ADM3251E

Rev. E | Page 11 of 16

THEORY OF OPERATION

The ADM3251E is a high speed, 2.5 kV fully isolated, single-

channel RS-232 transceiver device that operates from a single

power supply.

The internal circuitry consists of the following main sections:

•

Isolation of power and data

•

A charge pump voltage converter

•

A 5.0 V logic to EIA/TIA-232E transmitter

•

A EIA/TIA-232E to 5.0 V logic receiver

DECODE

RECT

REG

V–

C4

0.1µF

16V

VOLTAGE

DOUBLER

C1+ C1– V+

C2+ C2–

R

T

VOLTAGE

INVERTER

GND

ADM3251E

OSC

ENCODE

ENCODE

DECODE

*INTERNAL 5k

Ω PULL-DOWN RESISTOR ON THE RS-232 INPUT.

0.1µF

C3

0.1µF

10V

C2

0.1µF

16V

0.1µF

C1

0.1µF

16V

Figure 13. Functional Block Diagram

ISOLATION OF POWER AND DATA

The ADM3251E incorporates a dc-to-dc converter section,

which works on principles that are common to most modern

circuit that switches current into a chip-scale air core transformer.

Power is transferred to the secondary side, where it is rectified

to a high dc voltage. The power is then linearly regulated to

about 5.0 V and supplied to the secondary side data section

external circuitry.

Because the oscillator runs at a constant high frequency

independent of the load, excess power is internally dissipated

in the output voltage regulation process. Limited space for

transformer coils and components also adds to internal power

dissipation. This results in low power conversion efficiency.

The ADM3251E can be operated with the dc-to-dc converter

enabled or disabled. The internal dc-to-dc converter state of the

internal dc-to-dc converter is enabled. To disable the dc-to-dc

mode, the user must externally supply isolated power to the

the ADM3251E then continue to operate normally.

(GND). It is coupled across the isolation barrier, inverted, and

then appears at the transceiver section, referenced to isolated

RS-232 signal levels that are referenced to isolated ground.

The digital signals are transmitted across the isolation barrier

using iCoupler technology. Chip-scale transformer windings

couple the digital signals magnetically from one side of the

barrier to the other. Digital inputs are encoded into waveforms

that are capable of exciting the primary transformer of the winding.

At the secondary winding, the induced waveforms are decoded

into the binary value that was originally transmitted.

Once the dc-to-dc converter is active, the input voltage

must be decreased below the turn-on threshold to disable

the converter. This feature ensures that the converter does

not go into oscillation due to noisy input power.

+

+

C3

0.1µF

10V

+ C1

0.1µF

16V

+ C2

0.1µF

16V

0.1µF

+

C4

0.1µF

16V

EIA/TIA-232E OUTPUT

EIA/TIA-232E INPUT

V+

C1+

C1–

C2+

C2–

V–

ISOLATION

BARRIER

CMOS OUTPUT

CMOS INPUT

4.5V TO 5.5V

GND

0.1µF

ADM3251E

Figure 14. Typical Operating Circuit with the DC-to-DC Converter Enabled

+

+

C3

0.1µF

10V

+ C1

0.1µF

16V

+ C2

0.1µF

16V

0.1µF

+

C4

0.1µF

16V

EIA/TIA-232E OUTPUT

EIA/TIA-232E INPUT

V+

C1+

C1–

C2+

C2–

V–

ISOLATION

BARRIER

CMOS OUTPUT

CMOS INPUT

3.0V TO 3.7V

3.0V TO 5.5V

ISOLATED SUPPLY

GND

0.1µF

ADM3251E

Figure 15. Typical Operating Circuit with the DC-to-DC Converter Disabled