The TI LM3409 is a DC-DC constant-current buck driver IC. This is a step down driver that can provide a constant current source to power LED(s). The circuit I designed around it is optimized for low ripple, fast switching capability, and current up to about 6A continuous. 

Design specification as built:

• Vin up to 20v
• Vout 3v up to 17v (3v dropout max, 2v typical)
• Output current up to 6A continuous (tested)
• Less than 1% ripple (simulated and measured)
• Clean switching up to 120kHz (measured)
• Analog current adjust using ADJ pin (takes digital pwm inputs and filters using an RC)
• Digital PWM/signal pin (turns off on pin HIGH)
• Easily controllable using an arduino (or anything really...)

Schematic on KiCad:

PCB Layout on KiCad (BOM is included):

And a build video:

PSpice simulation circuit:

Recommend you use a heatsink on the bottom of the board, and on the LM3409 itself as pictured. Also recommended you use my 3D printed mount:

https://www.thingiverse.com/thing:2461948

As pictured, I used a Cree XM-L2 (single 3.3v output), and a wall wart 6.5v 3A PSU. The LED is mounted on a CPU heatsink

That's all for now :)