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LM3409: Different start-up time on LM3409

Sherman Xin
Prodigy 350 points
Part Number: LM3409

Hi Team,

Nice to talk with you. We meets a problem on LM3409.
The LM3409 is used in a 12V power supply, and it is found that the consistency is not good when it is turned on, the lights are on at different times. and the voltage waveform across the LED is as below:

         

The quicker one's start time is about 3.94s, and the slower one's start time is about 7.44s.

We exchange the LM3409 IC in two boards, as a result, the one that starts fast slows down, and the one that starts slow gets faster. It means this is an IC problem, right?

So could you help us review on this and give some comments here?

Best regards,

Sherman

  • 0
    TI__Mastermind 18890 points

    Hello Sherman,

    What is the intent here?  Is is to turn on the LED faster?  3.94 second is a long time.  You should be able to driver the current to the LED a lot faster then that.  Please share schematic and LED load and current control level. 

    From your comments above, the input voltage is 12V and from scope capture.  From the scope capture you are showing 40V output...This is a Buck controller which steps down the voltage which would make output voltage less than 12V so I a really confused here.

    Thanks Tuan

  • 0 in reply to Tuan Tran74
    TI__Prodigy 350 points

    Hi Tuan,

    Thanks for your reply! Sorry that I input the incorrect unit. It should be 12W power rate. And input voltage is 55Vdc, output 39Vdc. 

    In this project, customer designs 3 seconds to start to low brightness status(1% full brightness), and totally 8 seconds to start to full brightness status. They control the output voltage 27V to realize low bright status and output voltage 39V in full brightness status. They use 10 LEDs in parallel and always 2-3 LEDs turn on to low brightness status with a long time for 7s-8s, this situation is random and follows with ICs(LM3409). Could you help review the schematic? I have sent it to you by email. Thanks! 

    Regards, Sherman

  • 0 in reply to Sherman Xin
    TI__Mastermind 18890 points

    Sherman,

    We will close this tread and handle it via email.

    Thanks Tuan