Working! LT1308 constant current step-up for LS

[**DONOTDELETE**]

Sounds very intriguing!

Advantages:

<UL TYPE=SQUARE><LI>Excellent, detailed spec sheet
<LI>Relatively efficient current regulator
<LI>Efficiency might improve even more using Zetex diode
<LI>Uses relatively small 5uH inductor
<LI>All parts available from Digi-Key
[/list]

Drawbacks:

<UL TYPE=SQUARE><LI>Chip costs $6.50 from Digi-Key
<LI>A dozen components---large PC board required
<LI>Requires 0.92 volt to operate
<LI>Eight pins less than 1mm apart---hard to solder
<LI>Spec sheet shows efficiency only around 83% with Iout=350mA and Vin=2.4v
<LI>Chip footprint is relatively large at 5x6mm
<LI>Can't safely use 10v tantalum when driving 3 or even 2 LS's[/list]

 

[bikeNomad]

I'm working on a constant-current booster circuit for driving Luxeon LEDs (or lower current LEDs as well).

This is a design using a Linear Technology LT1308B (datasheet at http://www.linear-tech.com/prod/datasheet.html?datasheet=415) switchmode converter to power a Luxeon Star high-output LED. It drives a constant current through the LED, to avoid damaging it and to track the best power point over temperature and manufacturing variations.

I just tried it and it hit 80% efficiency driving one Luxeon Star at 310mA from 2 AA batteries.

It will drive three LS lamps from 2 AA batteries (though not for long...)

It should work (with one LED) down to about 1V input, or up to 10V input. Step-up only. Maximum output voltage is 36V, current is set by the resistor R5 that's in series with the LEDs (200mV across this resistor at desired operating current).

The secret here is to use the low-battery comparator (LBI and LBO pins) to sense the current. When the voltage across R5 is lower than 200mV, the LBO pin is pulled low, causing the output voltage to increase. So the output is a sawtooth of a few mA around the current setpoint.

You can see it (as well as the above text) here on my web server

I'd be curious to find out what you think of it, and to hear suggestions and criticisms.

 

[bikeNomad]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Duggg:
Sounds very intriguing!

<LI>Chip costs $6.50 from Digi-Key
<LI>A dozen components---large PC board required
<LI>Requires 0.92 volt to operate
<LI>Eight pins less than 1mm apart---hard to solder
<LI>Spec sheet shows efficiency only around 83% with Iout=350mA and Vin=2.4v
<LI>Chip footprint is relatively large at 5x6mm
<LI>Can't safely use 10v tantalum when driving 3 or even 2 LS's[/list]<HR></BLOCKQUOTE>

Well, let me address these:
<UL TYPE=SQUARE><LI> the DigiKey price is too high. You can get up to 100 of them from the Linear Technology website for about $4.50 each.
<LI>the board is larger than I wanted, but it's still only 0.9 x 1.0 inch and has some extra space on it.
<LI>I probably will be using two cell designs; the 0.92V minimum is fine there.
<LI>I find soldering the SO-8 parts to be fairly easy; they're easier than the finer pitch parts, and easier than 0402 parts.
<LI>I'm getting about 80% efficiency on my board.
<LI>You can't have easy to solder far apart pins and a small chip. Pick one.
<LI>I'm using a 16V 10uF ceramic output cap, which is giving me good enough results (that is, I'm getting 310mA average with 350mA peaks)
[/list]

 

[bikeNomad]

Well, I finished a board and have tested it. You can see the details as well as pictures at my web site.

I'm getting 80% efficiency driving one Luxeon from 2 AA cells. I can drive three Luxeons in series just fine from the same cells (though I haven't measured the current, I suspect that it's very high; in the 1200mA range, probably).

And the toner transfer technique using plain glossy inkjet paper worked great (my first time!).

 

[Mercator]

Hello, bikeNomad....

Nice job on your circuit.

I noticed that you used the Coleman 2 AA cell flashlight for your mod. I was wondering about this flashlight ever since I first saw it. The battery compartment looks much larger than it should be for AA cells. Actually when I first saw this flashlight I thought it might me using the 123 cells. Is there some sort of sleeve inside? I gusee what I'm getting at is... Would it be possible to fit a battery with a diameter of the CR123 cells inside the body? Do you have some means of measuring the ID of the battery compartment? Also could you show more detailed photos of the flashlight constrution? I'm sure you'll agree that this flashlight size and design lends itself to many easy modifications.

Thanks,
Mercator

 

[bikeNomad]

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

I noticed that you used the Coleman 2 AA cell flashlight for your mod. I was wondering about this flashlight ever since I first saw it. The battery compartment looks much larger than it should be for AA cells. Actually when I first saw this flashlight I thought it might me using the 123 cells. Is there some sort of sleeve inside? I gusee what I'm getting at is... Would it be possible to fit a battery with a diameter of the CR123 cells inside the body? Do you have some means of measuring the ID of the battery compartment?<HR></BLOCKQUOTE>

I haven't quite figured out yet how to get the circuit inside the flashlight, but...

the diameter of the battery tube is 0.825". However, there are three ridges running the length of the tube; these are about 0.1" high. So the effective diameter is about 0.625". My AA cells measure about 0.550" in diameter. (sorry about the inch dimensions, but it's my only caliper).

The bulb passes through a 0.24" thick brass plate at the end whose OD is about 0.9". This has a 0.29" hole in it for the bulb. This plate threads into the bulb end of the battery tube.

The weakest part of this flashlight is the tailcap switch assembly; I had to remove mine and bend out the tabs (that contact the tube) before it would work (!).

The reflector/lens assembly threads into the adjustable head. Inside the head is about 1.375" in diameter. If you mounted a bare LS on a thin circuit board on the brass bulb plate, you could trim a little reflector off and get it to focus (I think). The small bulb that the flashlight uses protrudes a very small amount into the reflector at focus position.

 

[**DONOTDELETE**]

Sounds pretty cool. Where do you get micorscopic resistor and such? I dont think the crackshack sells them. Any thoughts? Also what is a good way to make your own pc boards for you mods?

 

[JollyRoger]

I've been able to fit a luxeon and MAX757-based step up regulator (with an adjustable trim) into the head of the Coleman 2AA flashlight. I'll try to post some pics soon.

 

[bikeNomad]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by thepaltx:
Sounds pretty cool. Where do you get micorscopic resistor and such? I dont think the crackshack sells them. Any thoughts? Also what is a good way to make your own pc boards for you mods?

<HR></BLOCKQUOTE>

I got all the parts from Digi-Key. Another good source is Mouser.

I describe how I made the boards here on my web site.

 

[jeff1500]

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


I describe how I made the boards here on my web site.<HR></BLOCKQUOTE>

That's really good work.

 

[MrAl]

Hello,

Looks like a pretty good circuit.
I might give that one a shot too.
0.9 v input min isnt too bad, but
i guess i'll have to look up alkaline
battery data again and see how much
storage capacity is left at less then 0.9v.

I wanted to look up that schottky diode and
compare it to the Zetex diode; do you know
the manufacturer? The Zetex 2000 diode is
the best one so far.

I would bet that most of the eff loss in
in the output bipolar transistor they use.
It probably has a v sat min of 0.4 volts or
higher.

You might try a hand wound inductor if you
feel like it. You may wish to try several
types of materials as we did with the
Zetex project. It pays off to some extent.
Maybe you could gain another 5%.

If you can get a spice model of the Schottky
in that schematic ill check it out too.

Good luck with it,
Al

 

[MrAl]

Hello again,

In addition to the above post:

Interesting AA Alkaline battery data:

Storage capacity between voltages of 0.90 and 0.75 volts
varies with current drain. With high current drains (800ma)
about 30% capacity remains after the cell reaches 0.90 volts
that would otherwise be utilized if the cell could be
taken as low as 0.75 volts.
With low current drains (10ma, not really useful to us here)
lost capacity is only about 6 or 7 percent.
With medium current drains (300ma) lost capacity is about 20 percent.

Short table:

Note: if the above table is hard to read,
copy and paste it into Wordpad, then select
the entire table, then set the font to
"Courier New". This doesnt work with
Notepad though, only Wordpad.


Using NiCd or NiMH cells this is a different story:
The amount of storage capacity lost as a result of
turning off at 0.9v instead of 0.75v is
5 percent or less.

 

[bikeNomad]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by jeff1500:
That's really good work.<HR></BLOCKQUOTE>
Thanks! I just finished a Coleman 2-AA flashlight mod using it; I put the pictures up too.