Designing a LED Driver Board with LM3414

Thread Starter

franzm1985

Joined Dec 17, 2023
4
Hello there, i am trying to design a driver Board for LEDs with an LM3414 IC, I used the Datasheet as a reference and basically build the provided circuit for my first test. I am pretty new to PCB Design, and i know there will be a lot of things to improve and i am always open for tips. But mainly i am looking to get this circuit to work.
I also used the Datasheet to calculate the values for all of the components. They provide a pretty good instruction there.
Here are some Pictures of the PCB.

My Circuit Design

Screenshot 2023-12-17 130516.png

The Top Layer

Screenshot 2023-12-17 130704.png

The Botom Layer

Screenshot 2023-12-17 130715.png

It is used to Drive LM301H Evo Leds from Samsung, they are on a separated board, i used 8 of them in series and 5 rows in parallel. I aimed to drive the System with 24V, i realized that there could be a problem with the Voltage, but i tested the Board with the LEDs itself an the LEDs stared to light up at around 20V and that should be enough given the 10% overhead in Voltage specified in the Datasheet. But connecting the Board with the LEDs and driving it with the 24V resulted in nothing happening, the Board drew about 2-3W of Energy, so i guessed something was happening. I tested erverything for continuity, checked the resitances and also measured the different Voltages. I got 5V at the VCC Pin and 1.25V on the Sensing Pins (eg. IADJ and FS) and also of course my 24V at the VIN. But Checking the Voltage between VIN and LX only gave me around 0.25V, not enough to drive the LEDs and way less the the about 20-22V I was expecting. I left the PWM Pin unconnected, since it is specified that this should just make the IC operate at full power then, also Pulling the PWM Pin to ground dropped the Energy consumption to 0W, so i guess that is working as intended there.

I would be really thankfull if someone has an Idea what i am doing wrong here and could help point me in the right direction. If any more Information is needed please ask, i will provide it.
 

Thread Starter

franzm1985

Joined Dec 17, 2023
4
Thanks for your answer.
I will take a look at the Datasheet tomorrow.
With the inductor you found a poor part choice of mine, that really would not be up to the Task since there will be just under 1 Amp of current at the maximum and the Inductor they choose for their example in the Datasheet is way more capable. But I just tested it, the Inductor is still working, and if the IC had worked the Inductor should be dead, so i assume the problem is somewhere else.
 

Ian0

Joined Aug 7, 2020
9,802
Thanks for your answer.
I will take a look at the Datasheet tomorrow.
With the inductor you found a poor part choice of mine, that really would not be up to the Task since there will be just under 1 Amp of current at the maximum and the Inductor they choose for their example in the Datasheet is way more capable. But I just tested it, the Inductor is still working, and if the IC had worked the Inductor should be dead, so i assume the problem is somewhere else.
You are unlikely to destroy the inductor, but if the inductor saturates it isn't going to work, and is likely to destroy the IC (and it saturates at 55mA).
 

jim_TX

Joined Jul 17, 2023
7
Ok. After looking at what you are attempting to do, I have a couple of ideas / suggestions.
First, you are using a Buck C-C IC to create your LED driver. Buck C-C drivers reduce the input voltage to a lower voltage to drive the LEDs (or whatever). As opposed to a Boost C-C driver that uses an input voltage and increases it, or boosts it to a higher voltage, to drive the LEDs (or whatever).

Buck circuits need some overhead voltage to work. That is, if your LEDs operate at - as you state - 20-volts, you need about 26 or 28-volts - or more. In their design example, the Input voltage is 48V and the LED voltage is 35V, a difference of 13V.

My first suggestion is to increase your input voltage to 32 or 36-volts and see if the LEDs get brighter. I say 'get brighter' because I think they might be lit with only 24-volts input - they're just too dim for you to see.
You might also try attaching only one (1) LED string to the output to see if a single string of 8 LEDs works.
Lastly, maybe 8 is too many LEDs to drive. Remove one LED, and place a shorting wire across the footprint, making it a 7-LED circuit. Try that. (You will have to adjust the current-limiter resistor)

What is the current you are trying to drive each string at? Looking at the LED datasheet, I hope it is 65mA per string (give or take). That means your board is set to 65mA * 5 = 325mA. (I didn't go through the math to see what your 3.48k resistor sets the current at.)

As others have mentioned, your Inductor choice could have been better. You need an Inductor with low-series reststance. Your Inductor is over 2-Ohms series resistance.
The Inductor used in the datasheet is the style recommended...
1703265549616.png
As you can see, it has a series resistance of about 0.2-Ohms.

My second suggestion:
Scrap your current PCB layout and start over. This time follow the layout guidelines in the datasheet (See page 22). Your layout is 180-degrees backwards to what the datasheet says to do.
TI even makes an Evaluation Board ( LM3404HVEVAL/NOPB ) that you can look and use as an example of how to do the layout.
1703262625828.png
I have designed several Boost C-C regulators... and they work. But - I followed the layout guidelines. These types of circuits are finicky. If you don't follow the rules... well, you can see what happens.
 

Thread Starter

franzm1985

Joined Dec 17, 2023
4
Well first of all, thank you both for taking a look at my problem.
I tried changing the Inductor and also had no luck there, im still only getting 0.2Volts between the + connector and the LX pin, so i guess something is completely wrong in my design. i also tried up to 30V input voltage and nothing happened or changed.
The Question regarding the current for the LEDs, my Plan was to limit it to a max of 200mA per Row since that is the maximum they can handle and dimming it down with the PWM Signal. but i guess i will reduce that a bit, since 200mA maybe is a bit lot there.
Also i got on a little journey on learning a lot about inductors.
So i guess i will take your advice and start over with all the new knowledge i now gained.
I will share what i make next.
 

PadMasterson

Joined Jan 19, 2021
63
I'm wondering how you got the pad under the part soldered since that's a ground pin to the device? If you didn't get that soldered or have a really bad connection, that may be what's causing your issue also? I'm not sure how you're doing your PCB, but that looks like the type that routes out copper and leaves the traces, etc. as it goes, so I'm also guessing you don't have soldermask on it either? If you ran solder from under the board through the vias and wanted that to connect to the bottom, I don't think you will be getting a good connection. Are you able to use solder reflow the joints? If you have plated through holes, (or even if you don't) I would use a larger drill under the part such that you can get your soldering iron into the hole and against the part and solder it to the hole/via, (a large hole, like .062" or so if you have the room) to make sure you have a good connection. Just a thought... Good luck.
 
Last edited:

Thread Starter

franzm1985

Joined Dec 17, 2023
4
I'm wondering how you got the pad under the part soldered since that's a ground pin to the device? If you didn't get that soldered or have a really bad connection, that may be what's causing your issue also? I'm not sure how you're doing your PCB, but that looks like the type that routes out copper and leaves the traces, etc. as it goes, so I'm also guessing you don't have soldermask on it either? If you ran solder from under the board through the vias and wanted that to connect to the bottom, I don't think you will be getting a good connection. Are you able to use solder reflow the joints? If you have plated through holes, (or even if you don't) I would use a larger drill under the part such that you can get your soldering iron into the hole and against the part and solder it to the hole/via, (a large hole, like .062" or so if you have the room) to make sure you have a good connection. Just a thought... Good luck.
Hey, thanks for your thoughts and you really have some good Tips how to solder PCBs. I ordered a Prototype from JLCPCB with a Solder-mask, so that's taken care of. And for soldering i am using solder paste and Hot Air, so there should be a sufficient Connection on the Pad under the IC. Measuring the Ground Connection with the Vias also shows a connection, hard to check how uniform it is tho.
Thanks :)
 
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