So! Good news on two fronts.
One is that I finally got my brain-cells lined up and did the freq calculation. There are three main factors we control which affect the frequency (remember we are capped at 2Mhz):
- inductor L (inductance in units Henry)
- input voltage minus output voltage (a wider gap means higher freq)
- output voltage (higher means higher freq)
Clearly when building a driver one must consider the #2 and #3 factors when choosing what to use for the inductor (#1).
From what I can see, the absolute minimum inductor we could use while driving a single XM-L (call it 3.4v) from an 18v source would be ~1.414uH. Working in the other direction, I find that if we use a 3.3uH inductor that puts us at about 856kHz with the same XM-L and 18v power source.
Driving an MT-G2 at 8A it seems to me that a 3.3uH inductor will give us a frequency of about 545kHz.
Two is that as far as I can tell, with some help over here, there are perfectly good Schottky diodes in the 8-15A range in a package called POWERDI5 which will fit on this PCB just fine. I borrowed an Eagle package from Robert Starr and added some legs to it to better represent what we’re actually laying down on the board. I think everything fits quite nicely. (At 5A many packages are available, there’s plenty of space here for any with this general shape.)
