Driving and dimming numerous LED strings together

Thread Starter

MythicalCoder

Joined Aug 28, 2015
83
Hmm, I'm having a hugely difficult time finding any suitable through-hole LED drivers here. Went to DigiKey, Mouser, even Element14, Newark, Arrow, and a few others, but no dice on any of them. The closest I could find was a DigiKey unit that has a minimum order quantity of 1,000 -- far too large for a project this small. Might have to hand-solder an SMD one, which could prove difficult. What kind of SMD footprints are the easiest to hand-solder?
 

Thread Starter

MythicalCoder

Joined Aug 28, 2015
83
Why not buy a ready-made module such as this? Cheaper than just the IC.
Where's the fun in that? I want to learn, not just assemble ;)

Not to mention there's a couple of potentiometers on it that allow the end user to adjust the voltage and something else (current?). It tops off at 50V, and if it dips any lower, the LED emitters don't fare as well. I'd prefer to take out the "human" element and just have this thing be plug-and-play. If that means I need to build it myself, I'd prefer to do that.

Although a thought occurred to me: instead of multiple individual boost converters, could I build my own standalone boost converter and just wire all the inputs to it? Seems to be the most logical solution. Something like this:

White_Test.png

Only question is, how do I build that circuit?
 

Thread Starter

MythicalCoder

Joined Aug 28, 2015
83
The Application Notes for whichever ICs you choose should have at least one example circuit and some guidance.
Okay, but I haven't found a suitable IC yet. With the new approach of a universal boost converter, do I still need an LED Driver IC, or would a generic boost converter do it? I'm basically looking for a way to take an automotive voltage, bump it up to a stable 50V, then tap into the higher voltage for each LED string.

I've been trying to find this circuit to no avail. Joule thief seems improbably low in voltage, boost converter seemed to be varying in output voltage, buck goes the wrong way, buck-boost seemed promising but I didn't get to research it heavily last night. I'm just trying to find something that can take, say, 9V to 15V with random voltage spikes, and provide a clean, stable, 50V output. I think the rest is fairly simple from my understanding of it.

What is this circuit even called? That will help in my search significantly. Bear in mind that I only need to build one per taillight, not one per LED string.
 

Thread Starter

MythicalCoder

Joined Aug 28, 2015
83
Anyone? I'm struggling to find the name of this circuit, or any clues as to how to build it. Really just need something that transforms a 12V battery input to a stable 50V. It appears to involve a torus and wound wires, but anything beyond that is still a mystery to me.
 

Thread Starter

MythicalCoder

Joined Aug 28, 2015
83
There are LED drivers that feature constant current output and do "boost" regulation to bump the low input voltage up higher..
I posted a few already http://www.linear.com/products/step-up_(boost)_led_drivers
Those are all SMD components, and I had massive trouble finding a through-hole guy, which goes back to the first question on this page: what's the easiest SMD footprint to hand solder?

One other bit of confusion: what exactly is an "output"? It it strictly limited to one single LED string? Or can it include other LED strings as long as the amperage is low enough? The best circuit here is one that would have four inputs that are mapped directly to four outputs.
 

mcgyvr

Joined Oct 15, 2009
5,394
Is this something you intend to sell?

IMO.. limiting to through hole is a big mistake.. 99% of the solutions out there are SMT..
FYI.. A quick search on digikey yields 4000+ solutions for surface mount boost LED drivers.. Then picking through hole and I'm down to 64 parts.

I'll direct you towards Diodes Inc, Rohm, and Infineon for now..
There are a few out there that are still through hole and 1 pc min buy..

Frankly I'm not sure you have even nailed down your requirements yet...
Is sequential functionality still on the table?
Have you decided on total number of LED's yet?
,etc...

So far it seems what you need is a through hole "step up (boost) LED driver"..
 

dannyf

Joined Sep 13, 2015
2,197
Tho
Boost converter' is what you want for a steady DC output voltage
Those devices work by varying it's output voltage so as to maintain a constant voltage between its feedback pin and ground.

So if you put a current sampling resistor between the feedback pin and ground, and LEDs from the output to the feedback pin, you have a constant current led driver.

Ie. The two are one and the same.

The same principle applies to linear regulators.
 

Thread Starter

MythicalCoder

Joined Aug 28, 2015
83
Is this something you intend to sell?
Not at the moment. If I can make a reliable enough part, I might, but I doubt I'll have much interest in something like this.

IMO.. limiting to through hole is a big mistake.. 99% of the solutions out there are SMT..
FYI.. A quick search on digikey yields 4000+ solutions for surface mount boost LED drivers.. Then picking through hole and I'm down to 64 parts.
I agree, which is why I repeated my previous question of "what's the easiest SMD footprint to hand solder?" That being said, it may be a matter of trial and error and thus unanswerable by others, so ignore that question. I'll start with the biggest pins and work my way smaller if necessary. I have a fairly steady hand, but I don't have that much fine-grain control, so I'll have to figure that one out myself.

Frankly I'm not sure you have even nailed down your requirements yet...
Is sequential functionality still on the table?
Have you decided on total number of LED's yet?
,etc...
I thought I had my requirements nailed down pretty well. :confused: But it doesn't hurt to be more explicit: Sequential functionality is desired, yes, although I already have a working schematic for that at the moment thanks to the other thread I made on the subject. For now though, I'm building the simpler circuit without the sequential part, for simplicity's sake. More on that in the next requirement.

Total LED count was one of the first things I decided. 3 strings of 16 LED emitters each per bulb replacement, so a total of 48 white for reverse, 48 red for parking, 96 red for brake lights (two bulbs), amber is really the only "funky" one, which is why I'm opting to exclude it from this current design as I will likely have to drive the strings in a much different way than with the whites and reds.

So basic requirements:
- Input: 12V battery
- Output channels: 3
- Output voltage: 50V (Minimum: 40V red, 44.8V white)
- Output current: 55mA
- Additional features: Allow dimming

Actually, I'm going to do some digging tonight to see what I can find. With literally thousands to choose from, I'm bound to find some good ones.
 

Thread Starter

MythicalCoder

Joined Aug 28, 2015
83
I think I found a good match! The ISSI IS32BL3554 (Datasheet)

I'm building the schematic as we speak, but I just want to confirm: is this a good IC to pick? It has four channels (one will be N/C), bumps automotive voltage up to 50V max, has a range of 20mA to 100mA per channel, it seems like a winner to me. Plus, it's in a fairly easy-to-solder footprint.
 

John P

Joined Oct 14, 2008
2,025
My immediate thought on looking at the "typical installation" on the data sheet was "That's a lot of external components". But if that's OK with you, then it looks like a good choice.
 

Thread Starter

MythicalCoder

Joined Aug 28, 2015
83
My immediate thought on looking at the "typical installation" on the data sheet was "That's a lot of external components". But if that's OK with you, then it looks like a good choice.
I thought so too, but it's mostly resistors and caps that won't take up much space on the back of the board. I'm probably going to make my own board for the power supply, and a second for the LED emitters, connecting the two with headers (which I have plenty of).

A couple things struck me as odd though. There's a few "NC" capacitors and at least one zero-ohm resistor (look near pin 3). Does that mean I can replace those components with a board trace and be done with it? I thought "NC" means "not connected", but they have a connection on both sides to ground? That's my only hitch at the moment. I don't understand the need for a zero ohm resistor, and I can't find a rating on the NC caps either.
 

Alec_t

Joined Sep 17, 2013
14,280
My only concern with that IC is the fact that the current in all 4 strings is set to the same value by a common control. Since you intend using both red and white LEDs, will the relative brightnesses for the two colours be what you want at the same current?
 

Thread Starter

MythicalCoder

Joined Aug 28, 2015
83
My only concern with that IC is the fact that the current in all 4 strings is set to the same value by a common control. Since you intend using both red and white LEDs, will the relative brightnesses for the two colours be what you want at the same current?
I have three strings per color, so each IC will only drive one color (the other output will be N/C).
 

mcgyvr

Joined Oct 15, 2009
5,394
You should be able to use a potentiometer in place of the Riset resistor to allow "dimming" without needing a PWM circuit to feed into the chip..
Just set it up properly (with some fixed resistors too or whatever) so you don't over drive them no matter the position of the pot.
 

Thread Starter

MythicalCoder

Joined Aug 28, 2015
83
You should be able to use a potentiometer in place of the Riset resistor to allow "dimming" without needing a PWM circuit to feed into the chip..
Just set it up properly (with some fixed resistors too or whatever) so you don't over drive them no matter the position of the pot.
I'll do that, thanks! Didn't even realize that was an option.

Back to this part though, can anyone answer the part I'm quoting below?
There's a few "NC" capacitors and at least one zero-ohm resistor (look near pin 3). Does that mean I can replace those components with a board trace and be done with it? I thought "NC" means "not connected", but they have a connection on both sides to ground? That's my only hitch at the moment. I don't understand the need for a zero ohm resistor, and I can't find a rating on the NC caps either.
 

mcgyvr

Joined Oct 15, 2009
5,394
I'll guess that NC = not critical
Total guess though..

personally I'd pick a different chip.. ISSI is to close to ISIS :D
 
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