What exactly is a buck driver?
And boost? I guess boost is when the output voltage is greater than the input voltage? Is that correct?
If so, are there 17mm boost drivers that can output 6 to 8 volt from a single 18650 cell?
Boost obviously boosts the voltage. 
“Buck” sounds like my language “buk” which means “duck” (not the bird), which is “lower”, so a buck-driver lowers the voltage (without burning off the excess voltage).
Boosting a single Li-Ion cell is not done often.
Probably not much choice out there, but i’m not too familiar with drivers (yet).
You could maybe consider 2x 18350.
What are you planning to do with it?
To expand on Jerommel's answer:
Buck and boost regulators are also known as 'switching' regulators. With a linear regulator (non-switching), any excess voltage is burnt off as heat. In a switching regulator, an inductor is used to either step the voltage up or down. When the desired voltage is reached (which is sensed via a feedback loop) the switching part of the circuit cuts the power to the inductor. When the voltage in the inductor drops too low (or too high) the switch opens again allowing power to flow and the process starts again. This happens really fast, thousands of times a second. It's obviously way more complicated than that, but in my opinion that's as simple as it can be summarized.
Switching regulators are efficient because instead of burning off the excess voltage, they control the voltage to sort of use only what's required.
If you want to boost a single Li-Ion cell, the best (and easiest to use) IC is the LM3410 from Texas Instruments. It will easily boost to 8 volts from a single cell, but it's output current is limited based on the total output power required. The larger the voltage input/out delta, the lower the output current can be.
- Matt
I’m fairly new at modding lights, not really planning anything, just trying to get a grasp of what’s available.
Of coarse running an MT-G2 of a single 18650 cell would nice so one could avoid having to get a bunch of 18350 cells, but it doesn’t look like that’s easily achieved.
Definitely not I'm afraid. You could easily run an MT-G2 from a single cell using the LM3410 IC, but your maximum drive current will top out at 500mA-700mA.
Most people around here just use two batteries and a modified NANJG driver or one of the BLFXX-Z drivers.
Ahh I see, thanks for the clarification. I was interested in size, so I guess 2 x 18350s is currently the only way to go for MT-G2s in a single 18650 size host.
Pretty much. Depends how hard you want to drive the LED, but most IMR cells will do you pretty proud ;)
I have here somewhere a boost driverwhich is rated 3A (5A peak) output current.
It’s for a single cell to make 5 Volts, but you could probably change some resistors to increase boost.
However, it’s CV not CC (constant voltage not constant current, which you want for an LED)
It uses a LR024.
But i have no idea of the workings…
Ahh, not CC. You wouldn’t happen the have a link?
I forgot a few questions in my original post:
What’s an FET driver?
I also see the term “alomst direct drive” when reading about drivers. Is direct drive the same as just wiring the LED straight to the cell through a switch?
I couldn’t find the original link…
Try ebay search ‘boost converter’ around $ 5.—
But maybe someone would chime in about the LR024 or something.
Can probably be used in a CC setup, was why i mentioned it.
A FET is a relay with no moving parts. It sits in the circuit between LED- and battery ground (battery positive is always connected straight to LED+ at all times), so any resistance in the FET will limit the current that'll pass through the LED. Some FETs are really good (low resistance when switched on) and hard to find, but most are average to really bad (high resistance) but easy to get and cheap. Guess which ones usually get used on cheap drivers...
Constant voltage regulators CAN be used, but you run the risk of serious damage if something goes wrong in the LED. This is because LEDs exhibit a massive change in current for even the smallest changes in voltage. So if the LED overheats and the voltage goes up even 0.1V, it could try and pull serious current from your regulator. Things will go pop. It is possible to add current feedback loops to turn CV regulators into CC regulators, but it's tricky and you have to know what you're doing. I don't, but I have seen some members here and over at CPF manage to do it. They are typically complicated set-ups of course.
Remember that the larger your delta between VIN and VOUT, the lower the efficiency. Boosting a source voltage by 60% or more at 5A+ is going to require some seriously large inductors and very highly rated diodes and FETs. If you can get it to work it'll be very big, and very inefficient.
Honestly in a single 18650 host, 2x18350s and a Direct Drive or Linear driver is going to be WAY less effort than a boost driver and a single 18650 cell. Given the power you are looking for the DD/Linear driver may be just as efficient as a boost driver as well thereby defeating any real point in going down the more complicated route. This is because whilst the 18650 does have more capacity than the 18350 cells, this might be offset by the inefficiencies of the boost converter so your runtime may end up being pretty close between the two approaches.
Personally the difficulty in designing and building a boost driver capable of what you're after and that will fit in such a small host makes it not worth the effort. But that's just me - we'd be nowhere as a species if people just put everything in the 'too hard basket' all the time ;)
- Matt