ADM3251E(Rev0) View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

ADM3251E
(Rev.:Rev0)

Isolated, Single-Channel RS-232 Line Driver/Receiver

Analog Devices 

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

C1

0.1µF

16V

C3

0.1µF

10V 0.1µF

C2

0.1µF

16V

C4

0.1µF

16V

ADM3251E

VCC

0.1µF

ROUT

TIN

OSC

DECODE

ENCODE

C1+ C1– V+ VISO

VOLTAGE

DOUBLER

C2+ C2– V–

VOLTAGE

INVERTER

RECT REG

ENCODE

DECODE

R

RIN*

T

TOUT

GND

GNDISO

*5kΩ PULL-DOWN RESISTOR ON THE RS-232 INPUT.

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

power supply designs. VCC power is supplied to an oscillating

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

and to the VISO pin. The VISO pin should not be used to power

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.

ADM3251E

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

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

ADM3251E is controlled by the input VCC voltage. In normal

operating mode, VCC is set between 4.5 V and 5.5 V and the

internal dc-to-dc converter is enabled. When/if it is desired

to disable the dc-to-dc converter, lower VCC to a value between

3.0 V and 3.7 V. In this mode, the user must externally supply

isolated power to the VISO pin. An isolated secondary side

voltage of between 3.0 V and 5.5 V and a secondary side input

current, IISO, of 12 mA (maximum) is required on the VISO pin.

The signal channels of the ADM3251E then continue to operate

normally.

The TIN pin accepts TTL/CMOS input levels. The driver input

signal that is applied to the TIN pin is referenced to logic ground

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

then appears at the transceiver section, referenced to isolated

ground (GNDISO). Similarly, the receiver input (RIN) accepts

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

The RIN input is inverted and coupled across the isolation

barrier to appear at the ROUT pin, referenced to logic 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.

There is hysteresis in the VCC input voltage detect circuit.

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.

Rev. 0 | Page 11 of 16

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