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BQ79656-Q1: Troubleshooting

Asadbek Shamsiev
Prodigy 10 points
Part Number: BQ79656-Q1

Hello,

I am an electrical lead for the Virginia Formula SAE team and we are working with the TI BQ79656 chips. We are attempting to design a custom BMS for an accumulator made up of 100 cells in series. Our battery configuration is within the specs of these chips. We have made a prototype 84V battery for testing, and we have built the peripheral circuits on the breadboard. We are now trying to power the chip on and communicate with it using the TI USB-2-ANY module. 
With the following circuit configuration, we have had some success sending and receiving UART frames. However, the frames do not seem to mean anything and we cannot get meaningful communication going. 
Beyond the above Circuit configuration, we have had no success sending or receiving data from the chip. After implementing the NPN Transistor power path, we are unable to communicate. The same is true if we connect any of the VC or CB pins to the prototype battery. In addition, during the majority of this testing, the chip is struggling to maintain the correct voltage at its internal sources such as CVDD, AVDD, and LDOIN, especially after adding bypass capacitors. It is our theory that the device is not getting enough power to function properly, but we have been unable to fix it by adjusting resistor values into the NPN transistor power path. 
The full implementation of our circuit (which should be functional according to the users manual) is shown below. We were unable to get any sort of meaningful communication with this configuration. 

Please let me know if you can help at all based on the information that is provided. I would love to start a dialogue with someone who has technical knowledge of this product and can help me during the debug process. I am happy to call, email, or provide as much information as needed. Thank you for your time, and I really hope to hear from you soon.
  • 0
    TI__Intellectual 1845 points

    Hello

    It looks like you do not have any GND nets on any of your decoupling caps. Please see the below diagram and add gnd nets to all of your decoupling caps where GND is required.AVSS, DVSS, and CVSS are all GND reference, you need to actually attached them to a GND path as well. Also, it seems the NPN circuitry is incorrect. You should have a LDOIN cap referenced to GND near the pin and a capacitor on the collector of the NPN near the NPN collector as well referenced to GND. I believe the GNDing of the PCB is a problem since I do not see any GND net.

  • 0 in reply to Chase Fagen
    TI__Intellectual 1845 points

    On page 187 of the BQ79656 datasheet you should see the proper circuitry. Right now your capacitors on the NPN emitter and collector share the same node, they should not.

    Thanks,

    Chase

  • 0
    Prodigy 30 points

    Hello,

    I am another member of the electrical team working with Asad on this custom battery management system. Thank you for the reply. We do actually have all of those decoupling caps referenced to ground as in the datasheet, and our NPN circuitry is as you described. I simply forgot to add a GND net label in the schematic that I sent you. Our circuitry matches your descriptions in your response as far as I can tell. 

    We ordered new NPN transistors that are in spec with the TI evaluation module that you sell to test this chip, but that is the only lead we have on what to try next. Thanks again for your response, I hope to hear more from you soon about your thoughts. 

    Best,

    Silas Schroer

  • 0 in reply to Silas Schroer
    TI__Intellectual 1845 points

    If the circuit matches the schematic and diagram above, you are good to go. Since LDOIN powers the IC, it is essential the NPN is properly selected with sufficient headroom to power the device. If the regulators aren't working, then it is definitely a power issue. You need the capacitors on each pin and close to the pin of each device. It is important to build the entire circuit fully and then test rather than building subsets of the circuits piecewise.

  • 0 in reply to Chase Fagen
    Prodigy 30 points

    Ok, makes sense. I will check back in when we get the transistor. We are doing all of this testing on a breadboard before designing our PCB. Will this chip function on a breadboard/breakout board if the circuit is correct?

  • 0 in reply to Chase Fagen
    Prodigy 30 points

    Hey Chase, thanks for the reply. We got the NPN and it is well within spec to power the device. We are still having a lot of trouble with the regulators of the device. LDOIN pin is sitting at 4.5 volts, CVDD is sitting at 4.5 volts, DVDD is sitting at 400 mV. I have attached pictures of our breadboard test circuit connected to 6 battery cells in series. Do you see any issues with how we've set up the breadboard circuit? Do we need to design a PCB to meet the strict requirements for Coupling Capacitor placement and Grounding schemes?

  • 0 in reply to Silas Schroer
    TI__Intellectual 1845 points

    Hi,

    You can email me at c-fagen@ti.com

    thanks,

    chase

  • 0 in reply to Silas Schroer
    TI__Intellectual 1845 points

    Hello,

    It appears there is still issues with the NPN circuitry since LDOIN is only 4.5V when it should be 6V. I would try isolating just this subcircuit and debug from there.

    Thanks,

    Chase