Car electronics, low power mode & normal mode

Hi all,

I am trying to build a circuit that will power an arduino when a my car is running, then flip into a low power mode when the ignition is switched off. In low power mode a button or switching the ignition back on should be able to 'wake' and bring the system back on to normal power mode.

The end goal is to not drain the car battery if left in low power mode for a week or so.

Im a little stuck as to whether powering the arduino from a buck converter would be a good idea, and if any of them have a low power mode.

You input would be greatly appreciated.