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Original question:

TLC5946: Junction to Package TempCo

TLC5946: Some question about the application of TLC5946.

Lumina Gui
Genius 3850 points

Part Number: TLC5946

Hi TI Teams,

We encountered some problems when debugging the driver of firmware TLC5946.

Hope to get support an answers for the below two questions, please convey it.

 1) Setting Dot Correction mode. The LED brightness can be adjusted normally only when 1(dot correction value) step is sent. But when the value is greater than 1, the LED brightness cannot be adjusted normally. And the maximum PWM value is only 33/4095.

2) Grayscale PWM control reference clock (GSCLK) have to be provided by MCU?

By the way, the schematic diagram is shown as below:

Thanks,

Kind Regards

  • 0
    TI__Mastermind 23490 points

    Hi Lumina,

    Glad to help you.

    1) Deduced from you description, I guess a possible reason is that VLED is too low or you put too many LED in serials (Vf is too large), and therefore you don't meet the headroom voltage requirements of the current sinks. That may make the current sinks unable to sink larger current. Also, this may cause channel output voltage below LED open detection threshold and trigger a false LOD detection. This will further trigger "AUTO OUTPUT OFF" function (please refer to datasheet Page 19), which turns off OUTn at the 33rd falling edge of GSCLK after the falling edge of BLANK. I guess this is the reason why you see a maximum PWM value of 33/4095. 

    Further steps:

    • You can check whether a false LOD detection is triggered by monitoring the XERR pin.
    • If so, you can try to increase VLED to see if the issues will go away.

    2) GSCLK has to be provided externally.

    Best Regards,

    Steven

  • 0 in reply to Steven Li
    Genius 3850 points

    Hi Steven,

    According to your feedback, the current of VLED has been increased. Setting to RIREF = 1.3kΩ.

    Let me describe the situation on my side. Only one channel of LED output is connected here and other output channel are empty. When setting Dot Correction mode value to more tha13, an error will be reported. 

    When the error occurred, XERR pin has low level occurs, and the maximum PWM value is only 33/4095. The waveform of XERR pin status is shown as below.

    Is there a certain error when there is a output channel that is not connected?

    Thanks,

    Kind Regards

  • +1 in reply to Lumina Gui
    TI__Mastermind 23490 points

    Hi Lumina,

    1. Sorry. My original meaning is that you can try to increase the voltage of VLED, not the current.

    Lumina Gui said:
    According to your feedback, the current of VLED has been increased.

    2. Please clarify the number below.

    Lumina Gui said:
    When setting Dot Correction mode value to more tha13

    3. LOD data are only checked for outputs that are turned on during the rising edge of the 33rd GSCLK pulse. So if you do not use some channels, make sure to set their GS data to 0 to avoid triggering unwanted LOD detection. Then please check if the problem of maximum PWM value still exists.

    4. By the way, do you connect any LEDs in serials?

    Best Regards,

    Steven

  • 0 in reply to Steven Li
    Genius 3850 points

    Hi Steven,

    Thanks for your suggestion, and we got the root of the problem. We connected the LED lamp to a resistor in series in mistake, which caused excessive current.

    Thanks,

    Kind Regards

  • 0 in reply to Lumina Gui
    TI__Mastermind 23490 points

    Hi Lumina,

    You are welcome. I am glad to see you find the root cause of the problem. The resistor will introduce extra voltage drop which may

    • make the headroom voltage requirements of the current sinks not satisfied. 
    • trigger a false LOD detection

    Best Regards,

    Steven

  • 0 in reply to Lumina Gui
    TI__Mastermind 23490 points

    Hi Lumina,

    If you no longer have any question, I will close this thread. Please feel free to contact me at steven-li@ti.com if you need further help.

    Best Regards,

    Steven