Hi, tried looking up Zetex AN & DN. Tried searching here for a 12Vdc to a generate a constant current source for LED lighting. Need to get upto about 20V to 30V to drive a string of 7 to 12 white leds.
Lots of DC to DC converters, but they all have regulated voltage as output.
Zetex show some CCS outputs into 1, or 2 or 3 LEDs from 1, 2, or 3 cells, but I could not find any for higher voltages to suit car battery and such like.
What ICs and circuits, very simple if possible, could give a DC to DC but with a controlled Current output?
Lots of DC to DC converters, but they all have regulated voltage as output.
Zetex show some CCS outputs into 1, or 2 or 3 LEDs from 1, 2, or 3 cells, but I could not find any for higher voltages to suit car battery and such like.
What ICs and circuits, very simple if possible, could give a DC to DC but with a controlled Current output?
Im not sure if the smps control ic knows or cares what you actually are controlling with the feedback - the current or the voltage...
Why doesnt any control IC do the job for you?
Just put a series resistor to your load and take the feedback from it, and if necessary amplify feedback to the control IC with opamp???
Why doesnt any control IC do the job for you?
Just put a series resistor to your load and take the feedback from it, and if necessary amplify feedback to the control IC with opamp???
AndrewT, most IC manufactures will have an inventory of parts that will do this for you and in a simple and efficient way, they are usually listed under their sections on LED lighting and in your case you'd be after a boost converter. I am sure if you look again using those as search/selection parameters you will find what you are after. Buck converters are step down only, boost converters are used when the output voltage needs to be greater then the input voltage and buck-boost (if configured correctly) converters will automatically switch between the mode of operation that is required.
As for boost converters I have used, the LT3517/8 work well but require an external FET. The zetex ZXLD1370 is similar to the LT3517 and also comes with a spreadsheet that designs the circuit for you - I picked it mainly because it's only £2.21 vs the £5.66 of the linear tech part over a RS. These drivers will work at lower output currents but are mainly for driving something more then a few mA.
If you need something for a lower output current, then the LT3519 works quite well, I've used it to drive 80mA into a string of 12 leds, it does not need an external switch if used without PWM dimming.
Obviously all of these parts (I think) are SMD.
As for boost converters I have used, the LT3517/8 work well but require an external FET. The zetex ZXLD1370 is similar to the LT3517 and also comes with a spreadsheet that designs the circuit for you - I picked it mainly because it's only £2.21 vs the £5.66 of the linear tech part over a RS. These drivers will work at lower output currents but are mainly for driving something more then a few mA.
If you need something for a lower output current, then the LT3519 works quite well, I've used it to drive 80mA into a string of 12 leds, it does not need an external switch if used without PWM dimming.
Obviously all of these parts (I think) are SMD.
It really depends on what you're doing.
LEDs are surprisingly tolerant. They strike at voltages below the specified Vf.
You'll see guys over on candlepowerforums.com powering 3.6V nominal white LEDs from a rechargeable LiPo with a fresh charged output of 4.2V with no dropper resistor.
This runs the LED pretty hot (may go purple) while the cell is hot, which reduces its life a bit, but the cell discharges and so what?
If you want to run a string of LEDs from say 12V, you can put 3 whites in series for 10.8V and drop the 1.2V using a couple of rectifier diodes. If it's a car battery use a 9V LDO regulator and put 3 diodes in series with the grounded leg. If the battery gets a bit flat and the supply drops out of regulation, so what?
The longer the string the better in some ways.
This is not necessarily what you want to do if you're designing for high reliability or a guaranteed lumen output for a client, but it's simple, cheap and it'll work within limits.
LEDs are surprisingly tolerant. They strike at voltages below the specified Vf.
You'll see guys over on candlepowerforums.com powering 3.6V nominal white LEDs from a rechargeable LiPo with a fresh charged output of 4.2V with no dropper resistor.
This runs the LED pretty hot (may go purple) while the cell is hot, which reduces its life a bit, but the cell discharges and so what?
If you want to run a string of LEDs from say 12V, you can put 3 whites in series for 10.8V and drop the 1.2V using a couple of rectifier diodes. If it's a car battery use a 9V LDO regulator and put 3 diodes in series with the grounded leg. If the battery gets a bit flat and the supply drops out of regulation, so what?
The longer the string the better in some ways.
This is not necessarily what you want to do if you're designing for high reliability or a guaranteed lumen output for a client, but it's simple, cheap and it'll work within limits.
So what? You are either purposefully damaging the LED or settling for a light that will have less then optimal output characteristics. Why would you actively do this if you can easily avoid it? LED drivers aren't that expensive, nor are they difficult to implement, they are also highly efficient and in the case of the LiPo batteries you'd probably get light for longer, with the driver, then without.
Depending on the exact source.
Damaging things is permitted when they belong to you. Optimal is not always the goal. Never heard of good enough?
Because of the low design effort, low parts count, low space occupancy and low cost? Taken individually or all together?
It all depends on the exact application. I've got a UV lightbox here built with LED strings on the basis outlined above, 12V supply, diode dropper. The LEDs will run at a milliamp or so up to 20 milliamps. It's turned on 5 minutes at a time, discovered empirically. The LEDs cost a few dollars. I could have built constant current supplies for each string, for each LED, but I chose to save the money and effort.
So what? My choice.
I'm offering an alternative that may be acceptable and may not occur to someone with limited experience of LEDs. It cuts across conventional wisdom, but it's based on experience, experience you evidently never took the risk to acquire.
Not everybody has the same concerns as you. I shouldn't have to point this out.
It has nothing to do with concerns of my own, what you're basically advocating is ignoring all conventional wisdom and guidelines/max ratings, throwing caution to the wind and abusing things just because it's easier/cheaper.
If you took the same mind set to other areas of DIY here, as in ignoring SOA curves, xmax ratings and maximum part ratings we'd have lots of projects which spontaneously combust or are prone to premature failure. Which ever way you look at it, this is BAD design, if you're okay with the things you build potentially going up in smoke, then hey good for you.
No I've taken the experience, I've abused LEDs and I know how quickly they can fail if you exceed their recommended operating conditions, either through over current or inadequate heat sinking. It isn't worth it most of the time and if you're going to the trouble of building something using LED lighting, then you're probably doing it for the right reasons, ie efficient lighting that lasts almost forever. If you overdrive the LEDs then you lose out on both of those advantages and using LEDs then turns into being a liability rather then something that should be a perk/bonus.
If even one basic resistor is too much then there is no hope for quality engineering 
There are minimal design requirements which have nothing to do with the "good enough" sentiment. Minimal requirements are there to let the device operate within its limits. Operating white LEDs till they glow purple ?!?! Geez...what will be achieved with that ? I hope you build cars bette.. oh wait. If discharging the cell is the goal next advice will probably be to short it ! No parts needed at all. " Good enough".
I often have to deal with people that defy such basic matters and when things fail you know to who they come with the broken device.
@AndrewT, could you live with 6 x 1 W LEDs in series ? This IC might be a good solution, it can drive 28 x 20 mA LEDs too:
http://www.xlsemi.com/datasheet/XL6003 datasheet.pdf
They can be found on Ebay:
http://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l2736&_nkw=xl6003
There are minimal design requirements which have nothing to do with the "good enough" sentiment. Minimal requirements are there to let the device operate within its limits. Operating white LEDs till they glow purple ?!?! Geez...what will be achieved with that ? I hope you build cars bette.. oh wait. If discharging the cell is the goal next advice will probably be to short it ! No parts needed at all. " Good enough".
I often have to deal with people that defy such basic matters and when things fail you know to who they come with the broken device.
@AndrewT, could you live with 6 x 1 W LEDs in series ? This IC might be a good solution, it can drive 28 x 20 mA LEDs too:
http://www.xlsemi.com/datasheet/XL6003 datasheet.pdf
They can be found on Ebay:
http://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l2736&_nkw=xl6003
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I was horrified when I first saw the way people 'abuse' LEDs
'You can't do that', I said.
I now know from experience that strings of (some) LEDs can be operated for thousands of hours with only minimal voltage regulation. In fact in one job we replaced electro-luminescent strip with just such, extending the service life of illuminated instrumentation by a factor of more than 100 and reducing the cost by a factor of more than 10. Some customers are cowboys.
Ignore this information if you choose.
I've pointed out that there are circumstances in which you may not wish to do this.
In the past I successfully designed equipment to meet the stringent in-service failure rates demanded by a major manufacturer of UMTS base stations.
Please try to get your self-righteous outrage under control, if only for the sake of your own health.
'You can't do that', I said.
I now know from experience that strings of (some) LEDs can be operated for thousands of hours with only minimal voltage regulation. In fact in one job we replaced electro-luminescent strip with just such, extending the service life of illuminated instrumentation by a factor of more than 100 and reducing the cost by a factor of more than 10. Some customers are cowboys.
Ignore this information if you choose.
I've pointed out that there are circumstances in which you may not wish to do this.
In the past I successfully designed equipment to meet the stringent in-service failure rates demanded by a major manufacturer of UMTS base stations.
Please try to get your self-righteous outrage under control, if only for the sake of your own health.
Mmm, I am not so sure who is being self-righteous here.
BTW the issue is shifting from "no voltage regulation" (which should be current regulation I guess) to "minimal voltage (or current) regulation" which is eh... different.
I ignore no information, I read data sheets because I can. There are already enough people messing up by "experience" while ignoring basic design rules. A good reason to have stuff designed and produced elsewhere. So what ?
BTW the issue is shifting from "no voltage regulation" (which should be current regulation I guess) to "minimal voltage (or current) regulation" which is eh... different.
I ignore no information, I read data sheets because I can. There are already enough people messing up by "experience" while ignoring basic design rules. A good reason to have stuff designed and produced elsewhere. So what ?
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I didn't know that advising someone against running components outside their rated spec was self righteous. As I said, I know from my own experience that LEDs do not take favourably to adverse operating conditions.
You can see for yourself how increasing the current and operating temperature can have a significant affect on reducing the LEDs operating lifetime by looking at CREEs LM80 data for most of their standard LEDs. It doesn't take genius to work out that pushing them harder then that is going to have quite serious implications depending on how the LED is built.
Is what you said though, some LEDs will survive being treated poorly. If you don't know which these are, then buying some and blindly hoping that they will work beyond their rated spec is poor advise to give to anyone. But as you're the one advising for pushing LEDs harder then they are specc'd to work, which LEDs do you know are capable of this treatment? Because if someone is thinking of doing what you're suggesting then it would be nice to know. It would also be nice to know from any point of view which are the more rugged LEDs out there, because those are bound to have better lifetime projections anyway, when run within their usual operating parameters.
You can see for yourself how increasing the current and operating temperature can have a significant affect on reducing the LEDs operating lifetime by looking at CREEs LM80 data for most of their standard LEDs. It doesn't take genius to work out that pushing them harder then that is going to have quite serious implications depending on how the LED is built.
Is what you said though, some LEDs will survive being treated poorly. If you don't know which these are, then buying some and blindly hoping that they will work beyond their rated spec is poor advise to give to anyone. But as you're the one advising for pushing LEDs harder then they are specc'd to work, which LEDs do you know are capable of this treatment? Because if someone is thinking of doing what you're suggesting then it would be nice to know. It would also be nice to know from any point of view which are the more rugged LEDs out there, because those are bound to have better lifetime projections anyway, when run within their usual operating parameters.
Jean-p
just bought 10 off xl for £5.20 incl postage.
So cheap, I hope they are not faked!
Now I've got to learn how to make a sop8 PCB with 5 added components.
Seems like I just change Rs to suit the LED current that I select.
Will start a new Thread if I need technical assistance on getting the XL to work.
Thanks to all.
just bought 10 off xl for £5.20 incl postage.
So cheap, I hope they are not faked!
Now I've got to learn how to make a sop8 PCB with 5 added components.
Seems like I just change Rs to suit the LED current that I select.
Will start a new Thread if I need technical assistance on getting the XL to work.
Thanks to all.
Hi AndrewT I would never have thought that I could give you usable advice !
Pad2pad is good software for developing SMD PCB's and it is free. You will have to have the PCB's made in their facility though. Quality is OK.
This is a small job so if you are not comfortable with layout software I could do this for you as an exception if I can have one or two of the boards.The data sheet is quite clear, I don't think you will run in to trouble with these chips as they are used in LED backlight applications for computer monitors. Large numbers + small mistake = large RMA
Anyway send me a PM if you like me to design a small PCB. I would advise on using good Phoenix screw terminals like MKDSN series.
Pad2pad is good software for developing SMD PCB's and it is free. You will have to have the PCB's made in their facility though. Quality is OK.
This is a small job so if you are not comfortable with layout software I could do this for you as an exception if I can have one or two of the boards.The data sheet is quite clear, I don't think you will run in to trouble with these chips as they are used in LED backlight applications for computer monitors. Large numbers + small mistake = large RMA
Anyway send me a PM if you like me to design a small PCB. I would advise on using good Phoenix screw terminals like MKDSN series.
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CREE LEDs arent exactly the best - to get around Nichias patents they use SiC; poor thermal resistance is a direct result.
the biggest problem with overdriving LEDs is that the light output is non-monotonic, in a bad way (doubling current gets less than twice the light). Assuming, of course, one does not melt the bond wires or the die. picking the optimal bias is non-trivial.
I've seen plenty of laser pointers that use battery impedance as the current limiter. and they all cost < $2 to buy. its actually pretty horrifying what happens inside a lot of commercial electronics. in many cases not exploding at power-on suffices for a pass/fail test.....
the biggest problem with overdriving LEDs is that the light output is non-monotonic, in a bad way (doubling current gets less than twice the light). Assuming, of course, one does not melt the bond wires or the die. picking the optimal bias is non-trivial.
I've seen plenty of laser pointers that use battery impedance as the current limiter. and they all cost < $2 to buy. its actually pretty horrifying what happens inside a lot of commercial electronics. in many cases not exploding at power-on suffices for a pass/fail test.....
It is clear from all the graphs that light output/watt is always better when run at much less than maximum current.
I want the most light for a set quantity of power. I will not be running them "hot". And besides that "hot" reduces life.
A double saving but not going to maximum current rating.
I want the most light for a set quantity of power. I will not be running them "hot". And besides that "hot" reduces life.
A double saving but not going to maximum current rating.
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