Adding to what Klaus stated, to minimum overhead IS about 3V but if you are using it as a constant current device, you have to add the voltage dropped across the load resistor. It is not the best device to use for constant current applications, a fixed voltage regulator works better.
If you want a constant current from a constant voltage, it will work well. You decide the voltage, make sure there is at least 3V more at the input pin and take the output through a fixed value resistor according to Ohms Law.
Brian.