Looking at both the LMZ120x and LMZ140x power modules datasheets, it is not clear to me what the minimum input voltage needs to be for a given output voltage.
Does Vin need to be greater than or equal to Vout?
For example the LMZ12001 datasheet says Vin can range from 4.5V to 20V and it also says Vout can range from 0.8V to 6V. This implies you can have Vin = 4.5V and Vout = 6V.