LED drivers

[Akya2120]

Hello,

I want to make an illuminated badge for my truck. I don't want to just wire the LEDs through a resistor to ignition power and smack the LEDs with 14.7 volts. I believe what I need is a constant current DC-DC buck converter. But I don't really know the first thing about what I am looking for, heck I don't even know what a buck converter is or how it works. I just assume it kicks off voltage to get to a lower voltage. How do I pick the right IC? I don't mind if there is PWM flicker, I see that on plenty of cars these days.

I appreciate any help. Thanks for your time!

[james_s]

What kind of LEDs are you planning on using? Buck converters are typically used when you're using power LEDs that are maybe 100mA+. For low power LEDs it's more common to use a linear driver or just a resistor.

[Benta]

What's an "illuminated badge"?
How many LEDs? Which type? What color? I could go on...

[Akya2120]

What kind of LEDs are you planning on using? Buck converters are typically used when you're using power LEDs that are maybe 100mA+. For low power LEDs it's more common to use a linear driver or just a resistor.

I was planning on using many single color small surface monunt LEDs. Probably about 100-200.

What's an "illuminated badge"?
How many LEDs? Which type? What color? I could go on...

If you google "illuminated badge" and click images you'll see what I'm talking about. It's just a logo like F-150, or Ford that lights up.

[james_s]

There are two ways you could go with this that I think would make sense. One is to do it the way LED strips do it and have groups of 3 LEDs in series and a resistor for each group. The other is to use much longer series string of LEDs with a constant current boost driver. For 100+ LEDs you're going to need a series/parallel arrangement for that too. That's going to be a lot of work to build by hand, you might also consider a much smaller number of LEDs with an acrylic light spreader.

[mariush]

You don't necessarily need dc-dc converter (step-down/buck) regulator style led drivers, you could use linear drivers, it would just be less efficient.

For example, let's say you design your device assuming the minimum voltage will be 11v and maximum 14v.
So, you could have 3 x 3.2v leds in series for a maximum forward voltage of 9.6v
At minimum voltage of 11v, your linear driver will only need to drop 1.4v  but at max input voltage you're gonna have nearly 5v drop.

A step-down / buck regulator led driver will be around 95% efficient so there's gonna be less heat generated by the driver.
However, you could also use a step-up/boost led driver and boost 10v...14v to whatever voltage you may need. For example, have 10 leds in series, for a total of 30-32v, and if needed you can parallel multiple 10 led chains. 

Would be fairly easy to go on Digikey's led driver category : https://www.digikey.com/en/products/filter/pmic-led-drivers/745

You can use the filters to see only linear drivers,  or step-up  / step-down drivers , filter further to see only drivers that support the battery voltage range (ex 10..15v) - a lot of drivers will be made for maximum 5.5v or something like that so you can rule those out.

Then I'd suggest sorting them by quantity available - if it's not a third party vendor with a lot of old stock, most likely that led driver is a generic one, or very used

edit: for 100-200 leds, maybe look at led drivers with lots of channels : https://www.digikey.com/short/48tbr4dm
most are linear and 5.5v but you could have a dc-dc converter to convert 10..15v down to around 4v..5v (you need just a bit above the forward voltage of the led, which is up to around 3.2v for majority of leds)

For a lot of these drivers you would need a small microcontroller (arduino would be enough) to send the initial commands to the driver to turn on the leds and set the maximum brightness as desired.

btw ... look your local laws about what led colors you're allowed to have... you probably would not be allowed to use blue or red ... blue because it's reserved for police and ambulances, and red because it could confuse drivers and think they're stop lights.

[james_s]

It is totally pointless to use an LED driver that needs a microcontroller when all you want to do is light up some LEDs. There is no advantage of that over a simple linear driver and some resistors.

[Benta]

If you google "illuminated badge" and click images you'll see what I'm talking about. It's just a logo like F-150, or Ford that lights up.

I can "google" a lot of things, that resemble something you might mean. Even illuminated lapel labels.

So please describe your requirements exactly, instead of leaving us "googling". We're not here to phrase your question. That's your job.

[tepalia02]

It depends on what LEDs you're using and what power supply you're using. If you have some 5mm LEDs in series, the voltage across them should not be more than 3 volts. So, if you try to use a 9V battery, you must step down the voltage. A Buck converter is not needed here. You just have to connect a current limiting resistor in series with all the LEDs.

[sleemanj]

K.I.S.S

Just put sets of 3 LEDs in series with a 330 Ohm resistor for somewhere around 15mA through the LEDs when the vehicle is running. 

It would be a bit dimmer when the vehicle isn't running but I doubt anybody would really notice.

[inse]

I'd be careful about the current definition as "100's" of LEDs running at 15mA each might put out an astounding amount of light.
Maybe you don't want a floodlight inside your compartment.
A possibility to switch between daylight and night operation could make sense, maybe even automatically...

[Akya2120]

It depends on what LEDs you're using and what power supply you're using. If you have some 5mm LEDs in series, the voltage across them should not be more than 3 volts. So, if you try to use a 9V battery, you must step down the voltage. A Buck converter is not needed here. You just have to connect a current limiting resistor in series with all the LEDs.

Power supply would be the same as any other 12V automotive system, an alternator and a battery. Healthy alternators put out about 14.7 volts, so being able to support 15 volts would be ideal. I'll probably be using small LEDs like 1206 SMD or so.

I can "google" a lot of things, that resemble something you might mean. Even illuminated lapel labels.

So please describe your requirements exactly, instead of leaving us "googling". We're not here to phrase your question. That's your job.

Thing is even an illuminated lapel label fits what I am trying to do. Think of a backlit keyboard, also same premise. Shine an LED through a piece of plastic so it glows. My question wasn't about any of that though. My question is really, what is an appropriate LED driver for about ~100 LEDs in an automotive application? Does it even need one?

K.I.S.S

Just put sets of 3 LEDs in series with a 330 Ohm resistor for somewhere around 15mA through the LEDs when the vehicle is running. 

It would be a bit dimmer when the vehicle isn't running but I doubt anybody would really notice.

Honestly you're probably right. On top of it being simple I don't need to know any more than I already do. Might just tune the resistance or use an input diode to adjust the brightness.

[pqass]

With a little more complexity, using a constant current sink allows a consistent brightness (say 15mA) between 12-15V input voltages.
Stack as many LEDs such that their total Vf < 11V; leave ~1.5V for the CC sink.  Your battery (at rest) should be at least 12.5V.

Play with the voltage slider (on right).  Via SPST, pick either the single resistor or the CC sink and compare.
Hover over the transistors and resistors and see the power being dissipated (displayed lower right).
Both transistors can be any small signal 2N3904/2N2222A (Hfe=50) will do as the right one only tops out @53mW at highest input voltage.
I don't think low temperatures (@0°C) will affect it much; maybe down to 14mA.

http://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWcMBMcUHYMGZIA4UA2ATmIxAUgoqoQFMBaMMAKACVxCq8qwuRsKKMIAsvJFUnQELPkL7de-MCjwghAEzoAzAIYBXADYAXBobobwwqZFZzOvVQ-AZC6kFr1HT5yxElQsHaE8squzigoIu6eBiZmFlYBMLayIRFRERhCmjpxPon+1kEsAObgItEIKhVVldYsAA7q+CBiVGhq1fINAO4tau3OQ5As-Z0UNWD13VBpoR3ZzoIdHnneCX7Wgan2CgJoy4RquV7xvknFqdgYSlTYx8Nwl6PGIDxtfOBOIl+8IAwUNAMMRBNgEGQEIQECI8PUYIRoYQcGhiJBsNNEeRKGszsYWG8VINBO8qCISf8GAikSjIGiMSIsYEITg4dhfp0cGAkDjYt4WAAnb7E+Q-D73YhjUltWHS2GSKUfeXCtriqVEtrPfYjQXS7APaXi5DwRWKZxqm5m-VuJVfIqjIXarX8ZVUY4sADKtUm8nqruEekMAGc6MIUCxsGoqAAxIrMcBIJggABqAHsTLpSnQWEA

[pqass]

Alternatively, if you need to light-up a lot of LEDs rather than breaking them up into strings of n*Vf < 11V, use a boost converter to regulate to a much higher voltage (say 48V max).  The boost converter will be able to deal with the fluctuating input voltage between 12-15V and all you'll need is one resistor for the last few volts or so (Vout / Vf = 48V/3.2V=15, but leave one off, so, 3.2V/0.015A = ~220R for the series resistor of a 14 LED string lit from a 12V to 48V boost converter).

I'm not sure how temperature on a string of 14 LEDs will change its total Vf.  But, you could use a CC sink instead of a single series resistor per string (see my previous post).

Also, 100 LEDs with Vf=3.2V @15mA is 4.8W and probably very bright!  You're going to need to revisit the maximum current you'll need for the given area they occupy.  Maybe even require a brightness control (instead of 10K to Vcc, feed the 10K with PWM, see attached).

http://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3AWAnC1b0DYrQOwGYAOJSAVg0iSL0gxIhMhBKcZIFMBaMMAKACVw5EAUZgheAExRpCRggLTGMEjwDmgphIVimQpTwAOIJFgTzjWElvCKeAdwsgzC886j3HV7ZeuQeeHEYTJx1gtwh9ABcQDkknDAQNBATpCC4YSBwCCQkSLOycJDAcCOgMWgwcCSQ6SDwzPG4WEAATNgAzAEMAVwAbSJ5onXlGWKkR1JiYcrIqmrA6hqblJHwssjBZCTwSRIZWjp7+ngAnJJENL2k8JA8xp2D7q78HJ+sdZ49h1xdzPzP7ngMAoOMEgQpRPA-AFRnFwTE4slEhF3GJmBxclhqIkMZAFNiQFhNtAUDgyLQCI1isQ8DwWjFMU44AjIIlZIwpG0un0BvTYqynHEMWRBVJOYceTxijiCQghYQ2cEIAt4NgcAgySA+GwAM4ASx1kU6ADsAMZsO47caPMDWZBYF4xW0uG3WeGOrhuoFO6wSKruV7OkB+qRcJBi-0exnshki91qFmJeGxb3u9zqLhCJFOrMXfSmxMgZOMtNgTg4GIYMoSLY4ObcCSLbBwXiBt3AmJgjtRkUx4WmOJ+ARhiOhsDh4OyRRM8DMJTYFSvWXMjgxmMetcrteD-yJdp2FonAD2BgAOgBHPDQKQQJDQSScZZMe-k9bVAjlampKUYKRK6xKjgWBSPuh4nheV43jYpSQBIP5-kSQGODkiQgQex5npe17QTYGRwWICHBjWyH+mhYGYZBOEwfhv4kYRkgciAoEYRB2HKrhsA0fRjaOPCZEsVhUHsXh8G8R2YQ6Px4GCVRzZcY4Fz-goUkUWxOEiQRgiMPCYhyJJTHodJlHsdRom6cIohCEGKmsUJHGwWZ4g8eZfEGeRtmyRptHmSGWlFjxNkySZcmORyxE+aRbkCcZ6mcaF4BIT5xGBTFwlxZp5lBplSEpWpaUOQ4wQXEGeZfNYxV2jojpBggOjmbVDo8EewZEqkuS7OACAkpQkBgHQJD1ANVQwS20hiliPBAA