Driving many white LEDs – with some twists! - Page 1
Forum section: electronics / projects

author: foxtrot
author_rank: Contributor
timestamp: January 16, 2024, 05:53:05 am
content: Hey all! I have an interesting challenge for the forum, and I'm hoping someone with some experience designing LED illumination / lighting will be able to give me some advice!I'm working on a design that calls for two dimmable channels, each comprising 56 white LEDs (V_f = 2.67V, I = 55mA) to be driven with minimum power dissipation and excellent output uniformity (<1%) from a 36-60V input.In my design, the LEDs themselves will be on a board (subject to footprint constraints) that is separate from the board containing the driver (by a distance of ~50mm) – and I need to minimize the number of conductors going between the boards, primarily due to cost/mechanical constraints. BOM costs need to be minimized, as always.I was thinking about using a constant-current driver (such as TI's TPS92205x) in a series/parallel configuration (i.e. 8S7P), and an array of BJTs to act as a current mirror, as outlined in a couple of StackExchange posts (i.e.thisorthis) – but this approach seems like it has its own pitfalls. I also looked at the LM3466 (a seemingly *very* cool (but far too spendy) part. I also considered something like a backlight LED driver (i.e.ALT80600), with a channel per parallel string for current sinking, but it gets expensive/large quickly at 16 channels, and I've not found a backlight driver that works across my V_in range. I'm not sure if any of this is considered best practice these days when using lower-power LEDs, and if I'm just complicating this for myself.I would deeply appreciate thoughts, part recommendations, etc. from anyone that's tackled this sort of project. I know I'm not re-inventing the wheel, here, but it's almost immediately apparent to me that this is one of those areas in which there's a lot of subtlety to getting it right – so I'm appealing to the gods in this forum

author: Psi
author_rank: Super Contributor
timestamp: January 16, 2024, 06:25:27 am
content: Have a look at STLED524https://www.st.com/content/ccc/resource/technical/document/datasheet/6d/fb/4d/93/de/80/45/b2/DM00113642.pdf/files/DM00113642.pdf/jcr:content/translations/en.DM00113642.pdfIts a small BGA matrix'ed led driver with built in DC-DC boost converter so you can drive white led's from like 3V input if you want.It's a 5x24 matrix, so 120 leds with each led brightness controllable (0-255) as well as a global brightens control option, all over SPI.It's intended for a led display so designed to have good uniformity.One drawback, I don't think you can drive 55mA leds, i think it's for 20mA. There is a small chance youmightbe able to more than 20 if you dont need the build-in DCDC converter. Dunno, you'll have to check datasheet.Also its an ultra tiny BGA, so PCB is a little more complex.But it's an awesome little chip.Maybe instead of driving 56x 55mA leds you could drive 120x 20mA leds and achieve the same optical output power you need?We used it to drive 0402 LEDs and the main thing I remember about it was how damn bright it was.(We might have been pushing those leds a bit far for 0402 size, but still the chip impressed us with its power)

author: foxtrot
author_rank: Contributor
timestamp: January 16, 2024, 06:37:58 am
content: Quote from: Psi on January 16, 2024, 06:25:27 amI really appreciate the suggestion! I don't need individual control of the LEDs – just control of each of the two "channels" comprised of 56 LEDs. I think the matrix approach is interesting, but it either requires a lot of conductors to operate "remotely" from the driver board, or space that I probably don't have on the LED board, itself.It seems like parts such as the ISL97678, LT3760, TPS92638, or MAX16807 could be good choices for this application – even though they predominantly target backlighting applications. The problem is that none of them seem to like higher input voltages...

author: Psi
author_rank: Super Contributor
timestamp: January 16, 2024, 06:38:32 am
content: Another option is to look at what chips china is using for all their cheap LED matrix modules that are available on Aliexpress.I think they are mostly shift registers and output drivers, you do the PWM control yourself in software/hardware. but might give you some ideas.

author: Psi
author_rank: Super Contributor
timestamp: January 16, 2024, 06:40:57 am
content: Quote from: foxtrot on January 16, 2024, 06:37:58 amCant you just increase it from 2 dimmable channels to 4 dimmable channels, then you dont need such a high voltage.You can control the channels with the same input source, so the are effectively the same channel.

author: foxtrot
author_rank: Contributor
timestamp: January 16, 2024, 06:52:46 am
content: Quote from: Psi on January 16, 2024, 06:40:57 amThe "channels" in this case are simply logical channels – groups of LEDs that should be driven together. My post suggests that I break these into multiple strings that I could place in parallel with one another. I think the thing I'm looking for some expertise on from the forum is the best approach to ensure output uniformity and high dimming ratio while minimizing cost, given the high number of low-current LEDs and the high input voltage range of my system.

author: Psi
author_rank: Super Contributor
timestamp: January 16, 2024, 07:18:09 am
content: Are you planning to use bin'ed leds to get the best uniformity between all the leds?The current feedback on the switchmode drivers on each channel should keep the current the same at any input voltage.If this is super critical then maybe those switchmode drivers will need a better current feedback circuit with low tempco high accuracy etc..The uniformity between each series string is just the accuracy/tempco of the sharing resistor feeding each string.You could have all the Isense resistors on each string and each channel in the same location and thermally coupled so they all experience the same temp.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 16, 2024, 08:28:08 am
content: I don't think you realize how demanding this 1% requirement is, and doesn't play well with the cost minimization target. Maybe there is no other way but to add optical feedback to every LED and drive them all separately, which would be easily several $ for each LED. LED binning is normally much less accurate, like you can see +/-25% differences in luminous output within one bin.If optical feedback per every LED is out of question, then I think there is no other way but non-standard binning, either by yourself (measure each LED and bin), or by manufacturer. But even if you are able to bin within +/-1%, you have then used all of your error budget already and can't afford any more drift (LED aging) or temperature difference. You know, even just a few degrees of temperature increase brings the LED output down by 1%, given the same current.Obviously series connection guarantees exact same current through each LED, so then it's only about keeping them at closely similar temperatures and finding an LED supplier who can do specialized binning for your strict 1% requirement, and hoping for the best.I would not rule out a single 56-LED string: that would be 150V, with a boost ratio of 150/36 = tad over 4, easily achievable with bog standard boost converter, which can be regulated for output current.Is the 1% requirement set in stone? I mean, could you accept 10%? That would make the project reasonable.

author: Psi
author_rank: Super Contributor
timestamp: January 16, 2024, 08:56:20 am
content: Quote from: Siwastaja on January 16, 2024, 08:28:08 amJez, i knew it was bad, but not that bad.This was why I was suggesting something like STLED524, it gives options to fix output uniformity to some degree in software. How much is hard to say, 0-255 isnt a lot, probably want like 10bit not 8bit.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 16, 2024, 09:33:55 am
content: If you want a smooth dimming range from 0-100% you need at least 10 bits of control,ideally 12.Uniformity shouldn't be an issue if LEDs are all from the same reel. How visible any nonuniformity is depends a lot on the optical arrangement but unlikely to need anything near 1% unless it's for a speical application

author: Siwastaja
author_rank: Super Contributor
timestamp: January 16, 2024, 11:28:49 am
content: Quote from: mikeselectricstuff on January 16, 2024, 09:33:55 amI would agree, but it's possible OP is having a "special application" because they put this into an actual number and even underlined it by saying "excellent output uniformity". I think before suggesting anything we would need to know if this is an actual hard requirement or something that can be compromised.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 16, 2024, 11:32:01 am
content: Quote from: Psi on January 16, 2024, 08:56:20 amYeah, such LED-by-LED calibration is a good option. While 8 bits doesn't sound like a lot, you could use this "just for calibration" (e.g., just set the LEDs to say value 240 +/- calibration, in which case you have better than 0.5% step), and globally control the brigthness using the maximum current setting pin devices like this often have.

author: foxtrot
author_rank: Contributor
timestamp: January 16, 2024, 02:46:32 pm
content: Quote from: mikeselectricstuff on January 16, 2024, 09:33:55 amQuote from: Siwastaja on January 16, 2024, 08:28:08 amI think I can easily relax the 1% requirement pretty significantly – it's nice to have, but sounds like it's not necessary if (as you suggest) the optical assembly has a good amount of diffusion and the LEDs are arranged correctly. The most important thing is that the appearance of output is roughly uniform.Relaxing the requirements is the right move – so thank you for asking.Quote from: Siwastaja on January 16, 2024, 08:28:08 amMy original approach was going to be to drive each LED individually (using the TLC5948A) and apply dot correction; however, I increased the number of LEDs and cost and footprint became an issue. I was considering a 150V string, which actually works for my application; however, an open failure of a single LED takes out the entire string, which makes this option less attractive.Maybe I'm not thinking about this in the way I should be – but it seems like the ideal scenario for me would be to have four 37.38V strings of 14 LEDs each – which would yield reasonably-good tolerance to faults, and provide good efficiency (with minimal boost)? Would the current-mirroring technique using BJTs outlinedherework reasonably well for this?The thing that's interesting to me is that most of the boost LED driver ICs I find are rated for a lot more current than I need. LED backlight drivers, which handle multiple parallel strings and current balance, seem to be a really great fit – but I can't find any that work at my full input voltage range – and I suspect that's an indication that I'm really not thinking about this problem in the right way.This is to say that I deeply appreciate the feedback you are all giving me.