LDO selection in Thermal point of view
Why do you need an LDO (low drop-out) regulator if input is 15V and output is 5V? Any old regulator would fit the bill on this. For instance an 7805 (TO220 package and NOT a LDO regualator) will produce a decent 5V from anything between 7V and 30V. You have to watch power dissipation though because with 15V in and 5V out with 250 mA flowing there is 2.5 watts dissipated hence you'll need a heatsink.

It's the same for LDO regulators so get hung up on the term LDO. Maybe a 5V switching regualtor will better suit your needs if power/heat is a problem.


  why they state output current uptp 500mA in datasheet


They state 500mA output current for the small version of the 7805 because with a low differential voltage between input and output (say 2V) the power dissipation is only 1 watts and potentially doable in a small package with good copper on the PCB surrounding it.