Can I be explained what type of driver M21K uses? Is it linear as this VDO suggests? And super-turbo at 20 A is directly driven?
And @thefreeman’s answer (so it doesn’t get buried):
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It’s a buck driver, unsure about the 20A momentary turbo, if it’s still buck or FET (Simon has a tendency to give current ratings to his unregulated FET drivers). I gifted mine before having a chance to open it up.
I ordered another one for me and I’ll take a better look then.
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Like listing “6A” for the CC + FET driver of my S21E with 519a? I asked and was told it was max current, although this is a FET turbo.
Thank you for solving this puzzle for me (and @QReciprocity42 whom I also asked about the mystery). Now that I see your post, I am thinking the 6A and 20A designations must be the top of the CC part, and FET is just direct drive?
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I have M21K with LHP73B coming. Perhaps will have the answer when it gets here once I measure Turbo output?
Using the test curve by koef3 below, if I get much higher than 6000lm, then it indicates driver has Turbo = something higher than 20A, meaning FET direct drive?
If I get 6000lm, then Turbo could be the highest level of the Buck driver, 20 amps.
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I don’t think I’ll see 6k lumens, especially not out of a TIR. These big multi die leds love amps and low iR batteries so the more m you can feed them the better. In my experience, the SFH, SFN 9 and 24 die leds need about 30-60 amps to really start singing. The Nightwatch NI03S needed 50+ amps for 13,000 Lumens, limited only by the batery I had. You’ve got much higher output batteries now-tabless cells didn’t exist when I did my review. I’m going to estimate about 5k Lumens out of the M21K for 10 secomds before a steep thernal (timed) stepdown.
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I predicted 5250 lm and some 625 ANSI m for it here.
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I measured 5770 Lm on my M21K LHP73 5000K; 144k CD.
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I put it on the Map. The lights with * are predictions, the rest use measurements.
For what I consider the best review site (and excellent reviewer 
) I am surprised Simon didn’t include Tenpower with the light you’re reviewing.
When my M21K gets here, it will go straight from the package to my lumen tube. The suspense is almost too much :=).
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I did not realize @Sirstinky has a review website–what is it?
He reviews on 1lumen as Nick. Reviews a lot of Nightwatchs (among others) like this one and has cost me dearly.
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I haven’t got the M21K yet. I’m hoping it will have the high output battery though. I was referring to the Nightwatch lights I’ve reviewed. Tabless batteries weren’t mainstream when I did the NI03S review and the one included with the A54U and the NS14R were inconsistent.
There are a few options you can add to your order.
His is/will be a review sample though 
We always request the best battery or the default battery with samples. I’m hoping it’s either the JP40 or the RS50
Like listing “6A” for the CC + FET driver of my S21E with 519a?
From the excellent review of S21E here by @bilakos10, I found out what the listing above means. This driver is FET+1(7135). Turbo is indeed FET, and max current of FET Turbo at 4.2v is 6A.
So exactly as per thefreeman’s note, a max current listing from Simon, like 20A for M21K, doesn’t narrow down whether it’s Buck or FET.
It’s an asynchronous buck driver ( as expected), with reverse polarity protection.
High/top of the ramp is regulated at 9.85A.
Momentary turbo is regulated at 19.96A.
So it’s fully regulated, no FET direct drive channel for momentary turbo.
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do we know if its buck or linear?
Yes, it’s buck. This thread is too long and noone asked Simon so I did. It’s a buck driver.
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Asynchronous 
, anyway better then linear.