What is maximum Vout spec of ADP2389/90 ?
I could not find the spec in ADP2389/90 datasheet.
The MIN Vin will have a factor to MAX Vout so the input parameter is important.
Please use the "ADP2389/ADP2390 Buck Designer Tool" that can be found in link below. It will help you in the overall converter design that includes computing for Vout and Iout parameters.