LASER driver [adjustable up to 800mA+]

[DarkClover]

Hy! Just made a small SMD laser driver. It's based on a MC33269D (adjustable voltage regulator).
It can handle up to 800mA of output current without getting very hot. The design has two savety circuits built
in to protect the laser diode from being destroyed by false supply connection.
(although the diode must be connected the right way so DOUBLECHECK before running the device)
With the potentiometer you can set the current output from (almost) 0 to 800mA depending on the chosen resistors
in the design.
If you need more current just swap the MC33269D with some LM317 or equivylent device which also has current regulation abilities.

However it's just a small easy project to get a proper driver for my PHR-803T laser
(known as the diode from the external XBOX HD-DVD drive)


For schematic see attachment.

Datasheet: MC33269-D


NOTE: I do not take any responsibility for using this device. USE AT YOUR OWN RISK!
...just to prevent from being overwhelmed from "it killed my diode" mails

Greets DarkClover

[tedz]

I have looked at the schematics. There are a few points I do not fully understand, but which could be critical for stable and reliable function. My worries are below ( and the logic behind my worries):
1) MC33269 is a so-called low drop voltage regulator. Without suffix, it is an adjustable version, right? (MC33269D-5.0 would be 5V-version etc..). The pin you refer as GND, is the adjustment pin (ADJ) on that version, and the pin is named GND in  the "pre-fixed" versions.
2) MC33269 tries to keep the output voltage constant in relation to the ADJ / GND pin. It delivers "out" a current on that pin (max 120 uA according to the data sheet for the adjustable variant, unspecified for the fixed one). That current is feeding some of the internal circuitry of the chip. The rest of internal current usage (max 8 mA) in the adjustable version of the chip is delivered to the output (thus minimum load for adjustable version is 8 mA, and 0 mA for the "fixed" versions)
3) From the above it is clear, that in this kind of use, it really matters, if you specify the adjustable or fixed version to be used. In case of fixed-voltage version, you should also specify the voltage, naturally.
4) After analyzing your circuitry and the internal circuitry of MC33269, I am pretty sure, that what you do is outside the intended scope of the regulator chip. Therefore, I expect, that one might have some problems with factors such as repeatability (=performance varies greatly, depending on production-related variation on the regulator chips), and temperature stability (as the ADJ/GND-pin current may vary over temperature - it's unspecified.)

Therefore, I would not bet my diode on the circuit, without at least some modifications. However, MC33269 can be configured as a current regulator, and there is a suggested basic configuration on the data sheet for that. With only a slight modification towards the current-regulator configuration, I believe the current regulation of your circuit  could be improved, and the risks thus reduced.

tedz

[DarkClover]

Hy tedz... you seem to be very good in supply electronics

1) In my description at the first line there is the text "...based on a MC33269D (adjustable voltage regulator)"
I've changed the schematic. The GND was there because I used the package of one of the fixed-voltage versions.

2) For a constant minimum output current of 8mA a resistor of 150 Ohm should work.

3) Adjustable current regulation is only possible if you use the ADJUSTABLE version of this ic.
The fixed versions will constantly deliver 800mA or am I wrong?

4) There shouldn't be any problems as long as you use non medium class parts.
But which kind of modification would you perform on the device?

DarkClover

[tedz]

Hi,

Been a bit busy, so haven't have time to answer. Sorry about that.

Basically, the chip you are using is a voltage regulator. They try to keep the output pin voltage constant in relation to the "third pin" (ADJ or GND).

There are only two principal differences between the adjustable and fixed versions:
1) Variable voltage version tries to keep the output 1.25V above the ADJ pin, while the "fixed" ones target a higher voltage.
2) The regulator needs some current to operate its internal circuitry. It is taken from IN-pin. In fixed one, the current goes out through OUT-pin, in variable it goes out via GND-pin. That explains why the variable one's data sheet specifies a minimum load current (8mA).

However, a laser diode should be driven rather with a current source, not with a voltage source, due to its electrical characteristics. Therefore, to  convert a voltage regulator to a current regulator. And, you just need a resistor, which gives at the target current a 1.25V drop. Plus you have to change the wiring so, that the output current goes through the resistor. The principle is well visible in another simple laser diode driver at
http://laserpointerforums.com/f65/140-pin-out-59465.html
, using LM317. The resistor between OUT and ADJ defines the current. For MC33269D, using Ohm's law, the current is 1,25/R, or for given current I the resistor R should be R=1,25/I . So for 800 mA, the resistor is about 1,56 Ohms, and for smaller currents proportionally higher. With 10 Ohms, you get 1,25/10 Amps = 125 mA and so on.

By making the circuit a bit more elaborate, one can avoid to have the output current to go through the adjustment potentiometer. However, the principle is the same: A resistor convert the current to a voltage, which is compared with a reference to regulate the current.

-I hope this clarifies what I meant.

br tedz

[DarkClover]

Okay now I know what you meant. Thanks very much!
I'll change the schematic and build a circuit to test the proper work.

I really missed the fact that the whole current flows over the resistor-potentiometer
construction. And this can be very dangerous.