New 3A Driver, Very Cheap

Ped had his eye out for a new laser driver last month when he posted a link to a DC-DC
converter being sold on eBay for ￡0.99. At that price I just could not resist and bought 3
units. After doing some testing, I can say that it is suitable for some laser diodes right out
of the box. While it isn't perfect, (don't expect perfection at this price) there may be a
chance it can be improved (modified).

The first test was a PWM switching which makes it easier to see how it performs during a
clean on and off condition. This revealed that the "soft start" in the MP1584 is working.
There is no overshoot at startup.

The next step was to go straight to random bouncing intermittent contact. For lack of a
better method, I took the loose ends of some stranded wire in my hands and brushed
them together. With the driver set to 3A, I was unable to produce a transient greater than
4.9A after around 20 minutes of doing this. With some new firmware in the "torture
device", the time could probably be reduced, but there is no substitute for a real "makie
breakie."

Varying input voltages and diode drops were also tried. With an increasing voltage on the
input side, the output continued to climb until around 10V, after which it leveled off and
stayed rock solid at the setpoint (3A during the test). Increasing the diode drop on the
output seems to have the effect of reducing the current. This would make it very
important to get the test load as close to the actual voltage drop of the laser diode as
possible to avoid any error in the setpoint. This may be caused by the fact that the pot on
the board appears to adjust both voltage (when used as a voltage regulator) and current.

What all this means is that individual mileage may vary. While I would feel perfectly
comfortable connecting this to a 4W C-mount, I would think twice before energizing a
9mm with it. Clean power cycle transitions would have to be assured. A simple switch or
relay would not do. We're talking some type of fully debounced logic controlled MOSFET
or IGBT. The enable pin (pin 2) of the MP1584 may be another avenue of exploration.

Thermal is prettymuch going to be a non-issue. Running these at 3A a heatsink is
recommended.

So in all I would say that the price is very good. The more people we can have checking
these out and testing, the better.

What circuitry did you use or modify to obtain current regulation?

I don't think it's ideal as a diode driver. The output is constant voltage not constant current.

You should always check the specs of any circuit used with expensive laser diodes!

I have used this circuit board with some limitations . . .

It is a constant voltage design with some voltage overshoot.

It is only speced for operation @4.5 VDC.

Current limiting is 4 amps.

Because of these limitations, you should set the operating Voltage . .

and operating Current about 25% under the maximum desired for the diode.

Diode current will rise with temp increase.

Then, you must provide an adequate heatsink for the diode to minimize the temp rise.

Finally, the circuit is ~ $3 shipped from China, so you must allow a month for delivery.

The size is 22mm x 17mm, so plan accordingly.

LarryDFW

Placing a resistance in series with the diode will allow some current adjustment. It's starting to make
more sense now. The resistor in the test load causes varying current with changes in the voltage
adjustment pot. A lower value resistor should provide better efficiency, but also make the pot
adjustment more critical.

The next logical step would be to feed the voltage drop across the resistor back into the the MP1584 to
provide true current regulation. At 800mV, the reference voltage in the MP1584 doesn't exactly lend
itself to current mode, but is a heck of a lot better than the 1.25V reference of the LM317. I don't know
if this would even be possible. I'll have to review the datasheet once again.

I have used a ZXCT1009 and a low resistance shunt to convert voltage feedback supplies to constant current with minimal power loss

djQUAN said:

I have used a ZXCT1009 and a low resistance shunt to convert voltage feedback supplies to constant current with minimal power loss

Click to expand...

What he ^ said.

In essence, you could un-reflow these components, and put them back onto a proper current regulating PCB, add a ZXCT1009 (and probably replace those tiny 0603 parts because they'll be a pain to recycle), and you've got a 3A current regulator for under $2.

If you work the math on your ZXCT1009 resistors right, you could even re-use the same trimmer.

Not a bad approach at all.

Done that before too. However, I have noted that because the ZXCT is not passive speed, so if you don't have enough of a soft-start timer, your regulator will overshoot because the ZXCT hasn't fully turned on yet and give you a nasty current spike. I've seen it in previous versions of the BlitzBuck, which is why I moved away from the ZXCT to begin with.

Wolfman29 said:

Done that before too. However, I have noted that because the ZXCT is not passive speed, so if you don't have enough of a soft-start timer, your regulator will overshoot because the ZXCT hasn't fully turned on yet and give you a nasty current spike. I've seen it in previous versions of the BlitzBuck, which is why I moved away from the ZXCT to begin with.

Click to expand...

What's a better alternative?

There are a actually a decent number of similar ICs out there.

Sent from my Nexus 5 using Tapatalk

I have also used MAX4172 without problems

Yeah when I linked these, I never said they were perfect, I said they were cheap.
What djQUAN pointed out, that I never noticed, is that these are CV not CC.
But with the mods mentioned above, still not a bad deal.

This would be a great candidate for a "community" driver PCB. The IC comes in a package that wouldn't be difficult for beginners to reflow solder. Someone could make 5 of them for under $20 even after factoring in the extra parts from Digikey and shipping. The board itself, after going double-sided with the components and losing the duplicate wirepads, could probably hit 12 x 12 mm.

@rhd:

As far as I know, there really isn't a better alternative. All of those types of ICs are going to end up not working at passive speed. So unless you can build in a soft-start to your IC which is slower than the turn-on time (significantly) of the ZXCT, then you'll always have a spike. I think.

Wolfman29 said:

@rhd:

As far as I know, there really isn't a better alternative. All of those types of ICs are going to end up not working at passive speed. So unless you can build in a soft-start to your IC which is slower than the turn-on time (significantly) of the ZXCT, then you'll always have a spike. I think.

Click to expand...

The MAX4172 may be better given that it takes its power supply from the pre-regulator V+, instead of just the regulator output.

Hey, someone may as well give it a shot, and then have someone with a scope check it out.

The costs of developing this into a community driver are negligible given the price of the components.

Sent from my Nexus 5 using Tapatalk

I can give it a try but I don't have the time at the moment. I'll post some measurements once I get to it.

djQUAN said:

I can give it a try but I don't have the time at the moment. I'll post some measurements once I get to it.

Click to expand...

If you (or someone) would be willing to calculate the R3 and C3 values, then I'd be willing to put together an initial schematic and board design to get things moving / incubate the idea.

I would suggest that the first iteration simply live with the 0.8V drop, in order to prove the concept (constant current). Then we can miniaturize and boost efficiency by introducing a current monitor IC and a lower wattage sense resistor later. That 0.8V drop really only means about a 15-20 % driver efficiency loss. That needs to be addressed eventually of course, but it's fine for a usable first revision.

I would also keep the initial proof of concept to component sizes that are reasonably easy to reflow, like 1206, so that even beginners can give it a try in their toaster oven.