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power consumption TLC5941

Rene Wassenburg71
Prodigy 50 points
Other Parts Discussed in Thread: TLC5941, TLC5951

Hi,

I am currently working on a design with 6 TLC5941 onboard.

The system runs on 3.3V; SPI data and GSCLOCK comes from a PIC microcontroller. The 3.3V comes from a lineair voltage regulator that has 15V input.
This 15V input I also use to power my LEDs (strings of 5 white LEDs).

Everything runs pretty smooth, but something strange happens that I don't understand.

When I set the outputs of the TLC5941 to max (0xFFF PWM data), my voltage regulator gets very hot.

How is this possible?

The current that flows through the LEDs and the output transistors of the TLC comes from the 15V supply, not from the 3.3V supply.

Can somebody explain why my voltage regulator gets hot?

Thank you.

rene

  • 0
    Prodigy 50 points
    After some more testing I found the problem myself.

    I am driving my LED strings at 40mA, which gives me a total Vf = 14.7V for the type of LED I am using.
    With Vled = 15V -> Vout = 0.3V.
    This seems to be borderline for the TLC to be able to sink the current from Vled. Figure 5 in the datasheet shows this too.
    datasheet:
    www.ti.com/.../slvs589d.pdf

    It seems the TLC tries to get the remaining power from the system's 3.3V regulator, which causes it to get hot.

    I have increased my powersupply to 17V, and now the problem with my power regulator is solved. It doesn't get hot anymore.
    I now measure Vout = 1.7V.
    This caused a new problem: The TLC's are getting very warm. A quick temp measure on the housing of the (TSSOP) housing gives me a temperature of approx. 65 degrees celcius when all outputs are on @40mA.
    The powerpad is soldered to the PCB, but not connected to the big GND plane (the soldered surface is as big as the powerpad itself, as i did not expect any thermal issue). I didnt connect it to the GND plane, as I was not sure if I am allowed to connect the power pad to GND (datasheet is unclear about this).

    This gives me 2 new questions:

    1) is graph 5 in the datasheet incorrect and should the bend in the line at 40mA be at 1.7V intead of 0.3V?
    2) am I allowed to connect the powerpad to the GND plane?

    Thank you
  • 0 in reply to Rene Wassenburg71
    TI__Expert 3275 points

    Hi,

    For your question:

    1. The graph of the bend should be correct.

    2. It's OK to connect powerpad to GND.

    From your descripiton, it's very strange that the regulart is very hot when 15V is configured. Is that possible you can meansue the current to the Vcc of tlc5951.

    BTW, you mentioned that the regular is hot only when GS=0xFFF, how about OxFFE or 0xFF0

     

    Best regards,

    Feifei