Looking at the LT8609S as a pre-regulator outputing 6.3vdc @ 1.2 Amps.
Working through the datasheet it appears that a 22uH inductor at 400 kHz
is reasonable. I'd like to support an input range from say 9v to 12v
meaning duty cycles greater than 50% ... the datasheet notes:
Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5),
a minimum inductance is required to avoid sub-harmonic
oscillation. See Application Note 19.
Lmin >= Vin * (2 * DC - 1) / (Sx * (1 - DC))
However I don't see where Sx is given for the LT8609S in the datasheet.