Using a DC/DC converter to power the laser

woop said:

yay. it works!!!
first time i soldered the chip (in a toaster oven at 200C), one of the ground pins didn't connect properly. so i reheated it and tapped the board.
I am using tantalum caps for the input ant output filter. and a 3.8uF ceramic cap for C3. (all caps salvaged from random electronic crap)

supprisingly it works, although with only 68% efficincy with 2.9V in and 3V out at 300mA
I tested it using a 10ohm resistor as the load, which is approx the same load as a burner diode.
now i just need to get myself a real burner diode

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That is an excellent result!

I'm surprised it worked on the first go really, especially since you are using soldering paste on a board without a soldering mask. Without a solder mask the risk of shorts due to solder ending up where you dont want it to is pretty big, so i guess you'll still have to keep an eye on that for future runs.

Does anyone here know what the YSD is meant for? (525kHz)

It's not used in any of the design examples.. I have four of them, and i will test it, but it might need a larger inductor and capacitor..

woop said:

I used a toothpic to apply small ammounts to the pads and the main sink pad. supprisingly no shorts, the solder has a tendancy to bead up anyway so you must be putting on a lot of paste for it to short out.

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I must have done something wrong.. Maybe it was my design, that's why i changed it now... If you look at my pic, the lines for the IC look very fat.. It might have contributed to a short.. I didn't want to make them too thin, so they wouldn't fall away. So now i made them thin where the IC connections are, and wider further away from it.

Besides, it was the first time, that i was trying to do SMD elements on a board without a solder stop mask...

the following modification will work off 2 ni-mh AA or AAA's. next board i make will include that

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I didn't even notice that one, untill you pointed it out. If it could work from only two Ni-MHs, that would be great!

I just killed a board, by heating it too long, before transferring.. The copper separated in some places.. I hoped the rest would stil be usefull, but for some strange reason, it only transferred perfectly in those exact areas, where the copper sepparated...


Anyway, this is my final design. I did some changes, to make it less prone to shorting the IC. There are some areas, that might appear weird, but that's because i made places for different sized components if needed..

Also, for the Rsense, i made a wider strip, so i can solder multiple resistors in parallel there, to adjust the current... There is also place for normal through hole resistors below that area, in case i don't find the right values in the old DVD writers.. So if something looks weird, it's that way for a reason...

I'm gonna etch this one tomorrow.. It's still almost exactly the same as the evaluation board, so i hope it'll have similiar preformance.. The difference is, that it is compressed a bit, to make it smaller, which means some lines are thinner, but also shorter, so i hope everything turns out ok.

I really hope i get it up and running tomorrow.

BTW: Woop, where did you get a 0.665 Ohm resistor? I found some 1.2 Ohm resistors, and i'll solder them in parallel to get 0.6 Ohm.. The circuit is gonna be for an open can, so it will need a higher current.


Also, did you use the exact values of capacitors? Why didn't you use ceramic ones instead of tantalums?

Does anyone know, if tantalums are good for this as well, or should all be ceramic?

I used a .51 and a .10 in serial.
i usede tantalum for input and output but ceramic for C3 mainly because i couldn't find any others which where close enought to the value. and the design called for ceramics (low esr, low inductance, better with high freq)
it will probably work with either one. maybe even use both types for the output capacitor. just try it and see.

with your design, why don't you join pin 1 to the middle gnd directly for better heat flow? and why didn't you just follow the chip layout on the datasheet for the LLP package?

I like my layout. because it is nice and small. and after making one, i realise i could make it even smaller. also it works somehow...

woop said:

with your design, why don't you join pin 1 to the middle gnd directly for better heat flow? and why didn't you just follow the chip layout on the datasheet for the LLP package?

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I did have it joined before, but i wanted to make it more obvious where the pins belong.. That's why there's a separation..
I think there's enough "dogbone" for the heat flow..

EDIT: Oh, i see what you mean... I uploaded a pic, where the pin 1 doesn't go up to the central GND strip.. I have two, almost identical, only that in one it goes up, and in the other it doesn't.. But in both, there is a separation (hole) in the "dog bone" to make it more obvious where the IC pins belong.. Haven't decided which one to use yet.. I like this one better, but this preference is just in my mind.. It just looks better to me, but i know it doesn't make any sense..

Do you really think it would help the heat flow that much, if i use the joined one? There is quite a lot of GND area on both sides of the IC as it is...

I like my layout. because it is nice and small. and after making one, i realise i could make it even smaller. also it works   somehow...

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I like your layout too. Very much so.. I like it because its incredibly simple..  And i'm going to use it, when i get all the correct components..

But for now, it was simply too small. I wanted someting a little bigger, so i can put in different inductors and different resistors if i don't find the correct ones.

Do you really think it would help the heat flow that much, if i use the joined one? There is quite a lot of GND area on both sides of the IC as it is...

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probalby not a great deal. forget i said anything, put it together and make it work

I like your layout too. Very much so.. I like it because its incredibly simple.. And i'm going to use it, when i get all the correct components..

But for now, it was simply too small. I wanted someting a little bigger, so i can put in different inductors and different resistors if i don't find the correct ones.

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now its even smaller! I squashed everything up a bit for my next prototype. its 11.8mm x 25.2mm. I love smd

woop said:

now its even smaller! I squashed everything up a bit for my next prototype. its 11.8mm x 25.2mm. I love smd  

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I hope it will still fit all the components.. As i said, i like your circuit very much, because all traces are square and wide, there are no thin ones and it's very simple.. I think it should be very efficient because of that.

I'm definatelly going to use it, once i have all the right components.. As it is, i lost A LOT of time on improoving mine, just because it has to fit different sized components if necessary..


At the moment i'm desoldering SMD resistors and capacitors from old DVD and CD burners, then i have to measure all the capacitors and sort them, check if the right ones fit my circuit, and then i go etch again....


I found a better label backing for the transfer.. The one i was using now had labels for cassetes, which meant it had rounded cutouts, and these cutouts had indentations in the backing, which made lines, where the toner didn't transfer..

Now i found A4 labels, that have the backing in one piece.. Hope it's easy to print on them, or i'll have to scratch and scratch some more..


But i think i'll have at least one up and running today..


BTW: Do you think it would be better if i used all ceramic capacitors, like specified in the datasheet? Well, i don't have the correct tantalums anyway..

Oh, Woop, do you perhaps know the Wattage of your iron, that you're using for transfer?


Also, could you please tell me how big your C3 is? I found a 2.2uF but it's very small... The datasheet says it has to be capable of 25V.. I have no idea what it's ratings are, since i scavanged all these from DVD drives and just measured their capacity..

woop, have you improved efficiency? my prototype's efficiency is about 71-74% at 277mA and 2.74V across the load (3x1N4005+1N5819 in series) with input voltage from 2.6V to 4V. i tried two different inductors and saw no difference in efficiency, so the only thing left that affects it is the catch diode which i havent changed yet.
probing the output with a scope revealed a monstrous LD nightmare - switching noise/high freq spikes, as expected, perfect executors of a laser diode should they exceed its ratings, unless there is a super-short leads /or SMD at best/ ceramic cap directly on LD. my board has some critical traces that could be made smaller. they contribute a lot to the noisy output.
also, do not leave the output floating, as this will blow the LM3410. while it is trying to increase the voltage until the reference equals the voltage across the shunt, there comes a point where the Uds of the switching FET goes beyond its absolute maximum of 26V and it shorts out...I had to learn it the hard way, eventho it is mentioned in the datasheet along with a circuit addendum how to avoid this mishap.

phenol said:

also, do not leave the output floating, as this will blow the LM3410. while it is trying to increase the voltage until the reference equals the voltage across the shunt, there comes a point where the Uds of the switching FET goes beyond its absolute maximum of 26V and it shorts out...I had to learn it the hard way, eventho it is mentioned in the datasheet along with a circuit addendum how to avoid this mishap.

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Well, it makes sense.. Since it's a current regulator, without a load it tries to increase the voltage to reach that current, but it can't..
Woop already did this by mistake, and said his IC got very hot, but survived..


What kind of inductors did you use? (uH, DCR?) your efficiency sounds very good...


BTW: Phenol, i found all the required values ceramic capacitors for my circuit, but the datasheet says C3 is supposed to be rated for 25V...
I did find some 2.2uF in my DVD drives, but they are very small. You think i can have a problem with this?

Maybe i should go scavenge some more devices, i don't know.....


I also found a resistor suitable for R2, and a couple that i can use in parallel, to get R1, so i'm all set! Finally!

The last batch of PCBs came out very nice, so i'm starting soldering now.

phenol said:

probing the output with a scope revealed a monstrous LD nightmare - switching noise/high freq spikes, as expected, perfect executors of a laser diode should they exceed its ratings, unless there is a super-short leads /or SMD at best/ ceramic cap directly on LD. my board has some critical traces that could be made smaller. they contribute a lot to the noisy output.

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So did we just spend a lot of time building laser killers, or is there anything, that can be done?

The datasheet says, that higher inductor values reduce the current ripple (or was that another datasheet? i read so many, i forgot, but the same should apply here). Did you measure with different ones, since you said, that they don't affect efficiency?


Also, is it possible to clean the output with capacitors a little on the board itself combined with some SMD caps on the LD?


What does your board look like anyway?

if the ones you found are in a 0603 package or smaller, you may have a problem with them. look for 0805, 1206.... even if you cant find a big 2.2uF one, you can still connect 2-3 1-uF bigger caps in parallel and be fine /thats how i did it/.
IgorT, did you find solder paste? I soldered mine with a regular soldering iron and a modified sharpened tip. I had pre-treated the PCB with some flux prior to soldering. this way with proper heating and solder wire application, the tin flew under the tiny pads aided by the flux, which greatly improved wettability.

phenol said:

if the ones you found are in a 0603 package or smaller, you may have a problem with them. look for 0805, 1206.... even if you cant find a big 2.2uF one, you can still connect 2-3 1-uF bigger caps in parallel and be fine /thats how i did it/.

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Well the 2.2uF is only 2mm long, but i have several 1uF and some 0.5uF, that are 3mm long..

I don't know what those package sizes mean.. I usually get everything exactly to specifications, and i have never done something like this completelly by myself before (without the help of my engineers)...


So would you suggest using three of those 3mm 1uF, or maybe two 1uF and one 0.5uF, or is this value not that critical? I still have many capacitors to measure and sort...

IgorT, did you find solder paste? I soldered mine with a regular soldering iron and a modified sharpened tip. I had pre-treated the PCB with some flux prior to soldering. this way with proper heating and solder wire application, the tin flew under the tiny pads aided by the flux, which greatly improved wettability.

Click to expand...

I have several different solder pastes, besides with my modded PCB there should be no danger of shorts..

The first boards had traces, that were too thick, and i did a stupid mistake, while unsoldering the IC.. I'm not going to repeat that.

I now only have 3 XSDs left, and 4 YSD, but i still don't know what those are good for... I do know, that they would probably require a larger inductor and capacitor.. I'll test them, once these work nicelly.

I think i found sizes in milimeters for those packages...

http://www.bs-skyline.com/Products/scc1.htm

From what you said, even the 2mm (0805) 2.2uF would be good enough, right?

The 0603 ones are so small, that i didn't even start measuring and sorting them yet.... From that table, all of mine are 0805 or 1206 size, so i should be fine.


I'm quite excited now.. Soon i'll have the first working board..