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LM324: LED premature failure

Willer Justinvil
Prodigy 10 points
Part Number: LM324
Other Parts Discussed in Thread: TLV9104, TL074H

I am using a blue LED from Rohm in our design.

I have gotten some suggestions from other sites, but I wanted to give you all a chance to take a look at the design.

We have been having premature LED failure within a month from the field. I am not sure what's going with the LED driver. I have attached the driver circuit/datasheet for your review.

sli-sla-580.pdf

  • 0
    Guru 261180 points

    A DAC value of 5 V would result in 42 mA, which would burn out the LED. What input voltage range are you actually using?

    What are the LM324's supply voltages?

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Hi,

    Thank you for the feedback. See attachment for the LM324 Supply voltages.

    Thanks,

    WJ

    +12V  and -5V

  • 0
    TI__Guru*** 143096 points

    Hello Willer, welcome to the TI e2 forums.

    After the LED was replaced, does the LED driver preform normally? Meaning R59 pin 1 matches the DAC voltage. 

    Clemens has a good point that that current of the driver can exceed the maximum current of the LED if DAC is set to high.

  • 0 in reply to Ron Michallick
    Prodigy 10 points

    Hi Ron,

    Thanks for your feedback, yes the LED driver does perform normally after replacement. I agreed with Clemens in term of exceeding max current and I am planning on monitor the DAC voltage to see whenever the voltage spike happens (@5V).

    Willer

  • 0 in reply to Willer Justinvil
    Guru 261180 points

    I asked about the supply voltages because the LM324's valid common-mode input voltage range ends 2 V below the positive supply (typically 1.5 V at 25 °C). So with a +4 V supply, any DAC voltage of 2 V (typically, 2.5 V) or higher can lead to the LM324's output being incorrect, resulting in a LED current much higher than even 42 mA.

    I assume that you are using a 4 V supply because the DAC voltage never is higher. But if there can be voltages between 2 V and 4 V, then you should use opamps that allow voltages near the positive supply (e.g., TL074H) or that have rail-to-rail inputs (e.g., TLV9104).