Choosing an voltage regulator
Whether or not the regulator is safe to use depends on the input voltage and the ability of the package to dissipate heat. Suppose that your external voltage source is 12V. Then, the voltage drop across the regulator is (12V - 3.3V) = 8.7V. At 150ma, the regulator would dissipate 8.7V * 0.15A = 1.3W. That is a LOT of power and most regulators would get quite warm (hot actually) at that load current. But, if the input is 5V, then the drop is 1.7V and the power in the regulator is 0.255W and most devices WOULD handle that. 

The key specs to look for are the minimum input-output difference (also called drop-out voltage) and the rated package power dissipation.