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BQ40Z50-R2: Can LEDCNTLx pins be externally driven to test LED operation during production testing?

Charlie D
Intellectual 275 points
Part Number: BQ40Z50-R2

Hi,

Can the LEDCNTLx pins be externally driven to test the functionality of the LEDs?  I am envisioning attaching an external current source of 1-2 mA to the LED control pins.  Will this cause damage to the BQ40Z50-R2 chip?  This will occur after the PCB is fully assembled and programmed.  I realize that the BQ40Z50-R2 does have an "LED_TOGGLE" SBS command designed to verify the functionality of the LEDs.  We are also considering testing via that method but want to know all our options.

Thanks,

Charlie

  • 0
    TI__Guru** 105760 points

    Hello Charlie,

    You can change the current sink for the LEDs depending on your configuration in the LED config register.

    It is in section 15.2.1.7 LED Configuration of the TRM.

    I would set the bits to whatever current your LED/tests will use.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Intellectual 275 points

    Wyatt,

    I realize that the LED Configuration register can control the LED current sink.  However, what I want to know is if driving the LEDs with an external current source will damage the current driver on the BQ40Z50-R2 or the LEDCNTLA, LEDCNTLB, and LEDCNTLC lines.  We would be connecting the current source to LEDCNTLA and LEDCNTLB, then applying 2 mA to light one of our LEDs.  Then we would reverse the polarity, then do the same with lines B and C, etc...

    My interpretation of the "LED Drive" diagram (Figure 2) on page 6 of the Rev. B datasheet here: https://www.ti.com/lit/ds/symlink/bq40z50-r2.pdf is the following:

    The LEDs are charlieplexed.  There are 3 instances of the LED Drive circuitry, so there are RLOENA, B, and C, etc... connected to LEDCNTLA, B, and C.  If RLOENA is closed, it connects LEDCNTLA to battery voltage (VBAT).  If RLOUT is closed, then LEDCNTLA is connected to ground (GND).  If neither switch is closed, LEDCNTLA is in high Z state.  Therefore, when no LED is on, all three control lines are in high Z, and the appropriate line is connected to VBAT and GND to turn on the appropriate LED.

    Since we want to connect an external current source to the LEDCNTLx lines, we would be doing this when all control lines are in a high Z state.  Since the LED Drive pin equivalent diagram is just a high level view, not to mention I am not sure what RLIN is, I do not know if the actual circuitry can withstand being biased to a positive voltage.

    Also, changing the LED Configuration register to change the LED current sink, I believe, changes the duty cycle of the charlie-plexing.  The average LED currents listed are 1/6th, 1/12, and 1/24ths of 22.5 mA.  Therefore I do not believe that changing these settings would have any affect on the robustness of the circuit.

    Thanks for your help,

    Charlie

  • 0 in reply to Charlie D
    TI__Guru** 105760 points

    Hello Charlie,

    I understand better now, you're only wondering if the pins can be damaged by applying a external supply when the pins are in high Z or when they're on? or just when they're in a high Z?

    The pins can tolerate any voltages within the absolute max rating, so if you're applying a voltage to drive the LEDs and it's within the voltage range, it should be fine in either state of the LED pins.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Intellectual 275 points

    Hi Wyatt,

    Yes that is correct.  Specifically I am only wondering about when the pins are in high Z.  But it is good to know that these pins will also be safe when the LED pins are active.

    Thanks for your help,
    Charlie