Introducing the "FemtoFlex" - 8x8mm, 0.5V Dropout Driver

[rhd]

We've got a ton of linear drivers popping up these days, but I think this one is special enough to warrant a thread! Also, I know we see a lot of "vapourware" here, but I have a decent track record of actually bringing these drivers designs out in physical form (even though I don't really have a decent setup for selling any of them at decent prices in volume).

Here's the cool part. As long as it works as expected, I'm going to post the schematic and PCB files for the forum (that seems like the best solution to my inability to practically sell any of my own drivers). Everyone will be free to use the driver as they see fit. I would ask (honour system) that if anyone actually sells the driver (or a close derivative made from the files I post) commercially, that they toss $1 in my direction per driver they sell.

I designed it for the purpose of replacing my Mosquito Red drivers that use the AMC7135, largely because of the incredible difficulty of finding a reasonable North American source for the AMC7135. I was also frustrated with having to deal in 350mA increments, and as small as they are, using four AMC7135 ICs to achieve 1.4A adds some size.

This driver:
- Uses a single STCS1 IC
- Has a total driver dropout of 0.5V
- Measures only... 8mm x 8mm!

That's the neat part! At less than a tenth of a square inch, this guy is 42% smaller than a FlexDrive, and will handle current from 0 to 1500 mA. Specifically speaking, it will be great for two scenarios:

1) Driving Mitsubishi 300 and 500 diodes (at up to 1500mA for the 500s) from 1-cell.
2) Driving BluRay 405nm 12x at up to their absolute max current (600mA) from 2-cells.

More generally, it will work great for almost any Red or BluRay diode. It will also work fine for 445s, but since you're going to be dropping the extra voltage (even though the driver's dropout is low), you'd need to heatsink it. Otherwise, for BluRay or Red, no heatsinking should be required.

 

[jimdt7]

RHD again you rock man !

Jim

 

[Flaminpyro]

I'll have a few of these to work with

 

[benmwv]

Very cool! Only 8x8mm!

Do you have a real pic?

 

[rhd]

Very cool! Only 8x8mm!

Do you have a real pic?

This is V6 of a driver I've been working with for a few months. So while I have real photos of a the versions before, I didn't "compact" it into 8x8 until just now. So I'm waiting on PCBs from dorkbot to get up real photos of a bunch of these ones reflowed.

 

[benmwv]

Oh, ok. Can't wait to see it scaled with a coin or something.

I remember before you were having some trouble getting it to work, did it end up being something wrong with the EN, DISC, and PWM pins like you thought?

 

[rhd]

Oh, ok. Can't wait to see it scaled with a coin or something.

I remember before you were having some trouble getting it to work, did it end up being something wrong with the EN, DISC, and PWM pins like you thought?

One of the initial issues was actually in determining the orientation of the ICs I got from Digikey. The first batch I got was nearly impossible to determine the orientation of. The text was highly faded, there was no visible dot on either side of the IC, nor was there a visible line across the bottom row of pins. I think it was a fluke, because that issue hasn't repeated itself. Might have been a printer low on ink at the big ol' IC factory?

 

[benmwv]

That sucks!

Maybe you were the first to buy them in a very long time?

I'm going to have to try these out!

 

[rhd]

There are two downsides to the STCS1. One of them probably doesn't matter. The other, might (pending me making 20 or 30 of the drivers to test variance).

1) It's common (+), so it won't work for... well at this stage I guess only old-school LPC diodes are a problem. This probably doesn't matter.

2) They have a pretty large +/- of 10% precision on the current setting. To clarify, this doesn't imply that the current "wanders" between 10% +/-, but that from driver to driver there's a +/- of 10%. This means that for true "edge driving" of a diode, you'll still want to test on a test load first to see where in that variance you fall. If I set the driver for 600mA (edge safe current for a 405), I might get 540, or 660 (or anywhere in between). 540 wouldn't be a problem, but 660 probably would.

Now, in reality, I don't know how true or likely this 10% variance is. My initial tests have been pretty close to dead on the current I set. Sometimes those plus/minus figures are theoretical, and in reality the variance will be much smaller. But, according to the datasheet, that 10% is a possibility.

 

[benmwv]

Its a good thing most diodes are case neutral now.

Those +/- figures are usually exaggerated, just in case of the freak every once in a while. I don't remember where, but I read that someone tested a ton of "5%" resistors and they were all within 1% of their labelled resistance.

Could you email me your "schematic" eagle file of this?

 

[rhd]

Its a good thing most diodes are case neutral now.

Those +/- figures are usually exaggerated, just in case of the freak every once in a while. I don't remember where, but I read that someone tested a ton of "5%" resistors and they were all within 1% of their labelled resistance.

Could you email me your "schematic" eagle file of this?

Sent

 

[benmwv]

Here is the little modification I made:

Its using the stcs1 in the other package. (DFN8 3x3mm) The board is 10mm in diameter and sits directly on the diode pins flat against the back of the aixis module. The circle and three holes in the middle are for the laser diode.

The IC is connected to a fairly big ground plane for some heatsinking, but that definitely wont be enough for driving a 445 at 1.5A from two cells. Since this board slides on the diode pins with the parts facing down, you can use some thermal epoxy and heatsink the top of the IC to the back of the aixiz module.

Also, the IC has its connections as pads on the bottom (no pins) so it could not be soldered with a soldering iron, it would have to be reflowed.