LTC2876/LTC2877

19

28767fa

For more information www.linear.com/LTC2876

applicaTions inForMaTion

±60V FAULT PROTECTION

TIA/EIA-485-A specifies that ground shifts between two

devices on a network can be as large as –7V to +12V

during operation. Most RS485 transceivers cannot safely

tolerate voltages on their interface pins that are much

higher than this range. However, industrial installations

may encounter voltages much greater than this, causing

damage to the devices.

This requirement means that a driver and receiver sharing

communication on a network must be able to operate with

a signal common mode voltage difference of –7V to 12V.

Competing PROFIBUS transceivers can be damaged by

pin voltages exceeding these levels by only a few volts.

The limited overvoltage tolerance makes implementation

of effective external protection networks difficult without

interferingwithproperdatanetworkperformance.Replac-

ing standard RS485 transceivers with the LTC2876 or

LTC2877 can eliminate field failures due to overvoltage

faults without using costly external protection devices.

The ±60V fault protection of the LTC2876/LTC2877 is

achievedbyusingahighvoltageBiCMOSintegratedcircuit

technology. The naturally high breakdown voltage of this

technology provides protection in powered off and high

impedance conditions. Figure 15 further illustrates how

the driver and receiver inputs tolerate large voltages above

the supply and below ground without excessive device

currents. As shown, the driver outputs are reverse-diode

ground. The receiver inputs use resistive dividers that

toleratelargepositiveandnegativevoltages.TheLTC2876/

LTC2877 is protected from ±60V bus faults even with the

±25V EXTENDED COMMON MODE RANGE

TheLTC2876/LTC2877receiverfeaturesanextendedcom-

monmoderangeof–25Vto+25V.Thewidecommonmode

increases the reliability of operation in environments with

high common mode voltages created by electrical noise

or local ground potential differences due to ground loops.

This extended common mode range allows the LTC2876/

LTC2877 to transmit and receive under conditions that

would cause data errors or possible device damage in

competing products.

PB PA

20:1

DIVIDE

FROM DI

INPUT CIRCUITS

LTC2876/LTC2877

SIMPLIFIED DRIVER

OUTPUT STAGE

LTC2876/LTC2877

SIMPLIFIED RECEIVER

INPUT STAGE

20:1

DIVIDE

28767 F15

TO RO

OUTPUT CIRCUITS

Figure 15. Internal Circuit Structure at PA/PB Pins That Tolerates Large Positive and Negative Voltages