Driver selection for high power LEDs

Question

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.

Answer 1

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.

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.

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.

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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.

Answer 2

I'd use two LT3476 to drive eight strings in total..

These can go up to 36V, so you could use the same controller for longer strings as well.

The ICs come in a 0.5mm pitch QFN package, so check that your board house is capable of putting soldermask between those pads reliably, otherwise it will be rather difficult to solder.