Tarzan: I asked the designer your 2 questions and he had a thoughtful response to the L and C selection and an attachment to explain the frequency and transformer design:
Your Question 1 is from thread 46249 and repeated here::
1. I understand what you mean, for ADUM3070 trasformer inductance should be 60 to 100uH.
But I just wondered how the switching frequency affects the selection of transformer.
Because this device has flexible Fsw setting (200kHz-1MHz).
And for the selection of the LC components on the secondary side,
Don't you have any calculating formula?
I don't know why the datasheet says Cout=47uF,and L1=47uH.
I'd like to use smaller capacitor and inductor to adjust Fsw since the load current will be less than 10mA in our system.
Do you have an application note about ADUM3070, which explains more about how to select external components at the different Fsw?
1) Answer to question 1, It is a little bit complex topic. Small output current doesn’t mean to choose small inductance. Small inductance tends to make DC to DC converter work in DCM mode at low load current; this will make the transient response bad; if they don’t care load transient response they can try some inductances with small values. Attached is one document regarding magnetizing inductance and filter inductance for your information; both of them will affect the work mode (CCM or DCM), that is the transient response. The switching frequency affects the work mode, also while choosing transformer we need to pay attention to its saturation characteristic.
Typically the filter inductance decides the work mode and the bypass capacitance decides the ripple (△V= I*DT/C). Also the L and C relates to the loop compensation design, if they change the values, they can change the small signal response of the converter; but this is tricky, they can change the L and C, however try to maintain the stability of converter.
Regarding question 2), yes, the converter will be shut down for 2mS and the soft start process is about 8mS. The current limit threshold is designed inside the chip, it is fixed.
Regards, Brian
