I'm working on the design of an LED lamp house for my film scanner. I need RGB LEDs, plus IR LEDs. I'm figuring on a 2"x2" grid of SMD LEDs. Datasheets for the ones I'm using below. The current design is based on the common cathode schematic at the bottom of this post: https://electronics.stackexchange.com/a/64623
I've got this working on a breadboard with similar thru-hole LEDs, and will be making a PCB for the final setup. However, this is a lot of resistors and they take up a lot of space. I'd like to keep the size as small as possible – not a lot bigger than the 2"x2" LED array, if I can. I'm using a Quad PNP transistor array with an isolated resistor array, which has a fairly small combined footprint. There are also bussed resistor arrays for each row of LEDs in my prototype, which obviously saves some space over using separate resistors for each LED, but the smallest I can get my board design is something like 5" wide by about 3" high. I'd like for it to be smaller – as close to the 2"x 2" LED array size as I can get, ideally.
So I'm wondering if maybe I should use some kind of LED driver IC for this instead of the setup I have now. I'm basically controlling a total of 192 LEDs. In all cases, all the LEDs of a given color will be all on, at the same intensity, or all off. When I do white, it'll be a mix of RGB. I'm using an Arduino Mega (with an external power supply for the LEDs) and am planning on using some of the PWM pins to calibrate the intensity of each color channel as needed.
Any suggestions for how to approach shrinking/simplifying this, or specific ICs I should be looking at? I've never used an LED driver, and I'm not really clear on how they work, but from what I'm seeing/reading, it seems like it'll save me a lot of space. Is it possible to use one driver chip to drive all the LEDs of one color at once, or even with a driver do I still need one resistor per color per LED?
What about alternatives to the bussed resistor arrays? Anything I can use with an even smaller footprint?
Thanks!
Datasheets for my LEDs:
Lite-On LTST-G683GEBW RGB SMD LED: http://www.mouser.com/ds/2/239/S_110_LTST-G683GEBW-337108.pdf
Kingbright APT2012SF4C-PRV IR SMD LED: http://www.mouser.com/ds/2/216/APT2012SF4C-PRV-55109.pdf
Best Answer
According to the data sheet, the 3 RGB LEDs are electrically separate. This means that you can connect them in series, using a higher voltage with fewer resistors and transistors. Nick Alexeyev's answer then applies. Assuming a 36 volt power supply, and strings of 8 for green and blue, 16 for red, and 24 for IR, total is 18 channels. I would not go with Nick's suggestion of 48v/12x strings for green and blue, since there isn't enough excess voltage for the limiting resistors to operate reliably, particularly with the Vf variations given in the data sheet. I'd expect that you'd need to measure the voltage drop of each string and tailor the limit resistor values accordingly.
What I think you've missed is power. Assuming 20mA for each LED, total power is 3.6 watts each for green and blue, 2.3 watts for red, and 1.5 watts for IR. Total power is 11 watts in the LEDs. I have no idea how you're going to heatsink this. Well, I do, but it involves using a beryllium oxide substrate for your LED PC board, bonded either to a pretty hefty heat sink, or maybe a TEC cooler. You want the LEDs to run as cool as possible for better lifetime. But trying to do it with FR4 is asking for early death of your LEDs. Similarly, you would also need to calculate the dissipation in your limiting resistors, although for the values I've given I'd expect total dissipation in the 4-5 watt range, and this can be handled with forced air cooling. And with the cooling requirements indicating a certain amount of increased size, I don't think you really need to worry about minimizing the driver board size, although at 18 channels you shouldn't have much difficulty.