"Direct drive" drivers, like FET drivers, are the ideal solution for maximum peak output from matched emitter and battery combinations, such as 2 lithium-ion cells in series driving a 6V emitter such as an MT-G2 or XHP50/70 or a single lithium-ion cell driving a 3V emitter such as an XP-L or XM-L2.  Direct drive drivers provide "full throttle" performance, while still offering dimming (lower modes) and low voltage protection capabilities.