# 2 XM-L at 700mA or 3 XM-L at 200mA



## chicknpiza (Mar 15, 2011)

Would it be better to build A or B? in terms of battery life? In other words, would B have a longer battery life since it is running 3 XM-L's at a lower mA at the same output? If so, by how much, assuming a 9.6v battery with a B2Flex Buck drive.

A) 2 XM-L at 700mA had an output of 520 lum total (260 lum each)

B) 3 XM-L at ???mA have an output of 520 lum total (I'm guess 180mA would equal 520 lum output, but I could be starkly wrong).


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## doctaq (Mar 16, 2011)

there is almost no diminishing return up to 700ma on the xm-l, also 3 leds running at 200ma is going to be dimmer than two leds running at 700ma, this is true of just about any of the bigger leds out there


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## CKOD (Mar 16, 2011)

For 520 lumens, 
2 XM-Ls 260lm each, 700 mA nominal, as stated (using T5 bin, so you can go cool or neutral white) Vf of 2.9V or so, ~4W used (2.9*.7A*2LEDs)
3XM-Ls 173 each, (66% relative intensity) at ~450-500 mA and 2.85 ish volts for the Vf 3.84 watts used (2.85*.45*3) 

http://www.cree.com/products/pdf/XLampXM-L.pdf

You have a lumens goal, so divide that by the number of LEDs and the "standard" 700mA value for the bin youre using (in the case of the T5 bin, right at 260 lumens) that gives you a relative percentage, that you scroll down to the relative flux vs current graph, and find the drive current you need. Then scroll back up the the forward voltage vs current graph to get the nominal voltage at that current. 

That gives you your nominal lumens output, drive current, and voltage, which lets you calculate watts and efficiency. 
So youre looking at 130 lm/W vs 135lm/w not much difference, go with whatever fits your form factor/driver better. 

For your 9.6v battery, Are you using 8 Ni-Mh/Ni-Cd cells in series? Or 3 3.2v LiFePO4 cells? Either way the 3 LEDs if in series would be 8.55v. A tad high for a buck driver that needs whatever the LED voltage is + 1V, meaning that you're close to its limit even with your batteries at 9.6v. So that would dictate that you use the 2 LEDs at 5.8v, and 7v is well beyond discharged for both battery chemistries I mentioned. 

Hope this helps


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## vaska (Mar 16, 2011)

You may use information from here for your estimation, because one XM-L is full equivalent of two XP-G LEDs.

So two XM-L at 700 mA are like four XP-G at 350 = 145,37 lm * 4 = 581,48 lm.
Tree XM-L are like six XP-G > 581,48 / 6 = 96,9 lm which is between 200 and 250 mA for each XP-G of those 6, and is equivalent of approx 450 mA for three XM-L.
2 XM-L consume 4,2 W at 581 lm, and 3 XM-L consume 3,96 W at the same light output rate.


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## wesinator (Mar 27, 2011)

why would you put so little current through each led when they can handle so much more. put 2 amps through 3 leds.


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## flashflood (Mar 27, 2011)

CKOD said:


> For 520 lumens,
> 2 XM-Ls 260lm each, 700 mA nominal, as stated (using T5 bin, so you can go cool or neutral white) Vf of 2.9V or so, ~4W used (2.9*.7A*2LEDs)
> 3XM-Ls 173 each, (66% relative intensity) at ~450-500 mA and 2.85 ish volts for the Vf 3.84 watts used (2.85*.45*3)
> 
> ...


 
This reminds me -- I love Cree as much as anyone, but why on earth don't they provide a simple table of lumens vs. current in 0.1A increments? It's just goofy that the only way to get this data is start with the 280 lm @ 700mA figure and visually interpolate from a graph in which Y=100% (of 280 lm) means x=700mA.


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## hellokitty[hk] (Mar 27, 2011)

flashflood said:


> This reminds me -- I love Cree as much as anyone, but why on earth don't they provide a simple table of lumens vs. current in 0.1A increments? It's just goofy that the only way to get this data is start with the 280 lm @ 700mA figure and visually interpolate from a graph in which Y=100% (of 280 lm) means x=700mA.


It's just goofy to think that even a table with .1A increments will give you a precise value; they'll still be estimates.
Good point though I doubt it'd be too much effort on their part.


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## degarb (Mar 27, 2011)

My question is this. Forget whether current is more efficient (problably not under 700 milliamps). But for me I use ten dollar 6 volt belt agm (4.5 amp hour and cheap/fast to charge many at once). 

If the forward voltage is low enough(well under 3 volt), I wouldn't need some driver to drive two xm-l's. I am not a fan of driver since they suck a good part of the efficiency and runtime, over a voltage closely matched to the led. So, if the forward voltage is 2.9 times two, and current could be limited with some variable resistor to 400 milliamps; I could get 11 hours (.8 amp total from both leds= .8x6volt or 4.8 watts of pure xml efficiency, with no controller drag). 

This wouldn't work if 2.9 volts wouldn't push 400 milliamps. ?
This wouldn't be useful if throw were paltry despite lower lumens.?


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## flashflood (Mar 27, 2011)

degarb said:


> My question is this. Forget whether current is more efficient (problably not under 700 milliamps). But for me I use ten dollar 6 volt belt agm (4.5 amp hour and cheap/fast to charge many at once).
> 
> If the forward voltage is low enough(well under 3 volt), I wouldn't need some driver to drive two xm-l's. I am not a fan of driver since they suck a good part of the efficiency and runtime, over a voltage closely matched to the led. So, if the forward voltage is 2.9 times two, and current could be limited with some variable resistor to 400 milliamps; I could get 11 hours (.8 amp total from both leds= .8x6volt or 4.8 watts of pure xml efficiency, with no controller drag).
> 
> ...


 
I'm not a fan of boost drivers either, but there's nothing wrong with a buck driver. Good ones are around 95% efficient. A resistor isn't going to compete with that unless the voltages are so close that the resistor can be around 0.02 ohms or less (the I^2*R loss). The other consideration is that both Vf and the battery's voltage vary with load and temperature, and the buck driver can adapt.

That said, you might drop a note to Wayne of Elektrolumens. He's been making pure direct-drive lights for years and seems to have the art nailed.


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## SemiMan (Mar 28, 2011)

flashflood said:


> I'm not a fan of boost drivers either, but there's nothing wrong with a buck driver. Good ones are around 95% efficient. A resistor isn't going to compete with that unless the voltages are so close that the resistor can be around 0.02 ohms or less (the I^2*R loss). The other consideration is that both Vf and the battery's voltage vary with load and temperature, and the buck driver can adapt.
> 
> That said, you might drop a note to Wayne of Elektrolumens. He's been making pure direct-drive lights for years and seems to have the art nailed.


 

Good boost drivers are 95% efficient as well. Keep up with the times.

Semiman


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## CKOD (Mar 28, 2011)

SemiMan said:


> Good boost drivers are 95% efficient as well. Keep up with the times.
> 
> Semiman



Any examples? Even in the chip manufacturer data sheets ive seen, they dont claim that much, and over a wide input range, or big difference between input and output efficency starts to drop, I'd be nice to find some good boost drivers.


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## vaska (Mar 29, 2011)

CKOD said:


> I'd be nice to find some good boost drivers.


 
Low efficiency of boost converters is a myth based on the fact, that boost scematic is commonly used in one-cell lights, where one essentially cannot achieve high efficiency because of low input voltage. For example, if you compare buck converter with 6V to 4V input/outgoing voltage to boost with 4V to 6V input/output, you'll find them equal by efficiency.
I'm a headlamp manufacturer for about 6 past years, and in my lamps I use boost scematic to feed three or four LEDs in series from two series Li-Ion elements. This boost converter based on LT3477 has measured efficiency from 92 to 95% depending on the mode. In a month or two I'm going to change scematic for FET switch that will rise efficiency up to 95-97%.


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## flashflood (Mar 29, 2011)

SemiMan said:


> Good boost drivers are 95% efficient as well. Keep up with the times.
> 
> Semiman


 
A boost driver that's 95% efficient with low input voltage (e.g. 1.2V NiMH) like we have in flashlights? Got a part number to back that up?


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## vaska (Mar 29, 2011)

flashflood said:


> A boost driver that's 95% efficient with low input voltage (e.g. 1.2V NiMH) like we have in flashlights? Got a part number to back that up?


 
Why not 0.5V input tension?  Comparing buck at 4V input vs boost at 1.2V is like comparing sweet vs hot.
Try a buck converter at 1.2V input, and you'll have even lower efficiency, than with the boost scematic.


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## flashflood (Mar 29, 2011)

vaska said:


> Why not 0.5V input tension?  Comparing buck at 4V input vs boost at 1.2V is like comparing sweet vs hot.
> Try a buck converter at 1.2V input, and you'll have even lower efficiency, than with the boost scematic.


 
Fair point, but bucking 4V vs. boosting 1.2V to a target of ~3.5V is the actual problem that single-cell flashlights need to solve.


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## CKOD (Mar 29, 2011)

Fair enough point on the Vin/Vout for boost and buck converter. Though I suspect for the same output and same percentage boost or buck (I.E. a 25% boost or 25% buck to get to the same output), a boost will have at least slightly lower efficiency due to I^2R losses from the higher peak inductor current vs the buck. (with both in continuous conduction just to be nice I guess) Are you looking at synchronous rectification for the 95%+ efficiency, or is that still with a schottky? Though now that I think of it I'd like to see some good sepic/Cuk converters, to run a single LED of a single Li cell with regulation the whole time more then boost converters


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## Microa (Mar 30, 2011)

Will you consider Taskled's LFlex? Which is specially designed for a single cell to drive a single XML.


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## vaska (Mar 30, 2011)

CKOD said:


> or is that still with a schottky?



Still with a Shottky, input/output tension is high enough not to bother about using FET rectifier.



CKOD said:


> Though now that I think of it I'd like to see some good sepic/Cuk converters, to run a single LED of a single Li cell with regulation the whole time more then boost converters


 
For such a task, XM-L from one or parallel Li, I'm thinking of buck converter switching to linear mode while Vin comes near Vf.


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## SemiMan (Mar 31, 2011)

Why would someone even bring up an issue of boost efficiency at 1.2V input (and really you need 0.9V for single cell). You can't buck at all at 0.9V so why even try to make the comparison. That is just being argumentative for the sake of being argumentative.

The initial post was regarding 2 or 3 XML, so we know we have an output voltage of 6-9 volts (or so). To hit high efficiencies at these voltages, it would be a synchronous design, however, that would be true for a buck regulator too. Synchronous is more important for high efficiencies at low output voltages versus high output voltages.

Yes I agree that a boost regulator, all things being equal, will have higher losses. However, my original post was to a comment about not being a fan of boost regulators due to low efficiency. For all practical purposes, having a boost regulator that is 90% efficient versus a buck regulator that is 95% efficient is not going to make much of a difference in a flashlight application especially if the boost circuit provides higher flexibility and/or is an enabler. That said, hitting 95% with a boost circuit is possible.


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