Is it possible to have a single circuit with the LT1618 that outputs 3.3V with an input range of 1.8-18V?
You could make SEPIC for this wide range of input voltages.
I have made an example using an inductor with two coupled coils with 47uH. Don't use less inductance due to the high input voltage of up to 18V. Maybe a design expert from ADI can give you more advice if necessary.