When the Leaf is ON, DC-DC converter is always on and supplying 13.5V or Turn on only when 12V battery is low?
Yes, when the LEAF is on it will always supply ~13.5v to make sure the 12v battery is charged.
Should be clear between "Off", "ACC," "On," and "Ready" states. Only the "Ready to drive" mode engages the traction battery and continuously supplies 13.5volts even after the 12v battery is charged. What's called "On" state in the manual is arrived at after two pushes of the power button from "Off" without touching the brake pedal. All 12v accessories, power windows, windshield wipers, Carwings, etc. are available, but anything requiring 400v is deactivated. Unpowered devices include power steering, climate control (fan will run but no heater or A/C), traction motor (obviously), and DC/DC converter. From "On" you can move the transmission between 'P' and 'N' only. Useful for getting a tow after you've killed your battery...
One other fun bit of trivia. The 12v charging system is smart enough to notice when the lead-acid battery has reached full charge, at which point it backs off the voltage from 14.5v or so to 13.5v. This is really ideal care for these batteries -- the Leaf is really kind to its 12v baby. Given that the thing never does much more than operate a few relays and accessories (i.e., no cranking amperage), it should have a good, long service life. What other car checks that its 12v is topped up and charges it automatically every week or so?