Driving 2 luxeons from li-ion, need circuit

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rayearth

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I was originally intending to drive 4 luxeons from 7.2 v (2 li-ion cells) but realized that 2 would be less bulky and provide ample light output for close quarters work. I am currently driving them using a 7 ohm dropping resistor giving me about 250 ma of current. I will probably switch to a 5 ohms or so, but my calculations state that I'll lose about 0.6 watt in the new resistor which means only ~75% efficiency. Is there any chip out there using only a few components that is an efficient step-down voltage converter? I looked through Maxim's line and none of them seem to stand out.
 

lambda

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Other than switchers, take a look at the LM134 constant current source. By using it to sink a PNP transistor's base current, you can get very good regulation, with out generating hardly any heat.
 

bikeNomad

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How do you figure? Whether you're using a resistor or a constant current source, you're still dissipating the rest of the power as heat...

The only advantage of a linear constant current source is that it avoids the over-current that a resistor is likely prone to, extending the constant brightness time of your LED.
 

bikeNomad

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<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by ElektroLumens:

I am giving considerations to a good chip to use in a buck design, that would be small. Nothing has really stood out to me either.
<HR></BLOCKQUOTE>
You should perhaps look at the Linear Tech LTC1701. It has a low dropout voltage, and can output enough current for a LS. And it's in a nice small SOT-23 package. Because it has a 1.4MHz switching frequency, you can use a small ferrite-core inductor, and ceramic caps.

That is, if you want a voltage regulator. I still like constant-current designs.
 

ElektroLumens

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<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by bikeNomad:
Originally posted by ElektroLumens:
[qb]
I am giving considerations to a good chip to use in a buck design, that would be small. Nothing has really stood out to me either.
<HR></BLOCKQUOTE>
You should perhaps look at the Linear Tech LTC1701. It has a low dropout voltage, and can output enough current for a LS. And it's in a nice small SOT-23 package. Because it has a 1.4MHz switching frequency, you can use a small ferrite-core inductor, and ceramic caps.

That is, if you want a voltage regulator. I still like constant-current designs.[/QB]


Thanks bikenomad, I'll check into the LT1701. Voltage regulation works fine for me. I haven't tried current regulation yet. What is most important is the overall efficiency, to me.

grin.gif
 

ElektroLumens

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bikeNomad,

The LT1701 has an input limit of 5.5 volts. It would work fine for 3 batteries. It really doesn't fit the bill for what I want to do, which is use a minimum of 4 AA's. The LM2595 handles a maximum voltage of 45 volts in, and has a 1 amp current rating. I run 8 AA's, and 2 Luxeons. I would like to make a circuit that is one inch in diameter, and be all SMD.

Wayne www.elektrolumens.com
 
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You might want to consider the LTC1877.

Efficiency is over 90% with Vin=7.2v and Iout=350mA.

No external transistor or Schottky diode required.
 

lambda

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Yes, you are right; still generates heat, but unlike a resistor, as you mentioned, no massive thermal effect with higher currents when new batteries are first used.
 

ElektroLumens

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<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Is there any chip out there using only a few components that is an efficient step-down voltage converter? I looked through Maxim's line and none of them seem to stand out.[/QB]<HR></BLOCKQUOTE>


I am giving considerations to a good chip to use in a buck design, that would be small. Nothing has really stood out to me either. Anyway, for the time being, I have been using the LM2595 in a simple circuit. It works quite well with one or two Luxeons. I have used it to power 2 Luxeons at 3.2 volts (parallel), with 8 AA bateries (12 volts), no problems. I also am using this circuit in my headlamp conversion, which uses 4 AA's at 6 volts to 3.2 volts.

The only thing is that it is kind of big, I have it on a 1" X 2" circuit board.
 
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