I would never put them on a separate circuit. Too easy to forget to activate.Two good answers but why do you ask?
One town in my area requires that smoke detectors in residences be on with a lighting circuit. Probably so the resident will not turn the circuit for the smokes off.
I believe this is a good pratice as smokes usually won't tell you that they do not have AC power unlike a FA system beeping/flashing trouble.
Required 120 V w/battery backup smoke dectors in new homes. If the smokes are on a
seperate circuit someone should notice the incessant chirping if 120 V power is lost. But nothing
protects from the Darwin principle if home owner removes the batteries.
E'rr is this something new??
I have 120/battery smokes in my house now and they only chirp when the 9 volt battery is dead? oh and they are not supplied by 120 volts just mounted to the ceiling running on battery only.
I have the same setup with the exact same results.
The smokes have never chirped when I lost AC power due to me or the POCO.