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TLC5947 switching off unexpectedly

marci yakab
Prodigy 20 points

hello,


i have the following setup: one TLC 5947 conntected to 5 rgb leds (15 outputs are used) led current is set to 20mA)
vlogic = 3.3v, vled = 5v
the system is in steady state, serial data is written only once during startup all leds on (0xfff) then clock 0, latch 0, blank 0

the problem is that everything works for a few minutes, then leds start to flicker, then after a few minutes all leds are switched off

i thought that this could be a thermal issue, but the dissipated power is below the rating, chip doesn't get hot (~40degC) ( thermal pad not connected (to anything))
and i have the same behavior if i use ~10% pwm so it is not a thermal issue.

any ideas what could be the problem, what can i check?

is there a limit how big the led voltage supply can be, i mean how much bigger then the led voltage drop while still below 30V?

thank you in advance

ym

  • 0
    TI__Expert 4035 points

    Hi Maric,

    Has your problem solved now? according to your description, what's the max forward voltage of all the 5 series of LEDs @ Io=20mA?

    As for the appropriate LED supply voltage, please see below:

  • 0 in reply to Ryan Hu (BSR-MV)
    Prodigy 20 points

    Hi,

    The problem disappeared after I've tried a different setup with the thermal pad connected to gnd and big copper area. It is still not very clear to me why this had to be done, the chip didn't get hot

    and the calculated dissipated power was very small, well below chip rating. Maybe connecting the thermal pad to gnd had something to do with it? I don't know... I had the understanding that for low power application

    leaving the thermal pad unconnected was a valid setup.

    Anyway in the end i gave up on this chip, it didn't fit the project, the problem is that there is a visible flicker when changing pwm, and there is no way around it. As mentioned in some other threads this chip is design for mostly static pwms ( I don't understand what is the point of that, but .. welll...)