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BQ76952: Added LED for checking CELL BALANCING

sung jin park
Prodigy 90 points
Part Number: BQ76952


Tool/software:

I would like to design additional LEDs to check the presence/absence of cell balancing on the PCB board.
I added the LED as shown in the picture below, so please check if it is properly placed.

  • 0
    TI__Intellectual 2840 points

    Hello,

    Yes, that is fine.

    Regards,

    Rohin Nair

  • +1 in reply to Rohin Nair
    Intellectual 485 points

    Are you sure about this Rohin ?

    For LD108, the current flows from CELL6 to CELL5. So when balance FET Q108 conducts (during balance) then the current flows through R126 to set the LED current. But the LED needs to be reversed to conduct current from CELL6 to CELL5. It is the wrong way around.

     you will need to be mindful of the forward voltage of the LED when calculating your R106 to set LED current.

    If you have defined the BQ76592 to start balance at a cell voltage of 3.4V (say for LFP) and the LED forward voltage is 2V (typically, check the datasheet) then to get 10mA LED current you need to set the resistor appropriately. If you assume the balance FET has low RDson and ignore it, then R106 = (3.4 - 2) / 0.01 = 140ohms. I see your R106 gives a higher LED current which is fine but also consider the current will rise as the cell increases to charge termination voltage. 

    I like your idea of the LEDs for diagnostics or during development. They could be part of the PCB layout and be left off in production to reduce cost. Nice one.

    All the best
    Harry  

  • 0 in reply to Harry McNally
    TI__Intellectual 2840 points

    Hello,

    Good catch Harry. I don't know how I did not notice the direction of the LED. The location of the LED is fine where it is for it to turn on during cell balancing, but yes it will need to be flipped/reversed.

    Regards,

    Rohin Nair

  • 0 in reply to Harry McNally
    Prodigy 90 points

    Thank you. Harry