LTM2881 datasheet(13/24 Pages) AD

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LTM2881

2881fi

For more information www.linear.com/LTM2881

Overview

TheLTM2881µModuletransceiverprovidesagalvanically-

isolated robust RS485/RS422 interface, powered by an

integrated, regulated DC/DC converter, complete with

decoupling capacitors. A switchable termination resistor

is integrated at the receiver input to provide proper termi-

nation to the RS485 bus. The LTM2881 is ideal for use in

networks where grounds can take on different voltages.

Isolation in the LTM2881 blocks high voltage differences

and eliminates ground loops and is extremely tolerant of

commonmodetransientsbetweengroundpotentials.Error

freeoperationismaintainedthroughcommonmodeevents

greater than 30kV/μs providing excellent noise isolation.

µModule Technology

The LTM2881 utilizes isolator µModule technology to

translate signals and power across an isolation barrier.

Signals on either side of the barrier are encoded into

pulses and translated across the isolation boundary using

coreless transformers formed in the µModule substrate.

This system, complete with data refresh, error checking,

safe shutdown on fail, and extremely high common mode

immunity, provides a robust solution for bidirectional

signal isolation. The µModule technology provides the

means to combine the isolated signaling with our RS485

transceiver and powerful isolated DC/DC converter in one

small package.

DC/DC Converter

The LTM2881 contains a fully integrated isolated DC/DC

converter, including the transformer, so that no external

components are necessary. The logic side contains a full-

bridge driver, running about 2MHz, and is AC-coupled

to a single transformer primary. A series DC blocking

capacitor prevents transformer saturation due to driver

duty cycle imbalance. The transformer scales the primary

voltage, and is rectified by a full-wave voltage doubler.

This topology eliminates transformer saturation caused

by secondary imbalances.

The DC/DC converter is connected to a low dropout reg-

ulator (LDO) to provide a regulated low noise 5V output.

The internal power solution is sufficient to support the

transceiverinterfaceatitsmaximumspecifiedloadanddata

applicaTions inForMaTion

rate, and external pins are supplied for extra decoupling

(optional)andheatdissipation.Thelogicsupplies,VCCand

VL have a 2.2µF decoupling capacitance to GND and the

isolated supply VCC2 has a 2.2µF decoupling capacitance

to GND2 within the µModule package.

VCC2 Output

Theon-boardDC/DCconverterprovidesisolated5Vpower

to output VCC2. VCC2 is capable of suppling up to 1W of

power at 5V in the LTM2881-5 option and up to 600mW

of power in the LTM2881-3 option. This surplus current is

available to external applications. The amount of surplus

currentisdependentupontheimplementationandcurrent

delivered to the RS485 driver and line load. An example

of available surplus current is shown in the Typical Per-

formance Characteristics graph, VCC2 Surplus Current vs

Temperature. Figure 19 demonstrates a method of using

the VCC2 output directly and with a switched power path

that is controlled with the isolated RS485 data channel.

Driver

The driver provides full RS485 and RS422 compatibility.

When enabled, if DI is high, Y–Z is positive. When the

driver is disabled, both outputs are high impedance with

less than 10µA of leakage current over the entire common

mode range of –7V to 12V, with respect to GND2.

Driver Overvoltage and Overcurrent Protection

The driver outputs are protected from short circuits to

any voltage within the absolute maximum range of (VCC2

–15V) to (GND2 +15V) levels. The maximum VCC2 cur-

rent in this condition is 250mA. If the pin voltage exceeds

about ±10V, current limit folds back to about half of the

peak value to reduce overall power dissipation and avoid

damaging the part.

The device also features thermal shutdown protection

that disables the driver and receiver output in case of

excessive power dissipation (See Note 4 in the Electrical

Characteristics section).

SLO Mode

TheLTM2881featuresalogic-selectablereducedslewrate

mode (SLO mode) that softens the driver output edges to

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