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TL4242 as multistring LED current balancer/limiter

tibor.ecker
Prodigy 50 points
Other Parts Discussed in Thread: TL4242, LM3466, LM3464

Hi!

I have an external 350mA LED driver module (Vossloh-Schwabe ECXe 2350.037). I would like to use this module to drive 6×6 Seoul 5630 LED (100mA@3.2V). So basically, there are 6 strings of 6 LEDs with 59mA in each string.
 But I need some cheap and effective current balancing/current limiting, so I have an idea: I will use a TL4242 in each string with 2.5Ohm R_REF (~70mA current limit).

Is that OK? I think, this solution more effective, than a simple NPN current mirror/sink, but I do not know, how operate six TL4242 in "paralel".

I saw other solutions (as LM3466, LM3464) but those did not meet the requirements, because this is a LED Tube application and the strings will far to each other.

Thanks!

PS.: Sorry for my bad English.

  • 0
    TI__Expert 5565 points

    Hi Tibor,

    Just to make sure that I have understood your idea correctly, is this what you had in mind for the 6 strings?

    If so, then this solution will work. If you want to control all 6 TL4242 devices as a single unit, you can tie the PWM pins together to control all 6 strings synchronously.

  • 0 in reply to Smriti B
    Prodigy 50 points

    Hi,

    Yes, I would do this. But I won't use the PWM input (only wired to the input). My question is: Can I use the TL4242 as a current sinking application?

    There are a lot of problem with this application, so I try to use the LM3466 IC.

  • 0 in reply to tibor.ecker
    Prodigy 50 points

    Hi,

    So I designed an LM3466 based LED Tube, The current balancer circuit is

    The LED string pairs are thermally coupled, but the two pair of LED strings are far from each other. I designed the layout for a one layer PCB, so I need to use some jumper (ZeroOhm SMD resistor).

    I have some questions about LM3466:

    • X7R/100V capacitors (Cvin): I would like to use as small as possible capacitors, but 1uF capacitors are 1206, so I used 470nF capacitors (0805). Is this an acceptable solution? Can I use X7S capacitors (like C2012X7S2A105K125AB) to change the capacitor's value to 1uF?
    • The VEQ and COMM lines: These traces are long: 30-45 cms between two ICs. I think, these lines can pick up some noises from the environment. Can the noise make some trouble for me? If yes, how suppress the noise (decoupling capacitor?)?

    Thanks,

    Tibor

  • 0 in reply to tibor.ecker
    TI__Expert 5130 points

    Hello Tibor,

    For the Cvin capacitor, we use a 10nF (0805) in our EVM. So actually it’s unnecessary to use 1uF cap.

    For the VEQ and COMM lines, since there is not much current in both of them, it’s ok for the long traces.

    Normally we will suggest to put the ground of both devices as close as possible to avoid voltage difference. For you case, I would suggest you to use a wide ground metal to reduce the resistance.

    Another thing is that LM3466 is not a current source. It just distributes the current equally to each LED string. So you will need a current source to provide current to LED strings.

    Regards,

    Victor

  • 0 in reply to Victor Chen_HK
    Prodigy 50 points

    Hi Victor,

    Yes, I made a mistake: Not Cvin, but Cled. In your example (in the datasheet), this capacitor is 1uF/100V, but I would like to use a 470nF/100V (otherway, my schematic is wrong).

    I use a wide trace for GND, so I think it's OK (but there is a board-to-board connector in 1200mm Tube with 60mOhm resistance).

    I have an external 350mA LED driver module (Vossloh-Schwabe ECXe 2350.037). This is the current source.

    Thanks for the information,

    Tibor

  • 0 in reply to tibor.ecker
    TI__Expert 5130 points

    Hello Tibor,

    If your cable connecting the LED string is long, the parasitic inductance of the cable may generate noise. A high quality ceramic capacitor with relative large value will be suggested if that case happens. The footprint is OK either you use 1206 or 0805.

    If the cable is short in your cause, then use a 470nF cap should work well.

    Regards,

    Victor

  • 0 in reply to Victor Chen_HK
    Prodigy 50 points

    Hi Victor,

    Between the LED Tube and the Current source, there are about 1+1m long cable with 0.5mm diameter, but between the LED string's last LED and the LM3466's ILED pin, there is 1-2mm long trace on the PCB.

    I think, I need some uF caps near the tube's two end, between the inputs (the drivers positive and negative pins), and after that I hope, that the 470nF cap enough for the Cled.

    Thanks,

    Tibor