ADuM4472 datasheet(28/36 Pages) AD

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ADuM4470/ADuM4471/ADuM4472/ADuM4473/ADuM4474

Data Sheet

Rev. 0 | Page 28 of 36

TRANSFORMER PRIMARY INDUCTANCE AND

RESISTANCE

Another important characteristic of the transformer for designs

with the ADuM447x is the primary inductance. Transformers

for the ADuM447x are recommended to have between 60 μH to

100 μH of inductance per primary winding. Values of primary

inductance in this range are needed for smooth operation of the

ADuM447x pulse-by-pulse current-limit circuit, which can help

protect against buildup of saturation currents in the transformer. If

the inductance is specified for the total of both primary windings,

for example, as 400 μH, the inductance of one winding is ¼ of two

equal windings, or 100 μH.

Another important characteristic of the transformer for designs

with the ADuM447x is primary resistance. Primary resistance as

low as is practical (less than 1 Ω) helps reduce losses and improves

efficiency. The dc primary resistance can be measured and specified,

and is shown for the transformers in Table 17.

TRANSFORMER ISOLATION VOLTAGE

Isolation voltage and isolation type should be determined for

the requirements of the application and then specified. The

transformers in Table 17 have been specified for 2500 V rms

for supplemental or basic isolation and for 1500 V rms functional

isolation. Other isolation levels and isolation voltages can be

specified and requested from the manufacturers in Table 17

or from other manufacturers.

SWITCHING FREQUENCY

The ADuM447x switching frequency can be adjusted from

200 kHz to 1 MHz by changing the value of the ROC resistor

shown in Figure 48, Figure 49, and Figure 50. The value of the

ROC resistor needed for the desired switching frequency can be

determined from the switching frequency vs. the ROC resistance

curve shown in Figure 13. The output filter inductor value and

output capacitor value for the ADuM447x application schematics

have been designed to be stable over the switching frequency

range from 500 kHz to 1 MHz, when loaded from 10% to 90%

of the maximum load.

The ADuM447x also has an open-loop mode where the output

voltage is not regulated and is dependent on the transformer

turns ratio, NS/NP, and the conditions of the output, including

output load current and the losses in the dc-to-dc converter

circuit. This open-loop mode is selected when the OC pin is

connected high to the VDD2 pin. In open-loop mode, the

switching frequency is 318 kHz.

TRANSIENT RESPONSE

The load transient response of the output voltage of the ADuM447x

for 10% to 90% of the full load is shown in Figure 36 to Figure 43

for the application schematics in Figure 48 to Figure 50. The

response shown is slow but stable and can have more output

change than desired for some applications. The output voltage

change with load transient has been reduced, and the output has

been shown to remain stable by adding more inductance to the

output circuits, as shown in the second VISO output waveform in

Figure 36 to Figure 43.

For additional improvement in transient response, add a

0.1 μF ceramic capacitor (CFB) in parallel with the high feed-

back resistor. As shown in Figure 36 to Figure 43, this value

helps reduce the overshoot and undershoot during load

transients.

Table 17. Transformer Reference Designs

Part No.

Manufacturer

Turns

Ratio,

PRI:SEC

Constant

(V × μs Min)

Total Primary

Inductance

(μH)

Total Primary

Resistance

(Ω)

Isolation

Voltage

(rms)

Isolation

Type

Reference

CR7983-CL

Coilcraft

1CT:2CT

256

0.2

5000

Reinforced

Figure 48

CR7984-CL

Coilcraft

1CT:3CT

256

0.2

5000

Reinforced

Figure 49

CR7985-CL

Coilcraft

1CT:5CT

256

0.2

5000

Reinforced

Figure 50

TGRAD-560V8LF

Halo Electronics

1CT:2CT

398

0.8

5000

Supplemental

Figure 48

TGRAD-590V8LF

Halo Electronics

1CT:3CT

398

0.8

5000

Supplemental

Figure 49

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