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Well, it dissipates 16W per Amp whch means it will get hot even at quite low current. Presumably you have it mounted or soldered down to a heat sink to keep it cool.
You should also have a capacitor across its input and ground pins to ensure stability, especially if the wiring to the power source is longer than a few cm.
therefore, for using 78M12, we always should use copper aria as heat-sink? actually I used copper aria, but because of limitation, it is around 4x4 mm2.
I presumed that maybe it is because of some problems with my design!