In many lights that run with fully regulated voltages sources like the grid, you can just add enough emitters unless your LED VF gets super close to the wall voltage.
That’s one of the techniques used by Philips’ Ultra Efficient (180-210lm/W) lights: they just string enough LEDs in series until they get super close to the wall voltage. This allows them to make a super compact driver design that is super efficient.
In the context of flashlights, linear drivers are always more efficient until you get to ridiculous power levels.
This is where it gets very interesting in the next example with an LHP531.
Let’s say we have a 350mA linear driver and an emitter that has a forward voltage of 2.65V at that current with a 4.2V source.
That gives it an efficiency of 63%. Assuming the emitter has an efficiency of 200lm/W at 350mA, that’ll give us an adjusted efficiency of 126lm/W.
Driving that same LED at 350mA from a FET direct drive source using PWM at absolute maximum power gives us an efficiency of 68.2lm/W.
System efficiency of the 350mA linear driver with a VF of 2.65V and 200lm/W: 126lm/W.
System efficiency of a FET driver driven at an average of 350mA: 68.2lm/W.