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TLC5970 + Multiple parallel LED strings

ChrisB
Prodigy 20 points

I am designing an indoor LED lamp and consider using the TLC5970 driver.

The LEDs can sink 30mA each, so my plan was to use 3 in series and 4 in parallel for each output.

Has someone already done a setup using multiple parallel LEDs per output on the TLC5970 ?

I think it should work fine, however in the event of a LED failure, the LEDs in parallel might be damaged ?

In my understanding of the datasheet, the TLC5970 will attempt to raise the Voltage of its own DC/DC converter output, to keep the current level on the output ports, which may lead to over-current on the other LEDs.

  • 0
    TI__Intellectual 980 points

    Hello ChrisB-san,

    I heard someone tried to use parallel LEDs on output, but I didn't get any result yet.

    I also think that works fine.

    Regarding LED damage, it depends on LED's specification.

    Output current is decided by xeternal resister setting. LED supply voltage is controlled to adjust output voltage between 0.9V and 1.1V.

    So output current isn't increased. But when some LED is opened in parallel in 30mA sink current, current of other LEDs line is increased.

    Please look at attaching file.

    8105.TLC5970_VFB_control.pdf     

    If one LED line is remainded and three lines are opened,it runs 30mA on remaining LED line in worst case.

    If 30mA is under spec of LED, LED is no damaged.

    Best Regards

    K.Narisawa

     

     

       

     

      

      

  • 0 in reply to Kiyoshi Narisawa
    Prodigy 20 points

    Hello Kiyoshi Narisawa,

    thank you for your fast reply. It is as you mentioned in your PDF file, but I will use 30mA per LED, so 120mA per OUTx. The LEDs have a maximum current rating of 50mA. If two LEDs in different strings are open, the remaining two strings will receive 60mA each (which is over the rated  LED current).

    I did not find any way to set a maximum VFB Voltage, as I think this would solve the problem.

    If there is no way in limiting the DC/DC Controller Voltage Output, is there any way to limit the current per LED string using external components (i.e. current-dependent resistor / diode) ?

  • 0 in reply to ChrisB
    TI__Intellectual 980 points

    Hello CrisB-san,

    I'm thinking of your application and attaching file is best for you.

    5482.TLC5970_parallel.pdf 

    Upper limitation of VFB is 17V, so it need additional resister on each LED strings to adjust to 17V.

    If one LED is opened in 4strings in parallel, current on remaining LED strings is increased.

    But voltage drop on additional resister also is increased.

    In your case, voltage drop is lower than saturation voltage on remaing LED strings.So LED on remaining strings will be off when one LED is opened.

    LED is no damaged, but LED is turned off. 

    Best Regards

    K.Narisawa