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TLC5948A: output failure

jLED__BOARD
Prodigy 60 points
Part Number: TLC5948A


Hi,

I am designing a RGB LED Board with the TLC5948A drivers. I use 12 of them connected by the serial outputs to the next serial input, so cascaded as the datasheet described.

The LED Board is multiplexed 4 ways with 1 msec timeframes, so the grayscale clock is defined to do 256 cycles in this time. I don't care about the dot correction or the error output, so the mode is selected always to use GS register (TMGRST = 1, DSPRPT = 0). I drive them with 4 signals (GSCLK, SIN, SCLK, LAT). VCC is stable 3V3. 

My problem is that, some outputs TLC, one or two, are randomly get damaged and I can't find the reason behind it, because I use VGB= 4 VDC for Green and Blue LEDs and VR = 3VDC for Red LED, and current are Red : 42mA, Green : 34 mA and Blue : 15 mA

If I switch on each color separately (BC = 0x7F), no output is damaged, but I switch on all RGB colors, also (BC = 0x7F), then output is damaged. the output failure is a small current that passes through the output when output is blank, perhaps, leakage current through the protection diode inside IC

Have you got any idea, or tip about the conditions when this type of driver can be damaged?

Best regards,

  • 0
    TI__Genius 14295 points
    Hi,

    We have never seen such kind of issue.

    Since TLC5948A can handle 45mA per channel when Vcc<3.6V, it should not be damaged even you open all cahnnels.

    Are the damaged channels not bright again?

    Or do you try switch on like 2, 3, 4,...LEDs to see what happen?

    You can also test the output waveform especially for the demaged channels for detailed analysis.

    Thanks.
    Regards,
    Kenneth
  • 0 in reply to Kenneth Du
    Prodigy 60 points

    Hi Kenneth,

    First of all, thanks for your quick answer.

    Are the damaged channels not bright again?

    No, channel damaged work well, but when the channel should be off, a small current flows that turns on the LED.

    Or do you try switch on like 2, 3, 4,...LEDs to see what happen?,

    Rest of the chip's channels work perfectly.

    I attached you this pictures:

    VLED Blue = 4VDC

    Channel Blue LED failure

    Channel LED Blue OK

    Thanks for all

  • 0 in reply to jLED__BOARD
    TI__Genius 14295 points

    Hi,

    Can you also attach your schematic here for reference?

    Thanks.

    Regards,

    Kenneth

  • 0 in reply to Kenneth Du
    Prodigy 60 points

    Hi Kennet,

    It's solved!!!!, I've changed time multiplexing as MAXIM's aplication note about driver MAX6973, as conclusion, channel must be off, in my case 15 uS, before end and start the MOSFETS switches. I don't know because is solved, but with this solution channel no damaged, do you know why?

    Thanks,