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Original question:

LM3404EVAL: Flickering of the LEDs using an LM3404 especially at low intensity.

LM3404EVAL: LED flickering at low intensity levels Part 2

Gregory Dyczko
Prodigy 120 points
Part Number: LM3404EVAL
Other Parts Discussed in Thread: LM3404, LM3404FSTDIMEV, LM3402

The below schematic is the same one as in the original post.  I understand that the original post will be visible while looking at this related question.  

As originally discussed in the previous post we were driving multiple LM3404 drivers with the output of one AM26LS32.  The AM26LS 32 was excessively loaded down and could not produce an output signal capable of turning on the LM3404 sufficiently to turn on the LEDs at the lower intensity levels.  At higher intensity levels, longer PWM pulse width, the LEDs would turn on.  I'm guessing, but I would think it is the input capacitance of the multiple external mosfets that loaded down the AM26LS32.  

By using a additional mosfet to drive all the dim pins, that is drive the external mosfets, (on the right side of the schematic above) we were able to keep from loading down the AM26LS32.  All the lower intensity levels that did not work previously now worked.  All 15 different intensity levels now functioned and we were able to get the 25,000 to 1 contrast ratio.  

However, we still have a flickering at the lower intensities.  Our PWM base frequency is in the 200 to 250 Hz range.  We tried varying the base frequency thinking that multiples of 60 Hz could be the problem.  We used a function generator for precise control.  But when we thought we had a flicker free frequency, say 230 Hz, the next day the flicker would be back and a different frequency would appear to be flicker free. 

Questions:

1.  Do you have any idea on how to eliminate the flickering?  We have tried more than one power supply.

2. We do not have resistor in series with the gate of the external mosfets.  The TI eval board LM3404FSTDIMEV uses a 1 ohm resistor.  Do you feel we should add a series resistor to the external mosfet gate and how do you calculate the resistor value?

Thank you for your help.

  • 0
    TI__Genius 13001 points

    Hi Gregory,

    Please try to decrease the Ron to increase switching frequency. I think it will be helpful.

    Robin

  • 0 in reply to Yuting Shao
    Prodigy 120 points

    Robin, Thank you for your response.

    We followed your suggestion and raised the switching frequency.  This helped with the flicker but did not completely eliminate it.  We ran into the issue of reduced output voltage when the switching frequency is raised.  With a 36 volt input the output voltage was clipped when we powered a 6 LED string of green LEDs.  We split the 6 LED string into two strings of 3 series LEDs in parallel.  This eliminated the output voltage problem but we still have some minor flickering at low intensity levels.  Are there any issues we need to know about when using series parallel strings?

    Questions:

    1. Our PWM frequency, 250 Hz, is several orders of magnitude lower than the switching frequency but we still have minor flickering.  Do you have any additional suggestions that you think might help us with the flickering?

    2.  See question 2 above please.  Is the 1 ohm resistor on the gate of the external Mosfet, on the LM 3404 evaluation board, to limit the in rush to the external Mosfet or is there another purpose?

    Thank you for your time

  • 0 in reply to Gregory Dyczko
    TI__Intellectual 2925 points

    Our experts for this issue are currently celebrating the Chinese national holiday, and they will respond next week.

  • 0 in reply to Gregory Dyczko
    TI__Genius 13001 points

    Hi Gregory,

    Since LM3403 uses DIM to turn on/off internal FET to achieve PWM dimming, so large inductor is as large as 192uH, even FET is turned off, there is still small continuous current flowing through LED. Please decrease inductor and try again.

    Robin

  • 0 in reply to Yuting Shao
    Prodigy 120 points

    Good evening.

    By the Dim pin, do you mean pin 3 of the LM3404 or are you talking about the "Lamp Dim" on the right side of the above schematic?  From your answer I believe you are talking about Pin 3 of the LM3404.  We tie that pin high as you can see from the schematic. 

    We experimented with the AN-1839 LM3402/LM3404 Evaluation board and used both the original eval board 33uH coil and our 192uH coil and still had the random flickering..

    I keep asking about the 1 ohm resistor in series with the external Mosfet on the 1839 Eval board.  Is this necessary.  What is its purpose and should we add it to our circuit? See question 2 above.

    Thank you

  • 0 in reply to Gregory Dyczko
    TI__Genius 13001 points

    Hi Gregory,

    The resistor is not necessary and it's used to adjust the rising/falling time of external MOSFET. In theory, the rising/falling time is the shortest without this resistor.

    Robin