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BQ76952: DSG FETs shut off when under load

coco walsh
Prodigy 40 points
Part Number: BQ76952
Other Parts Discussed in Thread: BQSTUDIO

Hi Everyone,

We are using Test Mode, and DSGTEST() 0x0020 to manually test enabling the Discharge FETs We are able to enable our Discharge FETs when there is no load connected - only a voltmeter and oscilloscope. We see 48 volts on Pack+, it seems to work fine.

When we connect a load to Pack +/- and issue DSGTEST(), The DSG pin goes high for about 20uS, then the Alert Pin goes high for about 20uS, and the Discharge FETs do not turn on.  

When a load is connected while the FETs are already on, the FETs switch off.

There are no alerts/faults in any of the registers when we check via SPI. Presumably, this is due to the alert/fault state being present for only 20uS, and our SPI reads are missing that brief window. 

We have tried disabling all protections, but experience the same results. We have calibrated the Current Sensor to match our sense resistor (0.0001 Ohms), and we have tried raising the Short Circuit in Discharge Protection threshold to maximum (500mV).

Do you have any ideas about what could be going wrong?

  • 0
    TI__Mastermind 25700 points

    Hi Coco,

    Could you please provide both your .gg config file and a schematic for us to check?

    Regards,

    Max Verboncoeur

  • 0 in reply to Max Verboncoeur
    Prodigy 40 points

    Thanks Max, 

    Can you provide more information about how to obtain a .gg file?  We're not using BQstudio. We're communicating with the BQ chip via an SPI connection from a microcontroller using custom software. We've iterated through many different configurations in testing - all with the same results. Are there particular registers you're interested in? 

    The schematics are attached. The BQ is on the cell-bank-module, and its DSG and CHG pins are connected to the DSG and CHG inputs on the power-plane via wires/screw terminals. 

      

  • 0 in reply to coco walsh
    TI__Mastermind 25700 points

    Hi Coco,

    The schematic looks good, I just want to add that we recommend a 10MOhm pull-down on BREG and that having a thermistor on TS2 will make it so that you cannot use the SHUTDOWN mode functionality of the device.

    Sharing any settings that you are changing from the default values would be helpful.

    Could you please measure the voltage across the capacitor from CP1 to BAT during this behavior?

    Could you also capture a waveform of the DSG and ALERT pins exhibiting this?

    Regards,

    Max Verboncoeur

  • 0 in reply to Max Verboncoeur
    Prodigy 40 points

    Thanks Max, 

    Here's a screenshot: 

    Yellow is DSG, Purple is Alert, Light Blue is PACK+, Dark Blue is BAT+ 
    We might have some additional screen grabs later today. 

  • 0 in reply to coco walsh
    Prodigy 40 points

    Hi Max,

    Another interesting symptom: Before we run DSGTEST(), we disable SLEEP ENABLE.  After we run DSGTEST(), we find that the device has enabled SLEEP ENABLE again. Is there some sort of reset that might be happening? 

  • 0 in reply to coco walsh
    Prodigy 40 points

    HI Max, 

    We got past our impasse by disabling Test mode.  In test mode, issuing DSGTEST() - or some other setting changes - caused the BQ to "reset" as described in our previous reply. 

    We're now able to continue working. 

    Thanks for your help  

  • 0 in reply to coco walsh
    TI__Mastermind 25700 points

    Hi Coco,

    I'm glad to hear that it's working now!

    Regards,

    Max Verboncoeur