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Voltage for LED question

Drew Jensen
Prodigy 20 points

Other Parts Discussed in Thread: TLC5928

Hey all, I'm pretty new to all this but I had a simple question. The voltage that I'll be supplying to the LED (VLED), I should just use the common VCC (3V) right? If it ends up being 5V it shouldn't matter to the LED Driver correct? Just for heads up I'm looking at the TLC5928 with some blue led's 20ma 3V nominal. So the IRef will be 2.52K. The MCU Atmega328P can handle 1.8-5V for VCC so I'm just going to feed 3V for the whole systems so I wont have to have any regulators. Good idea or bad? Am I missing something here or is it really that simple?

  • 0
    TI__Genius 14890 points

    Drew,

    The simple answer is yes, it is that easy.

    There are some things you need to be aware of as you get all of the details finalized.  See Figure 12 in the TLC5928 datasheet that shows the required output voltage for various currents.  Your VLED must be high enough to satisfy this voltage plus the LED voltage (worst case).  Power dissipation is another area to investigate.  If you use 5V for the system, the output voltage at the TLC5928 will be 5V-3V = 2V.  Each output that is on will dissipate 2V * 20mA = 40mW.  If all 16 outputs are on, the dissipation is 16 * 40mW = 640mW.  The dissipation rating on page 2 of the datasheet shows this is within the operating range.  One more thing to be aware of is noise on the supply when you are turning the outputs on and off.

    Regards,

    Dick

  • 0
    TI__Intellectual 955 points

     Hello Jensen,

     Typical forward voltage of general blue LED lamp is about 3.2V at Ta=+25 degree C. TLC5928 need about 0.5V at OUTn (n=0~15) at least for 20mA constant driving according to figure12 (OUTPUT CURRENT vs OUTPUT VOLTAGE graph). But these are typical data. LED forward voltage is increased at lower temperature. The shifted voltage is about 0.3V in general. Also TLC5928 OUTn voltage is changed depend on temperature and IC production lot. I think it is better to set OUTn voltage to 0.8V. So LED lamp anode voltage should be UTn voltage + LED forward voltage = 0.8V + 3.5V =4.3V. I think you should set VCC voltage to 4.3V.

     Best regards,

     Kazuya Nakai     

     

  • 0 in reply to Kazuya Nakai
    Prodigy 20 points

    Kazuya, I need to ask for clarification. I was under the impression from some diagrams I've seen that the cathode of an LED is what connects to the TLC5928 "output". Is that correct or am I mistaken?

  • 0 in reply to Drew Jensen
    TI__Intellectual 955 points

     Hello Jensen,

     You are correct. The cathode of LED must be connected to TLC5928 output (OUT0 to OUT15)

    like the typical application circuit shown on page1 of TLC5928 datasheet.

     Best regards,

    Kazuya Nakai.