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Original question:

TPS92512: LED flickering problem

TPS92512: PWM duty cycle to LED output

Jay Kim
Intellectual 475 points
Part Number: TPS92512

Hello,

- Using TPS92512 evaluation board

- Input power to board : 41V

- LED : 12(Serial) * 18(Parallel)
  LED stack voltage : 36V    


Had test full range of PWM duty(0~100%) and getting output(Watt) as expected except one point.

When setting PWM duty as 25%, output(Watt) goes 14watt first but "gradually" down to 8.5watt.(I'm expecting 12watt).

Any reason to this?

Regards,

Jay

  • 0
    TI__Mastermind 28655 points
    Hello Jay,

    Can you measure Vout and Vin when this is happening? What is the PWM frequency?

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

    [Ampere measured at duty cycle 27%]

    [Ampere measured at duty cycle 25%]

    Hello,

    PWM frequency is 400Hz(I had also try 1kHz, 12.8Khz, 25,6Khz... but see the same issue)

    1. PWM Duty : 27%

    (1) Vin : 40.71V  

    (2) Vout : 32.40V

    (3) Output : 14W

    2. PWM Duty : 25%

    (1) Vin :40.85V    

    (2) Vout :31.84V

    (3) Output : 8.5W

    Duty change between 25~27%, there's no output change happens.

    Output change only happens above 2 specific points and I see

    there's ampere drop&increase. Please refer attached photos.

    BTW, your mention on my previous post helps me to resolve my issue.

    After increase Vin from 36V to 41V, LED flickering is gone. Thanks a lot.

    user:110892

  • 0 in reply to Jay Kim
    TI__Mastermind 28655 points

    Hello Jay,

    I'm not sure what I'm seeing in your oscilloscope pictures.  The waveform appears to be about 16 KHz, I see about 60 uS between pulses?  If you are PWMing at 400 Hz, what is your switching frequency?  And what is the 16 KHz from?

    Regards,

  • 0 in reply to Irwin Nederbragt
    Intellectual 475 points

    Sorry, attached pictures captured while testing with PWM frequency 16kHz.

    Anyway, same problem happened with any PWM frequencies. (I also had tried below 1kHz like 400Hz, 800Hz...etc).

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

    Can you send the pictures at lower PWM frequency. 16 KHz is beyond the recommend operating range and can interact with the switching frequency so I cannot tell what is happening. At 400 Hz or 1 KHz I should be able to see the current ramp up, start switching and drop back down.

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

    Hello,

    Tested with PWM Frequency 400Hz & 800Hz.

    "current drop" is happened at lower PWM Duty.

    1. Current drop - PWM Frequency 400Hz

    2. Current drop - PWM Frequency 800Hz

    Regards,

    Jay

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

    From your original post:

    "Hello,

    PWM frequency is 400Hz(I had also try 1kHz, 12.8Khz, 25,6Khz... but see the same issue)

    1. PWM Duty : 27%

    (1) Vin : 40.71V

    (2) Vout : 32.40V

    (3) Output : 14W

    2. PWM Duty : 25%

    (1) Vin :40.85V

    (2) Vout :31.84V

    (3) Output : 8.5W

    Duty change between 25~27%, there's no output change happens.

    Output change only happens above 2 specific points and I see

    there's ampere drop&increase. Please refer attached photos.

    The recommended PWM frequency range is 100 Hz to 1 KHz."

    The above pictures are not at 27% or 25% duty. You are at a very low pulse width where the compensation may not be able to be stable due to the delay in the ramp and drop of the inductor current. What is the pulse width of you PWM?

    This is from the datasheet:

    8.3.5 PWM Dimming (PDIM Pin)
    The TPS92512 incorporates a PWM dimming input pin, which directly controls the enable/disable state of the
    internal gate driver. When PDIM is low, the gate driver is disabled. The PDIM pin has a 1 μA pull-up current
    source, which creates a default ON state when the PDIM pin is floating. When PDIM goes low, the gate driver
    shuts off and the LED current quickly reduces to zero. A square wave of variable duty cycle should be used and
    should have a low level below 0.79 V and a high level of 1.45 V or above.
    The TPS92512 uses a sample-and-hold switch on the error amplifier output. During the PDIM off-time the COMP
    voltage remains unchanged. Also, the error amplifier output is internally clamped low. These techniques help the
    system recover to its regulation duty cycle quickly. The dimming frequency range is 100 Hz to 1 kHz and the
    minimum duty cycle is only limited in cases where the BOOT capacitor can discharge below its under-voltage
    threshold of 2 V (VIN is within 2 V of the total output voltage).

    If you are needing very narrow pulses you may want to look at a different part and shunt FET dimming.

    BR,
  • 0 in reply to Irwin Nederbragt
    TI__Mastermind 28655 points
    I have not heard back from you so I believe you may have solved the issue.
    The thread will close now but will reopen if you reply.
  • 0 in reply to Irwin Nederbragt
    Intellectual 475 points

    Hello,

    Reduce PWM frequency as 4kHz, problem solved but with this, noise sound generated from inductor(I guess).

    Currently looking into it. and I will open additional thread if need help.

    Thanks & regards,

    Jay