The LM339 is a comparator.
They are not often used in a power supply.

Are you thinking of the LM324 op amp, which can be used as part of the control loop?

I attached my schematic in Slovak language, anyway there is LM339. Please take a look. @crutschow


Mod: link to old thread.E
https://forum.allaboutcircuits.com/...with-both-ics-tl494-lm339.160918/post-1410273

 

The LM339's appears to be operating as an overvoltage detector that generates a PG 5V signal when an overvoltage is sensed.

 

The Lm339 is used to give the Power Good signal out , it checks when 12V supply is up and running.

The TL494 has built in op amp detection on pins 1,2 and 15,16 for Voltage and current sensing, so for extra protection to shut it down a positive pulse is applied to pin 4.