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BQ76940: TAKING CONNECTION FROM CELL15 FOR POWER SUPPLY MODULE

Syed Inamullah
Prodigy 40 points
Part Number: BQ76940

HELLO,

We are designing a 15S BMS using the BQ76940 AFE, we are having a ST32 MCU as a host controller and using UART and CAN for the communication. Due to the current limitations of the REGOUT pin of the AFE BQ76940, we are planning to use the external buck or LDO circuit for the power requirement of the MCU and CAN. I want to understand few things,

1- Can we take the tapping from the CELL15 for the power supply, does it effects the Cell voltage measurement and balancing.

2- As you can see in the schematic we are using the 6 pin and 10 pin connector configuration for connecting the 15S battery pack, as we are taking tapping from the CELL15 we have to get the output at BUCK OR LDO only after connecting the lower and upper connector (i.e 6pin connector and 10 pin connector) , but i have observed that we are getting the output at BUCK/LDO after connecting the lower connector (i.e 6pin connector), without even connecting the upper connector i.e 10 pin connector. Can u please explain why it is happening.

Note:- Buck/LDO circuit is not shown in this schematic.

  • 0
    TI__Genius 11315 points

    Hey Syed, 

    You can take directly from the top of the stack. It should not have an effect on cell voltage measurement and balancing. 

    Unfortunately the picture you provided has a very low resolution. Do you mind reuploading? It would also help to see the BUCK/LDO circuit. If you only have a tap on Cell 15, I'm not sure why it's getting powered when you only connect the bottom six cells. The device may be turning on with the voltage from the six batteries. The BUCK/LDO could be outputting if you have it connected to REGOUT. 

    Thanks,
    Caleb

  • 0 in reply to Caleb Perez
    Prodigy 40 points

    13S BMS REV-2 INTEGRATED.pdf

    Hello,

    Thanks for your valuable reply, i have attached the PDF of schematic for your reference. In the schematic single 16 pin connector is shown but on PCB we have used  6PIN (lower) and 10PIN (upper) connector configuration. As i have said before we are getting the output at the LDO just after connecting the 6PIN connector (lower), without even connecting the 10PIN connector (upper). 

    REGOUT is not connected to LDO/BUCK.

    BMS is working fine.

  • 0 in reply to Syed Inamullah
    TI__Genius 11315 points

    Hey Syed, 

    After looking at your schematic I don't think this is as issue with our device. I confirmed that your LDO is on the same NET as cell 15, so I too am confused as to why your LDO is receiving power. The only explanation I can think of is that the 6PIN connector is somehow shorted to the top of the stack, but there should be no electrical connection between the 6PIN connector and the Cell 15/BAT+ NET if the top 10PIN connector is not installed.

    Can you check to see if there is continuity on your board between Cell 5 input and BAT+/PACK+? Can you also check if there is current flowing out of the VC15 pin without the top 10PIN connector populated?

    Thanks,
    Caleb

  • 0 in reply to Caleb Perez
    Prodigy 40 points

    Hey,

    Thanks for your valuable reply. I have checked the PCB and it is working fine for all the logics (i.e Cell voltage measurements, Balancing, Charging & Discharging,  OV, UV, SC, OC etc), therefore there should be no issue at the connector side.

    Can u please check the circuit at the BAT pin. I think it is the BAT-VSS voltage that is appearing at the 15PIN before even connecting the upper 10PIN connector.

  • 0 in reply to Syed Inamullah
    TI__Genius 11315 points

    Hey Syed, 

    I think I've come up with a solution for you. It seems that when the bottom group of cells are populated, the device will turn on but will throw an XREADY fault on the account of the voltage being too low. This will prevent your MCU from setting the DSG bit, so your DSG FETs will be off. CHG has a load detect feature (see datasheet) which pulls down on the CHG pin. The pull down will come through your Q17 body diode, through R63, D23, and R84 into L/C-. That is how your LDO is being powered.

    Thanks,
    Caleb