Tarzan: My response to your questions in order below:
1. The max duty cycle is 0.4 for the ADuM3070 but in the Transformer Design section of the datasheet it is advised to use 0.3 typical for most applications to allow for variations in the design. The exception is the doubling circuit, where 0.35 is advised to use so that too large a turns ratio Ns/Np does not cause a large voltage stress.
2. Transformer inductance is advised in the datasheet to be 60 to 100uH min per winding. There are 2 primary windings,and if one winding is 60uH, the total primary inductance would be 4 times this or 240uH. The transformers in the datasheet meet this requirement.
3. For 5V in to 3.3V out:
Ns/Np = (Viso + Vd)/ (VDD1min*D*2) = (3.3 + 0.5V)/(4.5V*0.3*2) = 1.4
Since Ns/Np = 2 is > 1.4, this transformer can be used for 5V to 3.3V
Note, even though 5V in to 3.3V out is a step down, due to losses in the diode and the typical minimum duty cycle of 0.3, a turns ratio of at least 1.4 is needed.
4. The merit of using the push-pull topology in the ADuM3070 is to increase the efficiency over using the flyback topology.
Regards, Brian