ76952.

I have configured the VCELL_MODE, balancing parameter and cut-off levels. I have enabled autonomous FET control using FET_EN register.

I have tested a charge and discharge cycle and the BMS works all good, cutting off when required and giving proper current reading

However, on relax state, when the output terminal are open, I noticed that the CHG FET turns on and off on its own. I have configured and attached LEDs to the DDSG and DCHG pins and am able to visually see it. The DSG FET remains on.

Is this normal behavior? I have scanned all Safety Status and Alert register and no flag is raised. Please guide me on this behavior. 

  • 0
    TI__Mastermind 18955 points

    Hello Mrinal,

    This sounds like it may be the device’s body diode protection at work. It helps protect the CHG FET from being damaged. Section 5.2.3.1 FET Configuration in the BQ76952 Technical Reference Manual explains this feature more. 

    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    Prodigy 215 points

    As per the manual, if one of the FETs in off, due to a fault or in sleep condition, and diode current exceeds limit, let's say 50mA, the FET will turn on. When the diode current falls below 50mA, it will turn off again if the fault condition is still present. Does this imply that it will not turn off if there are no faults?

    In my case, the BMS is in relax state. There are no faults. If the current is 0A, both FETs should be on, right?

    In the BMS, the idle current, with output terminals open, is fluctuating between -5mA to something around -40mA. As per my understanding, since there is no fault, both FETs should stay on. If there was a fault, let's say COV, then CHG FET would be off. In such case if a discharging current of >50mA flows, then CHG FET would turn on. Similarly, if there was CUV fault, DSG FET would be turned off and turn on again if charging current > 50mA. For normal conditions, both FET should be on.

    Is my understanding correct? 

  • +1 in reply to Mrinal Mani
    TI__Mastermind 18955 points

    Hello Mrinal,

    If the FET was already OFF before it turned ON from the body diode protection, it will turn back OFF if the fault is still present. 

    Both FETs should be ON normally unless you send a command to turn it OFF or the device enters certain modes. For example, if the SLEEPCHG bit is not set in the Settings:FET:FET Options:SLEEPCHG , the part will disable the CHG FET during SLEEP mode while the DSG FET is enabled. This is mentioned in Section 5.2.2 High-Side NFET Drivers of the BQ76952 TRM, in the paragraphs after Manual Control. 

    Do you mind checking that bit to see if it's enabled or not?

    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    Prodigy 215 points

    SLEEPCHG was by default set to 0 and the system was in sleep mode as check in Battery Status Register Bit15.

    On setting SLEEPCHG to 1, the CHG FET is now continuously on.

    Thank you for helping me out with this.