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current sink Led driver + odd led in parallel with TLC59116

Nicola Briox
Prodigy 100 points
Other Parts Discussed in Thread: TLC59116

Hi, 

I'm working in a project in which I'm driving several led in series/parallel configuration.

Let's suppose I'm driving 8 leds: the led are connected as 2 parallel branch, each branch is the series of 4 leds.

In this case (setting the proper Rext value) 20mA current flows through each branch.

As far as I know, keeping the current value and the other variables fixed, everything works good until the number of led is even,

so if i would have 6 or 4 led, i can just put them in a 2 parallel branch, with 3 or 2 led per branch.

 

But, what if I have to drive 7 leds, keeping the same configuration (i.e. 2 parallel branch)?

The led power supply is not enough to power the 7 led series, so the parallel configuration is required.

I has an idea, but i would like to have some feedback from an expert.

I would put a 2 parallel branch, one branch with 4 led series, the other branch with 3 led and a resistor in series. 

The value of the resistor depends on the current flowing in the branch, and the voltage drop of a single led (Rseries = VdropLed/Ibranch). 

 

According to your knowledge, would this configuration work, or there are some other way to solve my problem?

thank you in advance!

nicola

  • 0
    TI__Guru**** 154206 points

    For the 4s2p + 3s configuration, with the high side of each string tied togther, it would be best to place a series resistor in the 3s string so that the resistor can drop the additional power. Otherwise, the current sink has to drop the power.  Since the brightness is a function of the current through, not the voltage across the LEDs, the LEDs in this configuraiton would have the same brightness.  The only drawback is the wasted power.

    If you can provide your input voltage, dimming requirements and end application, I can provide a specific IC that might work for you.

  • 0 in reply to Jeff F
    Prodigy 100 points

    Hi Jeff, thank you for the fast answer!

    I'm using a constant current sink (TLC59116), my only concern was about the use of the series resistor.

    I want 20mA flowing through each led series string, and that each led has Vf=3.6V.

    Do you confirm that the correct series resistor value (in the case of 4s2p+3s) is R=Vf/Iout = 3.6/0.02 = 180Ohm? 

    thank you again.

    nicola

     

  • 0 in reply to Nicola Briox
    TI__Guru**** 154206 points

    Yes.  In my opinion, that should work.