ADUM3471ARSZ datasheet(25/32 Pages) AD

25 / 32 page

ADuM3470/ADuM3471/ADuM3472/ADuM3473/ADuM3474

Rev. 0 | Page 25 of 32

APPLICATIONS INFORMATION

THEORY OF OPERATION

The dc-to-dc converter section of the ADuM347x uses a secondary

side controller architecture with isolated pulse-width modulation

(PWM) feedback. VCC power is supplied to an oscillating circuit

that switches current to the primary of an external power trans-

former using internal push-pull switches at the X1 and X2 pins.

Power transferred to the secondary side of the transformer is

full-wave rectified with external Schottky diodes (D1 and D2),

filtered with the L1 inductor and COUT capacitor, and regulated

to the isolated power supply voltage from 3.3 V to 15 V. The

secondary (VISO) side controller regulates the output by using a

feedback voltage VFB from a resistor divider on the output and

creating a PWM control signal that is sent to the primary (VCC)

side by a dedicated iCoupler data channel labeled VFB. The primary

side PWM converter varies the duty cycle of the X1 and X2 switches

to modulate the oscillator circuit and control the power being

sent to the secondary side. This feedback allows for significantly

higher power and efficiency.

The ADuM347x implement undervoltage lockout (UVLO) with

hysteresis on the VCC power input. This feature ensures that the

converter does not go into oscillation due to noisy input power or

slow power-on ramp rates.

A minimum load current of 10 mA is recommended to ensure

optimum load regulation. Smaller loads can generate excess noise

on the output due to short or erratic PWM pulses. Excess noise

generated this way can cause regulation problems, in some

circumstances.

APPLICATION SCHEMATICS

The ADuM347x has three main application schematics, as shown

in Figure 38 to Figure 40. Figure 38 has a center-tapped secondary

and two Schottky diodes providing full wave rectification for a

single output, typically for power supplies of 3.3 V, 5 V, 12 V, and

15 V. For single supplies when VISO = 3.3 V or VISO = 5 V, see the

note in Figure 38 about connecting together VREG, VDD2, and VISO.

Figure 39 is a voltage doubling circuit that can be used for a single

supply whose output exceeds 15 V, which is the largest supply that

can be connected to the regulator input VREG (Pin 20) of the part.

With Figure 39, the output voltage can be as high as 24 V and

the VREG pin only about 12 V. Figure 40, which also uses a voltage

doubling secondary circuit, is shown as an example of a coarsely

regulated, positive power supply and an unregulated, negative

power supply, for outputs of approximately ±5 V, ±12 V, and ±15 V.

For any circuit in Figure 38, Figure 39, or Figure 40, the isolated

output voltage (VISO) can be set using the voltage dividers, R1

and R2 (values 1 kΩ to 100 kΩ), in the application schematics

using the following equation:

ISO

where VFB is the internal feedback voltage, which is

approximately 1.25 V.

ADuM3470/

ADuM3471/

ADuM3472/

ADuM3473/

ADuM3474

1 X1

2 GND1

3 VDD1

4 X2

5 I/OA

6 I/OB

7 I/OC

8 I/OD

9 VDDA

10 GND1

20 VREG

19 GND2

18 VDD2

17 FB

16 I/OA

15 I/OB

14 I/OC

13 I/OD

12 OC

11 GND2

COUT

VCC

0.1µF

CIN

0.1µF

+5V

ROC

VFB

VISO = VFB × (R1+R2)/R2

FOR VISO = 3.3V OR 5V CONNECT VREG, VDD2,AND VISO.

VISO =

+3.3V

TO +15V

47µF

47µH

100k

Ω

Figure 38. Single Power Supply

ADuM3470/

ADuM3471/

ADuM3472/

ADuM3473/

ADuM3474

1 X1

2 GND1

3 VDD1

4 X2

5 I/OA

6 I/OB

7 I/OC

8 I/OD

9 VDDA

10 GND1

20 VREG

19 GND2

18 VDD2

17 FB

16 I/OA

15 I/OB

14 I/OC

13 I/OD

12 OC

11 GND2

COUT1

COUT2

VCC

0.1µF

CIN

0.1µF

+5V

VFB

ROC

VISO = VFB × (R1 + R2)/R2

FOR VISO = 15V OR LESS, VREG CAN CONNECT TO VISO.

VISO =

+12V TO

+24V

UNREGULATED

+6V TO

+12V

100k

Ω

47µH

47µF

Figure 39. Doubling Power Supply

ADuM3470/

ADuM3471/

ADuM3472/

ADuM3473/

ADuM3474

1 X1

2 GND1

3 VDD1

4 X2

5 I/OA

6 I/OB

7 I/OC

8 I/OD

9 VDDA

10 GND1

20 VREG

19 GND2

18 VDD2

17 FB

16 I/OA

15 I/OB

14 I/OC

13 I/OD

12 OC

11 GND2

COUT1

COUT2

VCC

0.1µF

CIN

0.1µF

+5V

ROC

VFB

VISO = VFB × (R1 + R2)R2

VISO =

COARSELY

REGULATED

+5V TO 15V

UNREGULATED

–5V TO –15V

47µF

47µH

47µF

100k

Ω

Figure 40. Positive and Unregulated Negative Supply

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