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LM3409: COntrolling parallel String of LEDs using LM3409

zunuran nasrullah
Prodigy 220 points
Part Number: LM3409
Other Parts Discussed in Thread: TPS92638-Q1

Tool/software:

I am interested in using LM3409 as a LED driver for my Project. I want to use an appropriate potentiometer on Iadj pin to control brightness of LEDs. I have total of 32 white LEDs and 12V supply voltage. Forward voltage of LED is 3.1V and Forward current is 20mA. I am planning to use 3 series-11 parallel LEDs combination. I cant use more than three LEDs in series due to input voltage limitation. Is this design feasible? If I can implement this design then how can I calculate value of potentiometer, selection of mosfet and Indutor? 

  • 0
    TI__Genius 11785 points

    Hi Zunuran,

    You cannot drive LEDs in parallel because of part-to-part variation in I-V curves. Essentially, placing LEDs in parallel means you cannot control the current and there will be significant brightness variation between LEDs in the string.

    Why are you looking at a switching regulator for 20mA? LM3409 is used for designs up to about 5A. At 20mA you can achieve this with a linear regulator that has multiple output channels.

    Use 2x TPS92638-Q1 with 3x LEDs per channel. Use five 3-LED strings for each device plus one more 2-LED string to spread the power. You can control the brightness with PWM dimming or with the reference pin resistor.

    https://www.ti.com/product/TPS92638-Q1 

    Regards,

    Zach

  • 0 in reply to Zach Olson
    Prodigy 220 points

    only reason to select LM3409 was because it has option to control dimming using potentiometer. Can I use potentiometer with TPS92638-Q1?

  • 0 in reply to Zach Olson
    Prodigy 220 points

    I tried to read datasheet of TPS92638-Q1 to figureout how to setup potentiometer for dimming control but I could not figureout what value of potentiometer should I use and how to setup schematic for that. can you guide me with some refference design or just explain how  i can connect potentiometer to the IC for brightness control?

  • 0 in reply to zunuran nasrullah
    TI__Genius 11785 points

    Hi Zunuran,

    Please refer to section 9.3.1 of the TPS92638-Q1 datasheet.

    The current for all channels is controlled by the reference resistor. You can use equation (1) to determine what value potentiometer you need to use for your reference resistor.

    Regards,

    Zach