Boost driver based on LM2621 (for laser diode)

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Hi guys,
Sorry, I know that I just posted a question on an AVR project a few days ago, but...

I've been long wanting to design a general-purpose boost driver for lasers, and I decided to have a go at it. After much researching, I decided to base the design off of an LM2621. I don't have much experience with electronics, so I'm not sure whether there is something better to use or whether it will even function.

Anyway, I mainly relied on the application example from the datasheet (http://www.national.com/profile/snip.cgi/openDS=LM2621), except I added a potentiometer for adjustable output.

The idea was to make something compact (thus, use SMD parts) that can boost the voltage from a standard Li-Ion (~3.7 V) to whatever a laser diode would need. Ideally, a constant current driver would be great, but I think what I've made is constant-voltage...
I'm not sure, though. I'm definitely lacking in understanding at this point.

Anyway, I've included the Eagle schematic and PCB images, so if anyone has tips, suggestions, spots a flaw, or knows a better design, it'd be great if you shared.

Thanks!

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I think other dear 'Freaks here may help you more than me, but IMHO, a simple current controlled power supply for Laser Diodes is not a good aproach. Such beauties (laser diodes, I mean) can came in a many different variants to do so. If there is a component that is sensitive to ESD or to anything, including star's alignement, those are Laser Diodes. 'Simple' diode modules includs usually a PIN or a 'semi-standard' photodiode to sense the emitted power, and the supply is in fact a closed loop that mantains emited laser power to a constant, usually well below the maximum.

Laser Diodes are really sensitive to temperature, moisture, ESD, voltage and current spikes, user's breath, weather conditions, star alignement, your Zoodiac sign and dust. Caution, clean and controlled environment, ESD prevention, well designed supply (soft start, current limited, voltage limited, protection diodes, temperature control, increased temperature mass, and such are mandatory. Breathing masks are also mandatory (like protective clothes that avoid dust, moisture and other particles to fall onto the diode) when working with the bare metal.

Things tend to be much, much more complex that it seems at first glance. So, be patient and be prepared to replace many laser modules. A deep pocked may be useful.

Guillem.
"Common sense is the least common of the senses" Anonymous.

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Thanks for your input. As much as I agree that laser diodes are extremely sensitive electronics, my goals aren't very complicated, actually. I'm not using any complex modules, no feedback system, nor am I worried about my breath damaging the diodes. I'm talking about cheap, yet perfectly capable, diodes which can be found in anyone's DVD and Blu-Ray burners. If one dies (as it most definitely will at some point- they're being overdriven purposely), I'll just get a new one.

The point was to make a small circuit which can drive the diode at a constant current, which I can set. It's the exact same idea as, say, a high-power LED driver. Just a simple boost mechanism, with which I can use a single 3.7 V battery.

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AFAIK, laser diodes like around 2 volts, driven from constant current of course...

You might want to consider a constant current buck converter...
I have heard they can be damaged with even the slightest over current so I would imagine it requires cycle by cycle current limiting...

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The safest way to kill a Laser diode is overdriving it. Usually, the smaller the more sensitives they are. AFAIK, DVD's and Blu-Ray (probably >3.6V, since they are green-blue) have a power feedback for control, like any other little laser LED I know of. Big modules used to pump Diode Pumped Solid State (DPSS) lasers are the ones that are not sensed directly (but usually indirectly), but those have usually big (and grounded) copper thermal mass to stabilize their temperature (and liquid cooling is not rare), and are driven well below they maximum power. Voltage and current on those are still well monitored, and overvoltage plus overcurrent protections as well as ESD protections are used. Usually a relay shorting Anode and Kathode is the latest 'defensive barrier' used.

Control of power is ALWAYS performed by CURRENT.

Guillem.
"Common sense is the least common of the senses" Anonymous.

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You can use LM317 as a constant current source.