Hi,
It´s not clear to me what is your
* input votage (range)
* output voltage range
As far as I understood you need a constant current at the output.
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The easiest method with this controller is that you use a shunt (connected to GND) wiith 1.25 Ohms (= 1.0A. Higher value for lower current).
The other side of the shunt is
* connected to the load
* and to the FB input of the controller.
The power dissipation of the shunt is about 1.25W .. use a shunt with a higher power rating. Maybe 3W. Maybe you need a heatsink, because it becomes hot.
Use a low impedance shunt, no "wire wound".
The shunt is in place of R_FB1. don´t use R_FB2. (datasheet, page 1 schematic).
Don´t forget to feed the load current through the shunt.
Klaus