LED driver DC-DC converter

[zachtheterrible]

Hi all. I've been searchin for the perfect LED driver for the last hour or two and can't find what I'm looking for, figured someone would know of something. It's for driving one of CREE's XR-E LEDs.

I need something that can put out 1000mA at around 3.7 to 3.9V.

Input voltage should be less than 4 volts - 23V at the least.

Since there's probably nothing that's going to meet up to my specifications perfectly, I'll probably need something that's adjustable. Oh, and efficiency is a large concern.

Thanks for the help

 

[Oznog]

Less than 4v and up to 23v?? That's a buck-boost and that's quite complicated. Current-mode buck converters like the HV9910 or ZXLD1360 are quite good. The buck ones have reliable, uncomplicated current regulation.

 

[justDIY]

if you can narrow down your supp. voltage requirements, there are many a switcher to help you. check out dealextreme.com, they have many switching modules for driving the cree's, luxeons, etc... however they're designed mainly for battery powered apps.

 

[ronsimpson]

All these companies make “LED” drivers or PWM. Most companies have selector guides to help you choose a part. I mostly use Zetex but I am not in love with them. Right now I love FAN5330 through FAN 5333 but they will not work in your application. Most if these regulate the voltage on the LED which is a bad idea! Some regulate the current! If you are going for max power you may need to monitor the LED temperature.

ZETEX design note handbook DN81
ZXLD1360, ZXLD1362
**broken link removed**

NATIONAL
**broken link removed**
**broken link removed**

ti.com

fairchildsemi.com

on semi

 

[nickelflippr]

Only a couple of hours? ...just kidding. The flashlight and auto folks seem to have the leds figured out.

With making your own power supply it least you will know how it works, most of the cheapie "Mystery" current regulators have no data sheets. With the 1000ma drive you will need a good sized heat sink. Making a variable current drive would be a real nice option. I think getting 23V tolerant premade regulator is going to be hard to find or expensive.

**broken link removed**
http://autolumination.com/led.html
http://www.candlepowerforums.com/vb/index.php?

 

[zachtheterrible]

I just had a flash of brilliance, but it's making me feel really stupid. I probably should've explained what I was doing first. I want to run this LED off a Li-ion 22.2V (6 cell) battery pack. The reason I'm wanting to do it this way is so I can keep the LED at a constant brightness until the battery pack is completely drained.

So for my flash of brilliance: Being that these are Li-ion cells, they can't be discharged down to less than 2.3V, this being the case, my battery pack as a whole can only dishcarge to 13.8V.

Obviously this significantly decreases my specifications. So what is THE most efficient IC that I can use? The more efficient this is, the less of a heatsink I'll have to use too.

 

[audioguru]

Good. Zach is back.
Hi Zach,
It has been a long time without you.

 

[Oznog]

Obviously this significantly decreases my specifications. So what is THE most efficient IC that I can use? The more efficient this is, the less of a heatsink I'll have to use too.

HV9910 OR ZXLD1360 is still the answer.
Efficiency is NOT so much a matter of the chip itself. The inductor, external transistor (for the HV9910 but there is none in the ZXLD1360), and diode will affect the end efficiency. However, it's not hard to be over 90% efficient with "reasonable" component choices so don't spend too long sweating about +/- a few percent here. For the HV9910 use a "hexfet" mosfet transistor, one of those in an 8-SOIC pkg.

 

[zachtheterrible]

Hey Audio, long time no type! Ya I've got a little project I need some help with. Haven't done a whole lot with electronics in a while. I bought this truck october before last without realizing it was gonna turn into my life for the next year or two. I've since found out I'm a pretty okay fabricator and have learned SOOOO much about gears n axles n engines and what not.

But I've still got that electronics itch i gotta scratch every once in a while!

 

[zachtheterrible]

Those would work oznog. I'm also very open to using PWM, I would imagine this would be more efficient because of the lack of an inductor...

 

[justDIY]

some switch mode converters use pwm ... you still need the inductor to carry / provide current while the switch is off.

 

[zachtheterrible]

Oh. Well which one would you guys recommend I use?

 

[audioguru]

Those big wheels on your nice truck give it a lot of ground clearance.
You're in California, Zach?
Then the truck is up so high that the lizzards (or Mexicans) can't jump in.
Can you jump in?

 

[zachtheterrible]

Yes I am, and no they don't have a chance hehe. I felt kinda bad the other day, I was supposed to give a ride to a rather hefty woman and she could barely get in my truck.

Here in california there's soooooo many trucks lifted way higher than mine. It's rediculous, but the funny thing is they have zero offroading capability, all for show.

How's everything goin up there in Canada?

 

[Oznog]

Those would work oznog. I'm also very open to using PWM, I would imagine this would be more efficient because of the lack of an inductor...

Buck converters like this USE pwm.
There's 3 possibilities here:
1. linear regs or resistors-they just burn up the extra voltage as heat and efficiency is relatively low
2. PWM where someone thinks that switching 10V on and off at 25% duty cycle drives a 2.5V LED. It doesn't work and will burn out the LED.
3. PWM where you use a inductor to convert the voltage. That can be near 100% efficient and ones designed for LEDs inherently limit the current.

Inductors do not create inefficiency. Much the opposite.

There are design documents surrounding the HV9910 and ZXLD1360 that will guide you in selecting the correct inductance. You will need to look at the coil resistance of the inductor too. For example a 1 ohm winding resistance is going to lose 1V and make 1W of heat which is probably way too much. For a given package size, less inductance means less wire length and maybe thicker wire so the resistance is lower. However, if the inductance is too low for the chosen frequency the ripple will be high. You also MUST stay well within the "saturation current" of your inductor at all times.

 

[audioguru]

How's everything goin up there in Canada?

Winter suddenly came for a visit. There is a trace of snow and the temp is a little above freezing in the daytime and a lttle below freezing at night.
Next week will be warmer but raining.
The sun rises at 7:35AM and sets at 4:42PM.

A rather hefty woman?
If you like them big then get a winch to haul them aboard.

 

[bryan1]

Hiya Zach,
Log time no hear mate anyway over here in our RE world a bloke came up with such a simple circuit for powering led's correctly for the component count it's hard to believe how well it works. Personally I've made a high brightness led light using 2 strings of 4 high brightness led's (825,000mcd) and I've tested the circuit on my 30 volt benchtop power supply. The Rs resistor is what sets the current and the Rg resistor isnt critical I just used a 1 meg resistor. My circuit outputs 95mA which is 5mA below the maximum spec's. As the forward voltage on the led's are 3.5 volts so for 4 in a string any voltage under about 13.5-14 volts tends to dim the led's. Now when the voltage test wentup to 30 volts the current stays at the max 95mA. Another bloke used the exact same circuit to power 3 of the cree 80 lumen led's with sucess. Below is the schematic and all credit goes to Commanda from the Feildlines forum. for a good read go checkout the LED masterclass threads in FL in the light forum.

Cheers Bryan

 

[zachtheterrible]

Oznog, that makes sense! Guess I had the wrong idea. I think I'll probably go with the ZXLD1360 as it says I can get 95% efficiency and has a lower parts count.

Hey Bryan, that is a very nifty little circuit. I actually might use that to power leds that I'm going to use on my truck as rock lights, so I can see the boulders that are about to smash up my undercarriage I would imagine it's not as efficient as one of those LED drivers, but my alternator makes plenty of power so I'm not worried about that.

audio, i don't wanna get any false rumors started, i'm not into the big girls. i like mine petite hehe. Here in california it probably reached about 80 deg F today. I can't stand this time of year cuz it gets dark so early! Makes me lazy

 

[justDIY]

I want to run this LED off a Li-ion 22.2V (6 cell) battery pack. ... Being that these are Li-ion cells, they can't be discharged down to less than 2.3V, this being the case, my battery pack as a whole can only dishcarge to 13.8V.

I wouldn't push those cells below 2.8-3v, 2.3v is much too low, you'll be damaging the cells.

also in low load conditions, your lions will hover at their charge voltage for a while before settling down, so figure on 4.2 * 6 = 25.2 volts as your 'high' and 3 * 6 = 18 as your 'low' ... punch those number's into National's WebBench or any of the other manufs solution finders and you should have some good results.