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Artiet59, the drivers you mention are constant current linears, they use MOSFETs in triode mode or linear region. The principle of operation is like a gas valve (the MOSFET or MOSFETs) where the flow of gas (current) depends in how much the handle is turned open (MOSFET VGS or gate-source voltage). The driver MCU senses the current flow in the sense resistor, and tunes the MOSFETs VGS to limit the current flow accordingly. Of course, when the current flow is below the limit (battery too low, high emitter Vf, etc.) they limit nothing.

What you call “FET” mode is operating the MOSFETs in saturation mode, this is like opening the gas valve “all the way”. The current is then mainly limited by other reasons like spring, driver and battery resistances. If you can only operate the MOSFET blindly in saturation mode (open all the way), the only way to obtain lower modes is with PWM (quickly opening and closing the gas valve).

After checking out the thread CRX FW3A Mods & Teardown I see a driver with 8x 7135s plus MOSFET in saturation mode. Without wanting to be judgmental about the driver (great software I guess), it employs very outdated ways of doing the job. 7135s can only work at 350mA constant current, and so the only way to obtain lower modes is to PWM :facepalm: them. Honestly speaking, I don't see a reason to mix 7135s with a MOSFET onboard, as you can obtain lower modes by directly PWMing the MOSFET (and thus saving all the space occupied by the 7135s).

I prefer to always go regulated, but of course I know how to fine tune the sense resistor of a driver in case I need to lower or to increase the maximum current.

The FW3A has a very fine tuned driver, the Lume1 for FW3A/C/T - Constant Current Buck Boost + FET Driver with Anduril - Now for Sale! (not :X cheap, though).