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TPS92512: LED flickering problem

Jay Kim
Intellectual 475 points
Part Number: TPS92512

Hello,

- Direction : MCU(PWM Generate) -> PWM0 -> DIM0 -> TPS92512(PDIM)  -> J10(Output to LED)

[Case 1]

- Input to device from SMPS : 36V

- LED : 12(Serial) * 12(Parallel)

LED flickering upon PWM duty 60% ~ 0%

[Case 2]

- Input to device from SMPS : 45V

- LED : 15(Serial) * 12(Parallel)

No LED flickering

Any component change can help [Case 1] problem?

Regards,

Jay

  • 0
    TI__Mastermind 28655 points
    Hello Jay,

    What is the LED stack voltage? If there are 12 LEDs in series and they are 3V forward the stack will be 36V.

    Regards,
  • 0 in reply to Irwin Nederbragt
    Intellectual 475 points

    Hello,

    Yes correct. LED package's forward voltage is 3V. so Stack voltage would be 36V.

  • 0 in reply to Jay Kim
    TI__Mastermind 28655 points

    Hello Jay,

    If Vout is 36V and Vin is 36V you cannot regulate current.  Have you looked at the current waveform and switching node.  If the output and input are near the same the MOSFET can be on or switching at a very low frequency.

    Regards,

  • 0 in reply to Irwin Nederbragt
    Intellectual 475 points

    Hello,

    In my previous posting,  [Case 2] was

    Input to device from SMPS : 45V and LED : 15(Serial)*12(Parallel)

    so Vout and Vin are also near the same but is working well with no flickering.

    Attached photo is current measure when LEDs are flickering.

    Any component value change can help?

    1. PWM 250Hz, 100% duty

    2. PWM 250Hz, 90% duty

    3. PWM 250Hz, 80% duty

    Regards,

    Jay

  • 0 in reply to Jay Kim
    TI__Mastermind 28655 points

    Hello,

    Can you try a few things:

    Either short out one of the series LEDs (all that are in parallel) to make the stack 11 LEDs instead of 12.  Look at and compare the switching waveform of 11 and 12 LEDs.  It is unstable with 12 LEDs but that can be for a few reasons such as hitting maximum duty cycle.

    Or try raising the 36 volts up to 39 volts or so and see what difference there is in the switching and PWM waveforms.

    If the LED stack is 36 volts and you are providing 36 volts this will not work.  Also make sure the power source is stable.

    If you think this is a stability issue you can try and increase the comp capacitor value but suspect that will not fix this.

    Regards,

  • 0 in reply to Irwin Nederbragt
    Intellectual 475 points
    Hello,

    Regarding your comments,
    1. Raising input voltage to 39volts using digital power supply(not SMPS) actually helps problem.
    But we can not modify given SMPS so no further test could be done.
    2. We still didn't increase comp capacitor yet.

    Besides, we had change TPS92512's switching frequency and see what happens
    and changing switching frequency to 190kHz(it was 400kHz), flickering (which can be observe by human eyes) seems completely gone.

    But we're now making "flicker-free" products, more enhancement needed and asking which approaches we can do to reduce flicker.
    Using flicker meter(I think it's measure around 120Hz signals), flicker percent measured now is about 10~12% and we need to achieve below 3%.

    Regards,
    Jay
  • 0 in reply to Jay Kim
    TI__Mastermind 28655 points

    Hello Jay,

    The input voltage and output voltage cannot be the same on a buck converter.  Either the input voltage needs to be higher or the output strings needs to have a lower voltage.  If you run the 12 series LED from 45 volts it probably will work correct.

    As for measuring flicker, the buck converter has to be operating correct first.  It looks like it's oscillating but that can be from having the input and output voltage near the same.  Do some testing with higher input voltage, 45V input for 12 series load and see if it operates correct, then you know the circuit is working.

    Does it appear to start working better after it is on for a while (heating up)?

    Regards,