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BQ40Z50-R2: Uneven LED brightness when using 4 (not 5 LEDs)

Benjamin Danziger
Prodigy 20 points
Part Number: BQ40Z50-R2

I'm using a BQ40Z50-R2 battery fuel gauge, and while the system seems to be working properly, we are seeing an issue where 2 of the 4 LEDs are a little dimmer on some of our assemblies. I am seeing LEDs 1 and 3 at the same intensity, and then 2 and 4 are at the same intensity. If I swap LED pairs (so the bright two are replaced with the dimmer two), the bright two are now dimmer and vice versa. I think it's related to how you can drive 5 LEDs, and I'm only using 4.

However, this is not happening on all of the BQ40Z50-R2 ICs, but when it does happen, it's repeatable and I can't seem to fix the issue without swapping out the BQ40Z50-R2 for another one. I have this configured just like the example image in the data sheet (figure 35), except LED5 is not present.

Has anyone else encountered this? Is there a fix for this?

  • 0
    TI__Mastermind 42535 points

    Hi Benjamin,

    That seems rather unusual. The only thing we do with the drive is charlieplex it. So, they will dim exponentially as the current across them is reduced. You may want to check if the drive strength varies by connecting a scope and a resistor to measure the current when they are driven.

  • 0 in reply to Batt
    Prodigy 20 points

    By putting a scope on each line, we were able to detect the issue, and it's a fundamental failure due to how Rev2 multiplexes between the three pins. I am missing LED 5 (connecting pins A to C), but the chip still tries to drive the 5th LED (by alternating through pins A, B, and C, and by switching each one from current sink, driver, or disabled). When the chip tries to turn on LED 5 (C is high, A is sink, B is disabled), instead, LEDs 2 and 4 are unintentionally activated. So, LEDs 2 and 4 are being cycled on longer than 1 and 3. This is resulting in the LEDs appearing brighter.

    I do not believe there is a firmware solution.

    A possible solution is to respin my board and put on a diode to act as an LED.

    There are too many complications to change to Rev 1 (which has it's own dedicated line for each LED), and switching to Rev 3 would also result in a more comprehensive redesign.

  • 0 in reply to Benjamin Danziger
    TI__Mastermind 42535 points

    Hi Benjamin,

    Yes, a diode will work as it has a nonlinear resistance to it. Thanks for replying back here with what you observed in your test.