Oops, found another problem. This is much worse I think, because we can’t just tweak the layout to fix it.
Section 2, Maximum ratings
Table 2. Absolute maximum ratings
Ptot / Power dissipation at TA < 60 °C / 2W
Well, that explains it. We’re way beyond that trying to drive an XM-L2 at 4A.
An XM-L2 needs 3.6v at 4A. That’s 14.4w of output. Input voltage to output voltage delta will make a big difference in efficiency, but I’ll estimate our efficiency using TaskLED’s 3A chart for the b3flex. That puts us at 85% efficiency or worse. I would say approximately 80, but we’ll use 85. At 85% efficiency your input is about 118% of your output. Thus your dissipation is about 18% of your output wattage. 14.4w * 0.18 = ~2.6w That’s bad.
Actual dissipation with the MT-G2 depends strongly on efficiency.
4A * 6.6v = 26.4w
at 90% effic that gives 2.9W
at 95% effic that gives 1.39W
~93% efficiency is the crossover point, at least for the absolute maximum rating of 2W