How to boost my Lipo battery automatically when connected to a load?
You want to regulate LED string current rather than output voltage.  So place a resistor at the bottom of your LED string, use an amplifier to gain up the signal, and send that back to the FB pin.  That's the general idea.  You could just pick a resistor large enough that the LED string current created the regulation feedback voltage, but I suspect the power lost on the sense resistor would be too high.  (P = I^2*R).  An op amp lets you break the problem in half, pick a resistor for lowest power dissipation, pick an op amp gain to increase that sensed voltage. Good luck.