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LTM2882-3

Dual Isolated RS232 µModule Transceiver + Power

Linear Technology 

LTM2882

Applications Information

Overview

The LTM2882 µModule transceiver provides a galvanically-

isolated robust RS232 interface, powered by an integrated,

regulated DC/DC converter, complete with decoupling

capacitors. The LTM2882 is ideal for use in networks

where grounds can take on different voltages. Isolation in

the LTM2882 blocks high voltage differences, eliminates

ground loops and is extremely tolerant of common mode

transients between grounds. Error-free operation is main-

tained through common mode events greater than 30kV/

μs providing excellent noise isolation.

ANY VOLTAGE FROM 3.0V TO 3.6V LTM2882-3

1.62V TO 5.5V 4.5V TO 5.5V LTM2882-5

EXTERNAL

DEVICE

VL VCC

ON

DIN

T1IN

R1OUT

T2IN

R2OUT

LTM2882

VCC2

DE

DOUT

T1OUT

R1IN

T2OUT

R2IN

µModule Technology

GND

GND2

2882 F05

The LTM2882 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 core-

less transformers formed in the µModule substrate. This

system, complete with data refresh, error checking, safe

shutdown on fail, and extremely high common mode im-

munity, provides a robust solution for bidirectional signal

isolation. The µModule technology provides the means

to combine the isolated signaling with our advanced dual

RS232 transceiver and powerful isolated DC/DC converter

in one small package.

DC/DC Converter

The LTM2882 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 at 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 regulator

(LDO) to provide a regulated low noise 5V output, VCC2.

An integrated boost converter generates a 7V VDD supply

and a charge pumped –6.3V VEE supply. VDD and VEE power

the output stage of the RS232 drivers and are regulated

to levels that guarantee greater than ±5V output swing.

Figure 5. VCC and VL Are Independent

The internal power solution is sufficient to support the

transceiver interface at its maximum specified load and

data rate, and has the capacity to provide additional 5V

power on the isolated side VCC2 and GND2 pins. VCC and

VCC2 are each bypassed internally with 2.2µF ceramic

capacitors.

VL Logic Supply

A separate logic supply pin VL allows the LTM2882 to in-

terface with any logic signal from 1.62V to 5.5V as shown

in Figure 5. Simply connect the desired logic supply to VL.

There is no interdependency between VCC and VL; they

may simultaneously operate at any voltage within their

specified operating ranges and sequence in any order. VL

is bypassed internally by a 2.2µF capacitor.

Hot Plugging Safely

Caution must be exercised in applications where power is

plugged into the LTM2882’s power supplies, VCC or VL,

due to the integrated ceramic decoupling capacitors. The

parasitic cable inductance along with the high Q char-

acteristics of ceramic capacitors can cause substantial

ringing which could exceed the maximum voltage ratings

and damage the LTM2882. Refer to Linear Technology Ap-

plication Note 88, entitled “Ceramic Input Capacitors Can

Cause Overvoltage Transients” for a detailed discussion

and mitigation of this phenomenon.

2882ff

12

For more information www.linear.com/LTM2882

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