1.5 W LED Headlight, running off Li-Ion Battery
3 Li-Ion replacement batteries for Nokia 3310. Capacity 1500mAh each, integrated protection electronics, made in China
Power conversion by Philips TEA1208 step-up converter
Light source 1W Luxeon LED
The 270R resistor limits the current through the (internal) switch (and the inductor, too). Modifying this resistor affects the power delivered at low voltage. Excessive current damages the IC. Refer to the datasheet.
At first glance it is desireable to maintain the nominal LED current throughout the life of the battery. However, tolerating some LED current drop below a certain input voltage dramatically increases the battery life and avoids a surprisingly sudden blackout at the end of the battery.
So it's generally better for the user, the regulator and the battery not to select the 270R resistor for a flat-response LED current vs battery voltage curve.