Thanks. The datasheet is here. In the project I use several LEDs with a monochromatic camera to get a multispectral image of a film. Since I also have to use a filter, I ran some tests with a power supply and figured out that I need to run the LED at 10A.
I am willing to bet your Buck converter cannot respond to a 10A load that appears and disappears in milliseconds. You need a vary large capacitor with very low ESR. Remember the photoflash devices.
Please expand your explanation of your goal. Pulsing an LED on for µS is a lot different than operating for seconds.
Transient response of the current source
If my guess is correct you wish to use these LED as sort of a color filtered flash then you are best using a simple resistor and Mosfet for on off. You will need a Mosfet driver as the Arduino cannot supply enough current at its outputs to switch a Mosfet very fast.