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Old 20th January 2004, 07:44 PM   #1
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Default switching regulator as a current source

I am looking for a switching regulator to drive a 700mA LED. It must operate as a current source. Vin must be at least 40VDC. L6902D would be an excellent candidate, but Vin is only 36VDC. Does anybody know a part that would be a good solution for my application?
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Old 9th February 2004, 11:29 AM   #2
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Default Lm317

Do a search at national semiconductors site for the LM317. In the data sheet they so how to use it as a constant current source. I think it's rated at 40v thou I don't think it would matter because in their fore mentioned schematic it is not connected to ground(regulating current not voltage). By the way, I use this set up to charge nicads...Works good.
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Old 9th February 2004, 02:36 PM   #3
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Thanks for the reply. L317 is a linear regulator. In my application I need to drop 30V at 700mA. Power dissipation is 21W , which is not acceptable.
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Old 9th February 2004, 02:55 PM   #4
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Switching regulator, not linear. I guess for efficiency, but 40V supplies usually have plenty of current to spare. And what LEDs take 700mA???

Tim
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Old 9th February 2004, 02:59 PM   #5
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Linear Tech has an application note for "Current Sources for Laser Diodes" -- you will find schematics, the entire shooting match for CCS using both linear and switched regulators.

If it is a design where "metrics" are important you will probably want to consider using the Linear LT1533 slew controlled switching chip with post regulation.

by the way, Linear now sells their stuff, fewer than 500 pieces, direct to the DIY'r -- and prices are about 40% less than Digikey -- shipping is via FedX.
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Old 9th February 2004, 03:00 PM   #6
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Quote:
Originally posted by Sch3mat1c
Switching regulator, not linear. I guess for efficiency, but 40V supplies usually have plenty of current to spare. And what LEDs take 700mA???

Tim
Luxeon emitters., even the Agilent super-brights can be pushed to 100ma.

depends upon how many candela you want.
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Old 9th February 2004, 03:34 PM   #7
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unrelated questions: do we know if LEDs wear out? assuming that they are working within their spec.

I personally haven't experienced any worn LEDs but others have.
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Old 9th February 2004, 03:55 PM   #8
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Quote:
Originally posted by millwood
unrelated questions: do we know if LEDs wear out? assuming that they are working within their spec.

I personally haven't experienced any worn LEDs but others have.
Agilent has some data on "life cycle" on their website -- it's obviously a function of current, but also the heat exposure of the diode -- we are talking tens of thousands of hours at 20 ma, and a few thousand hours at 40ma.
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Old 10th February 2004, 05:39 AM   #9
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Default Hmmm

Ya, my bad....I was focused on the word "LED" not "700ma". I gotta agree with Sch3mat1c......700ma is alot for a LED, unless your planning on putting a bunch in parallel. Since you got so much voltage why don't you hook em' up in series. You could daisy chain quite a few LED's on 40v.
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Old 10th February 2004, 05:51 AM   #10
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Default And by the way...

And another thing, a linear regulator would be just fine because "40V supplies have plenty current to spare". I mean.. designing a 21watt constant current (SWITCHING!) supply for a LED is not worth it, and why constant current anyway? If a constant voltage is put across a LED, a constant current would flow thru it. nuff said.
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