I have some questions about ADUM3070.

Since I don't have experience using isolated DC/DC,

It would be great if anyone could give me some advice.

(sorry to ask many questions at the same time)

#1 Question,

I'm trying to calculate Transform ratio by using the following formula

Ns/Np=(Viso+Vd)/(Vdd1min*D*2)

But I don't know what duty cycle should be.

it says Duty should be 0.3, but I dont where this number come from.

Please advise me on how to estimate duty cycle.

#2 Question,

In order to select Trans inductance, Cout and L1 inductor,

Are there any calculating formulas?

The datasheet recommend value like 400uH for Trans, 47uF for Cout and 100uH/400uH for L1.

Should I use these figures?

Since the output current for my model could be less than 10mA so I'd like to use the smaller capacitor and inductor.

#3 Question,

The datasheet says that it's okay to use 5V to 3.3V for Ns/Np=2.

But I don't think Ns/Np is calculated in 1.2 for 5V to 3.3 V.

Could you make sure if it's correct?

=================================================================

[From page 15]

For a similar 3.3 V input to 3.3 V output, isolated single power

supply and with V DD1 (MIN) = 3.0 V, the turns ratio is also N S /N P =

- 2. Therefore, the same transformer turns ratio N S /N P = 2 can be

used for the three single power applications (5 V to 5 V, 5 V to

- 3.3 V, and 3.3 V to 3.3 V).

=================================================================

#4 Question,

It looks like the most low power isolated DC/DC incorporate Fryback converter.

But ADUM3070 is push-pull converter(using two FET inside the IC).

I think Flyback can reduce external components like Inductor on the second side.

So I just wonder what is the merit of push-pull converter for the low power isolated DC/DC.

Best regards,

Tarzan

Tarzan: A response from the designer:

It is a little bit complex topic. Small output current doesn’t meant to choose small inductance. Small inductance tends to make 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 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.

Regards, Brian