A FET can be run as a regulator instead of PWM, that’s basically what is inside a 7135. You could drive the LED at a constant current and therefore the LED would be high eff but theoretical downside is you are burning voltage with the FET. Basically a FET run this way would be the same as infinityx7135 lights.
The aim of the D4 (and Meteor) was to cram as much power in a comparably tiny package. Buck or Boost with drivers (with similar power) which require an inductor would inevitably increase the total size of the light or require significantly more complex machining (like creating a recess in the Emitter shelf to accommodate the taller components of the driver.
The FET solution is slim, cheap and gets the job done. Yes, there are drawbacks, but each other technology would have drawbacks as well, just different ones.
I completely agree with you. There will be compromise on both ends.
So now that the more affordable, compact, peak lumen D4 and D4S are available, it would be a great complement if Hank released a slightly longer, more expensive, regulated efficient version with the same emitter options. The driver on a buck boost is indeed larger and more complex, but Hank was able to make the smallest pop can light in the M43 that is still relevant today, so I have no doubt that he can reproduce his magic on a single cell version.
I think thats one of the things that makes Emisar’s so successful, its the fact that you have so many great emitter choices. As far as I am aware, there are no options like that for an efficient regulated light via buck/boost with the exception of the Noctigon Meteor M43 and quite possibly the Zebralights (not quite as customizable as the Emisar in emitter options).
If people are owning multiple Emisars of the same model in different colors and emitter options, I think these same people will also buy a regulated buck/boost version.
The opportunity for Hank is there and I think the demand is as well 
While HWang is taking djozz’s suggestion regarding regulated D4 permutations into consideration, Nichia E21a/Optisolis versions, perhaps even mules could be offered.
A current regulated aspheric D4 with 4+ emitters of the category, and 18350 tube would be tempting.
I’m hoping that a single emitter D4 (D1/4th?) will be created. I love my D4 with Nichias but it doesn’t get as much pocket time as I thought it would because of the extreme power and the possibility of accidentally turning on. I keep it in a Convoy holster but that keeps it from being an EDC light.
Less power, but still small and having a great UI would be a welcome addition. Maybe with an XHP35 HD or HI?
I also hope that they try to revive their 7 emitter design, but not with a trio of 18650s like they had planned, but with the capability of using a high drain 21700 like the 30T.
True dat. A single emitter, wide OP/TIR D4 non-hotrod would be a real Convoy S2+ killer.
A Nichia E21A D4 would be glorious.
I swapped in a Lexel TA 3 channel driver for my D3W1R. With the fet off its regulated around 550 lemons.
Linear FET drivers like that exist. For example, led4power makes some. They’re typically more efficient than a PWM FET driver, and slightly more efficient than a FET+N+1 driver… but less efficient than a boost or buck at most levels. Because instead of burning off extra voltage as heat, a buck driver converts the extra voltage into usable power.
what is so interesting on sst20 led? if we compare it with xpl……my another dilema which one to choose:)thanks
The SST-20 has more throw/lumen than the XP-L HI.
In lights like the D4, more throw is useful since you do not need to lessen battery life pushing + power to see far away.
And you can get the SST-20 in 95+CRI, which is nice.
thank you very much…bluesword…so now we can mix sst20 and floody optic…is it the best combination what do you think?
I agree.
At direct drive the FET isn’t burning off any voltage, you really can’t get any more efficient then that (at the same current level) with a buck boost because now you have an inductor in series with the LED in addition to a FET, (unless you have a bypass FET). I would like a buck boost, but peak power is kind of the selling point of an Emisar D4. My use case doesn’t require longer run time and I’ll take my zebra if I do require it. I’m not going to say that its worthless to develop a buck boost D4, and I would probably buy one, but I wonder what I would have to give up in size, brightness, etc.
The D4 driver is very flat and fits in a very shallow cavity in the D4. A buck/boost driver would certainly add at least 4mm to the length of the D4.
A single emitter D4 would also be longer because of the deeper optic. But there are a few exotic single emitter optics that are very shallow.
Yes, but at virtually every other output level, a buck/boost tends to be more efficient. And there’s no reason a buck driver couldn’t also have a FET for turbo. It could use whichever power channel is most appropriate for each level. The main drawback would be an inability to ramp smoothly all the way to turbo. Instead, it would have a gap in the ramp when it switches from the top of the buck circuit to the FET.
While trying to reassemble my D4 (after flashing Anduril) one of the e-switch solder pads got ripped off the driver 
. Is this repairable or do I need a new driver?
I did this on Lexel TA driver. With advice I hooked up E-switch to pin 2.
I am sure someone in the KNOW can tell you where to hook the E-switch.
Can you tell if its the one to MCU or ground?
Umm… Closest to pin 2 of the Attiny chip.
Looking at the photo of the driver in the 1st post of this thread, It sure looks like the switch goes to pin 2 on the MCU. You might be able to see the trace between the pad and the switch pin.
I can confirm the pad labeled SW+ is connected to pin 2 of the ATTiny85. The other goes to ground, which pin 4 is a convenient point to connect to ground. Or the adjacent pad of the closest resistor R?.