Yeah, I am learning as I go with the buck driver design, it is actually quite simple and ingenues once you wrap your head around it. The design of the actual buck driver is not a big issue as long as I can reverse engineer the MTN MAX driver to see how it is laid out to make sure I am getting the schematic right.
The only real hang up at this point that I see is the parasitic drain issue. Although like I said above I think this could be solved by powering the MCU directly from the batteries and thus not using the buck converter 5V regulator.
This allows us turn the buck converter off when the light is off by a small FET, thus eliminating any drain in the system besides the MCU. It would have a free pin to control the FET since it would only need a single output to control the buck converter. The FET would simply shut off anytime the MCU went into sleep mode.
This is the basis I am working on at the moment.
Someone like DEL would be the best to get an opinion from, I know enough to understand the basics but not how everything interacts.
The MCU does output a PWM but it is only used to tell the buck converter how much current you want. The buck converter does all the actual work.
I did a bunch of reading yesterday but can’t find some of the links that were good. Here is one that explains things:
A bit long but good.
Basically a buck converter is a normal FET circuit with an inductor placed between the FET and LED. The inductor stores energy when the FET is on and slows the voltage rise, then when the FET turns off the reverse happens. It releases it’s stored energy and causes the voltage to drop slowly (in comparison to instantly stopping).
What the buck converter does is watch all of this happen at up to 2mhz+. It then times the FET to turn on so that voltage/current increases to 5% over your target level. It then turns the FET off and lets the inductor release the stored energy until voltage/current (remember they are directly connected with LED’s) is 5% less then target.
It then repeats this process endlessly giving you a regulated and constant current with a 10% ripple (aka, 5% over and 5% under target). Better buck converters can obviously be much better then this but the one we use has a 10% ripple.
That is the basics to how it works, just an FET with an inductor, technically you could make a buck driver by simply adding an inductor to a normal FET driver. but the circuitry to actually monitor things and ensure constant current is a bit more complicated, which is where the buck converter comes in.
The ripple is not a big deal in most cases but if you add a large enough capacitor to the output from the inductor you can further clean up the signal. The old Skyray king drivers had this setup.
Far as I can tell the limiting factor when it comes to current is the inductor, small ones just can’t handle high amps and the larger the inductor the better. Since we have a lot of room in the Q8 this should not be a problem. The one I linked above is rated for 18A and would easily fit in the Q8 assuming we have at least 6-8mm of height clearance, which we should.
Honestly the biggest issue I see for a buck driver in a flashlight is size, trying to fit it on a 17mm board and make it supply high currents would be silly hard and you would need to overdrive the inductor causing a lot of heat.
I think I am going to move ahead with basically using the MTN MAX driver with an FET to turn the buck converter off when sleeping and see what the schematic looks like.
Anyone that has something to say, please do say it!
I pretty much need to finish these driver projects this week as my free time will be cut drastically after that.