Normally when we need to drive low power LEDs we don’t care much about power losses. What we do we add a current limiting resistor and that’s enough. For instance for 20mA LED we choose between 300Ω-1kΩ resistor when powering from 5V. But different situation is with power LEDs. The currents are much bigger here like 1A and more. Adding resistor to limit the current isn’t an option, because power losses become significant. Here you need a constant current driver to drive it safely without wasting energy. It happens that I have Cree XR-E Q5 XLAPM-7090 LED lying around. It requires 3.7V driving voltage and can take up to 1A current.