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BQ76940: BQ76940+BQ78350-R1+ BQ76200 Custom Board

R Madhavan
Prodigy 45 points
Part Number: BQ76940
Other Parts Discussed in Thread: BQ76200, , BQ78350-R1, BQSTUDIO

Hi Everyone,

I have made a custom board using bq76940, bq78350-R1, and bq76200 for my 14S battery pack configuration. I powered it 54V at BATT+ and BATT- pin and given all sensing input to it (C0-C15, While C13-C14 shorted). Before applying voltage at TS1, I measured voltage at following points

BATT: 54V, VC5X:3.8V, VC10X:38.7V, REGSRC: 2.8V, CAP1: 54V

VC0: 0V, VC1: 2.6V, VC2: 6.7V, VC3: 6.1V, VC4: 5.2V, VC5:15.94

VC6: 16.9V, VC7: 27.1V, VC8: 31V, VC9: 34.9V, VC10: 38.8V

VC11: 42.7V, VC12: 46.6V, VC13/14: 50.6V, VC15: 54.5V

Group VC0-V5X voltages are not proper and voltage sensing filter circuit of this particular group is getting heated.

Should I apply voltage at TS1 pin for bq76940 device boot up? What could be the possible reasons for this mismatch?

Thanks in advance.

  • 0
    TI__Guru 54715 points
    Hi,
    The 54V on CAP1 exceeds the abs max of 3.6V and has likely damaged the bottom group of the bq76940 causing multiple resistive shorts from pins to VSS. Look for a short on the board from CAP1 to BATT. If you have referenced a bipolar to CAP1 for an LED power supply similar to the EVM be sure it is a high gain NPN sufficient for the BATT voltage and is not damaged, a shorted transistor can damage the bq76940, bq78350-R1 and LEDs. Once the short is corrected you will need to replace any other damaged components.
  • 0 in reply to WM5295
    Prodigy 45 points

    Thanks for your reply. LED power supply NPN transistor was damaged. Now before powering similar board, I removed NPN transistor (will upgrade it with high voltage rating). After applying voltage from a 14S pack, initial measurements of new board are:

    • BAT: 54V
    • VC10X: 38V
    • VC5X: 19V
    • REGSRC: 18V
    • TS1: 0V
    • CAP1: 0V

    Applied voltage at TS1=2.6V, CAP1: 3.27V, REGOUT=2.5V and on the gauge side VCC=2.5V, MRST=2.48V, BAT: 2.38V, VEN= 2.25V, PWRM=0V

    Bqstudio is not detecting the hardware through EV2300. When I connect EV2300 it shows USB mark as active but soon as I connect it with SMBD and SMBC port, even USB connection mark disappears.

    What could be the possible reasons for this?

  • 0 in reply to R Madhavan
    TI__Guru 54715 points
    With PWRM 0 and VEN 2.25 it seems the bq78350-R1 gauge is not running. PWRM is normally high and VEN pulses. BAT input should normally be about 1V or less, you might check the divider ratio. To see if the gauge is running, check the I2C lines to the '940, you should see bursts of activity every 250ms. Continuous activity would indicate the gauge can't find the AFE, no activity would indicate the gauge is not running.
    Also look at the SMBus lines, they should be pulled to 3.3V by the EV2300. Higher voltage from the board would be some risk of damage to the EV2300. 0V should be a normal condition but perhaps could cause some reset of the EV2300. If the gauge is not working you must find out what is connected wrong or disconnected. Or if its damaged replace it.
    If you have a working EVM you might confirm operation of the EV2300 and bqStudio.
  • 0 in reply to WM5295
    Prodigy 45 points

    Hi,

    Thanks for your reply. I really appriciate it. Here are the observation based on your previous reply

    1. Corrected voltage divider ratio for BAT input
    2. SMBus lines at EVM 2300 side is pulled high to 3.3V
    3. No activity is seen at I2C line
    4. PWRM is still zero and VEN is pulsating.

    Looks like gauge is damaged. Replacing gauge for now. Meanwhile if you can please check my schematics once it will be very helpful. How can i send you my schematics file?

    Thanks in advance.

  • 0 in reply to R Madhavan
    TI__Guru 54715 points
    Hi,
    We corresponded offline and think you have a good approach to proceed.
    For other readers, several of the signals from the bq78350-R1 are open drain and must have pull ups. When debugging a design it is good to check the integrity of the pull up resistors.