I've tried to recreate design from 1st page of LTM4668A datasheet in LTPowerCAD and it looks strange.
1) Datasheet Vin range 5-17 V with output as low as 1.5 V - in LTPowerCAD it's visible that Ton@Vin max is way too low and out of range, and it will not work in that Vin range.
2) In datasheet switching frequency is 2.25 MHz, yet at LTPowerCAD it is 2 MHz fixed. Frequency can't be changed, but module could be synchronized to external source.
3) In LTPowerCAD desired BW is about 333 kHz - way high. Using 47uF as suggested output capacitors brings actual BW to like 20-30 kHz. Using lower capacitor values to get high BW yelds to low phase margin. Will low BW not cause instability and/or poor performance?
4) LTPowerCAD suggest Cff capacitor to about 10-15 pF, but datasheet suggest a value of 150 pF.