LM3409 (yet another high power LED question)

Hi,

Fairly new to electronics so I’m sorry in advance if I come off as completely confused. I’m trying to build a small plant system, with watering and a few more functions. All of that is figured out but now I’m at “Feed the poor plants light” part and I’m trying to figure out how to do it in an efficient way. The base parts for this part of the project is:

2 * UV - LST1-01G01-UV03-00 Vf 3.4V 500mA
10 * Deep red - LST1-01G01-PRD1-00 Vf 2.05 500mA / 700mA
4 * Royal blue - XPGDRY-L1-0000-00601-SB01 Vf 2.82 350mA / 700 mA
2 * White - XPGDWT-01-0000-00LE1-SB01 Vf 2.81 350mA / 700 mA

And for power: 12V 8.5A power supply with a max rating of 110W.

What I’d like to be able to do from the Arduino is to dim the lights (per colour) and have them on for a certain amount of hours per day (it will vary with how much light they get from the sun and at what stage they are in). Also, I’d like to run all functionality through the Arduino, that’s the reason for not just using a simple wall socket timer.

After browsing through what feels like a thousand similar projects I’m still pretty confused, but I think I’ve found that LM3409 is the IC I want to use to rule it. Based on the AN-1954 demo board I’ve attached a simplified schematic of it which I think cover my needs but I really need some input.

First obvious question is, is LM3409 the droid I’m looking for?

Assuming it is I’ll need 4 of them and get out 6.8V, 20.5V, 11.28V and 5.62V. The AN-1954 datasheet has a table on page 11 for “Alternate Designs”, and there they have some examples of values and components to change but my electronics skills are too low to be able to wrap my head around it. The 4 alternatives all have several changes so I’m having trouble seeing what affects what. The questions I have now, and I’m guessing there’ll be more when/if someone replies are:

  • How is the Vout decided? Is the step-down only by resistors? And does it only do step-down? (All examples and the base demo board all have higher Vin than Vout)
  • In the first section it says “The circuit can accept an input voltage of 6V-42V.However, if the input voltage drops below the regulated LED string voltage, the converter goes into dropout and VO = V IN ideally.” That sounds like step-down only, but I’m assuming I can’t just give it 42V and it’ll automagically set Vout to something that doesn’t kill my LEDs.
  • fsw, the switching frequency, is that something I really care about? Is that the speed of on/off or the max PWM frequency? As it’ll be steady light for 12-18 hours am I not really concerned by the micro seconds lag I’ll get when turning on/off and as long as the light doesn’t flicker during PWM isn’t that really something I care about either.
  • I don’t really understand the IADJ pin and how it’s controlled. Should I connect that to an analogue port on the Arduino and control there or do I hook it up like the example and have 350mA/500mA/700mA depending on which colour / current I want to run them on? If I do send an analogue signal to it, how do I know what amount of voltage sets what constant current?
  • EN - I’ll use that for PWM (if/when I want to dim), on of few things that seems straight forward. Or am I delusional? :slight_smile:
  • COFF, looks like I can ignore that? I’ll just not send any signal to EN when I want the LED off and that’s it?
  • I understand this is much to ask for, and completely see if this one is ignored: If someone could take one of my four LED groups and explain which components to change, to what and why it’d be great.

Thanks for reading the whole way through, hope my questions makes sense, I didn’t miss too many details and that I’m not completely lost.

LM3409.png

How is the Vout decided?

By the mount it needs to do to get the required current.

Is the step-down only by resistors?

No it is a constant current driver.

And does it only do step-down?

Yes that is what the "buck" means. If it stepped up as well it would be called a buck / boost converter.

but I'm assuming I can't just give it 42V and it'll automagically set Vout to something that doesn't kill my LEDs.

Yes you can. The components surrounding the chip are changed to set the current you want. Note you should not wire LEDs in parallel with this controller only series.

fsw, the switching frequency, is that something I really care about?

Yes the higher the better, over about 60Hz you will not see any flicker. But a too high frequency will radiate a lot of electromagnetic interference.

don't really understand the IADJ pin and how it's controlled

Leave it unconnected.

EN - I'll use that for PWM (if/when I want to dim), on of few things that seems straight forward.

It looks like it but the wording is a bit ambiguous, which is why we make prototypes.

COFF, looks like I can ignore that?

No it's part of the feedback circuit.

A 12V supply is not the best voltage to drive it. Basically add up the forward voltage of the LEDs until you get just below 12V. Then wire that many in series. You will need a board for each LED group.

For example your 10 * Deep red - LST1-01G01-PRD1-00 Vf 2.05 500mA / 700mA, so with 12V you can only have 5 of them controlled by one board.

Finally don't even think of using solderless bread board. Circuits like this must be laid out correctly on a PCB. It is quite a skilled job designing these and getting it right.

Thanks for all the input!

Starting with the last since that is the most worrisome:

Finally don’t even think of using solderless bread board. Circuits like this must be laid out correctly on a PCB. It is quite a skilled job designing these and getting it right.

Not on a solder less breadboard because it’s too hight current/frequency for them to handle it or for some other reason? Like I said I’m very new to electronics, and assuming I’d have a PCB board made as a prototype, is this really an advanced circuit? Thinking that it’s a fairly low amount of elements in it and as long as not trying to make it super tiny and instead use fairly wide tracings.

My plan was to use my lab power supply with much lower settings and weaker LEDs as a test setup and then if/when I get it to work move on to PCB.

COFF, looks like I can ignore that?

No it’s part of the feedback circuit.

Ah, thanks. Thought that was an alternative way of powering off but now reading through the two PDFs I see it’s tied to the frequency and not that…

Yes you can. The components surrounding the chip are changed to set the current you want. Note you should not wire LEDs in parallel with this controller only series.

Ok, yes my plan was to have the LEDs in series. Original plan was 4 boards in parallel from the supply but as you point out I’ll need 5 as the red ones will need two boards or find myself a 24V+ power supply.

is this really an advanced circuit?

Yes. Basically it is a switched mode power supply only with a constant current rather than a constant voltage output.
I have worked as a team leader and manager in electronics and I have worked with specialist switch mode supply engineers. I have never known one get a PCB right in less than three revisions of the PCB. The problem is that the circuit has to be made stable at all currents / voltages. You find that these specialists will get it to work first time but they become unstable under certain values of load. So revisions have to be made.

I would strongly recommend you to get a ready built unit, especially if you are a beginner.

A solderless bread board has too much in the way of stray capacitance, stray inductance and cross coupling to even begin to get anything stable. Also the current is way too high. The inductors and FETs are rather critical as well.

An easy way is a 24volt supply and constant current LED drivers.
These boards can drive/dim three strings of 3-6 3watt LEDs with 1 or 2watt (selectable) max.
Many CC LED driver boards like this on ebay.
Another option is Meanwell LDD-700 modules.
Leo…