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LM3409: What's will happens when the UVLO setting lower than LED forward voltage ?

Shaquille Chen
Intellectual 1770 points
Part Number: LM3409

Dear Team,

What's will happens when the UVLO setting lower than LED forward voltage ?

One of our customer feedback that their LED damage when they set UVLO lower than Vf.

Thanks

Shaq.

  • 0
    TI__Mastermind 28655 points
    Hello,

    The LM3409 is a peak current regulator. If Vin gets near the LED stack voltage the LED current can rise due to the shape of the inductor current waveform. When the applied voltage to the inductor gets small (Vin-Vled) the inductor charging starts to look like an 'R-L" instead of just and 'L'. The highest it can possibly go is just below the peak current trip threshold which is Iaverage plus 1/2 of the peak to peak current ripple. If the current ripple is not extremely high I wouldn't expect this to cause damage to the LEDs. What is their Input voltage range, output voltage, inductor value, output current, Roff, Coff, Rcs? Is Roff connected to the output? Is there any external circuitry such as shunt FET dimming?

    Regards,
  • 0 in reply to Irwin Nederbragt
    TI__Intellectual 1770 points
    Hi Irwin,

    Yes, Roff connect to output.
    They used shunt FET dimming.


    The spec. and component value as below:

    Input: 50V
    Output: 43.5V/1A

    Inductor : 15uH
    Roff: 33.2k
    Coff: 470pF
    Rcs: 60mohm

    Thanks
    Shaq.
  • 0 in reply to Shaquille Chen
    TI__Mastermind 28655 points
    Hello Shaq,

    So, their peak to peak ripple is quite high. As you Vin approaches Vled the current can go up by more than 50% at a very small input voltage window. This can be improved by going to a larger value inductor.

    Is Viadj at approximately 500 mV? That is what I back calculate from Iled = 1A and the ripple current.

    Thanks,