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I'm currently working in a project where I need to control the intensity of 24 high power LEDs in 5 levels. I'm planning to use LumiLeds L1CU-BLU1000000000 (https://www.lumileds.com/uploads/705/PB198-pdf) where the cut-off voltage is 2.83 volts and current is 350mA. As per the requirements, the power supply should be from an SMPS ( maximum output - 12v,6A).

I need to select a constant current driver with PWM facility ( 0-5v max ) to control the LED intensity from a micro-controller. What type of driver IC will be best suit for this application?

What I know so far is:

  1. I can use a constant current boost converter IC with PWM ( input-12v,3A max and output 23v,1.3A) and connect 8 leds in series and 3 such strips in parallel so that each parallel branch would get around 350mA and each LED in the strip would get around 2.8V. In this case I need only one PWM channel from micro-controller to dim all LEDs at a time. If someone could suggest a driver IC for this specification it would be great.Are there any hidden flaws in this Idea or is it less efficient in terms of power consumption?

  2. I can use a constant current buck converter IC with PWM ( input 12V, 3A max and output 9.5V,2.8A) and connect 3 LEDs in series and 8 such parallel strips. Each series strip gets around 350mA and each LED gets around 2.8v (with some resistors added to each series branch)

If I'm using 3 driver ICs and dividing up the LED load among those, is it still okay to use 3 pwm channels from the microcontroller for dimming all the LEDs simultaniously?

I've googled a bit and found these ICs, but confused in selecting the best one for the job.

  1. STCS2A (https://www.st.com/resource/en/datasheet/stcs2a.pdf)
  2. MAX16834(https://datasheets.maximintegrated.com/en/ds/MAX16834.pdf)
  3. LuxDrive 3021-D-I-1000 (http://www.luxdrive.com/content/3021-BuckPuck.pdf)

Please do suggest me a good design method and suitable driver IC.

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  • \$\begingroup\$ Is there a reason for limiting the supply to 12V? \$\endgroup\$ – Misunderstood Aug 15 '18 at 12:24
  • \$\begingroup\$ This project is gonna be an upgrade to an existing product, and as per the requirement specification document, I can only use 9-12V as input for this unit. \$\endgroup\$ – Rakesh Menon Aug 15 '18 at 13:06
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Rather than three strings of eight LEDs I would use three sets of 3 LEDs in parallel wired in series. This would balance the currents much better. Powering three strings of eight LEDs would require three separate drivers otherwise the currents would be unbalanced.


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UPDATE

I can only use 9-12V as input for this unit

Use 4 series parallel sets of 3 LEDs (3P4S) x 2.

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A very simple way to power the LEDs is with a Mean Well HLG-100H-24B AC powered LED driver. With this Type B you use a 0-10V PWM signal.

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UPDATE

I still think the best is to have a single serial string of LEDs. If you can only use a maximum of 12V then I still recommend a CC driver rather than CV.

For 9-12V use need 2 or 3 parallel strings. To keep the current balanced in each serial string you either use a separate CC LED driver for each string or use a single CC driver and balance the current through the strings.

The way to balance the strings is to use a Multi-String LED Current Balancer for Use with Constant Current Power Supplies such as the Texas Instruments LM3466.

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If you think you can run multiple strips with a single CC driver add a shunt resistor to easily measure the current in each string. I guarantee the strings will not be balanced. At least you can measure how unbalanced they are. I have seen where two string are driven in parallel the current mismatch was typically between 30% and 50%. I have seen them go into thermal runaway. This was using Lumiled Rebel LEDs from the same reel.



This is a fairly simple and inexpensive 350 mA, 95% efficient, Buck CC driver.
Use TI's WebBench to optimize for max current, input voltage, and output voltage.

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The Lumidled CZ Line is not very efficient. You may want to consider a cooler alternative.

The Cree XPEBBL-L1-R250-00302 has better efficacy than the Lumiled LEDs.

The Lumiled CZ Line 470 nm blue has an efficacy of 22 lm/W (twice the heat of XP-E2)
The Cree XP-E2 473 nm blue has an efficacy of 45 lm/W (50% less heat than CZ Line)
Or you could use 24 XP-E2 and reduce the current to where thermal management would be much easier.

You could use twelve XP-E2 LEDs for the same luminous output @ 350 mA as 24 CZ Line LEDs.

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