LT8610AX datasheet(12/18 Pages) AD

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LTC2879X

Rev 0

For more information www.analog.com

Note: Specifications in this section represent typical

values unless otherwise noted.

RS485/RS422 FOR HIGH TEMPERATURE

APPLICATIONS

RS485/RS422 offers high speed differential signaling for

robust communication between multiple devices over

long distances in noisy environments.

The LTC2879X was designed to operate in temperatures

from –55°C to 175°C, making it suitable for use in harsh

environments such as down-hole drilling and high tem-

perature industrial applications.

The LTC2879X is manufactured using special processing

steps to prevent intermetallic growth that can cause metal

voids on standard parts operating at high temperatures.

Low-drift, low-leakage circuitry and lack of a thermal

shutdown circuit allow the LTC2879X to meet critical

specifications, tested 100% at 175°C.

High Temperature Operating Life (HTOL), per JEDEC

JESD22, is used in the LTC2879X qualification to ensure

reliable high temperature performance. HTOL units

passed 3000 hours operating at 175°C, with no fails.

DRIVER

The driver is enabled when the LTC2879X is powered up

and DE is high. The polarity of A–B follows that of DI. That

is, when DI is high, A drives to a voltage that is greater

than B. If DI is low, B is higher than A. When the driver is

disabled with DE low, both outputs are high impedance

and the overall pin resistance is dominated by the receiver

inputs sharing pins A and B.

Driver Overvoltage and Overcurrent Protection

The LTC2879X driver outputs A and B are protected from

short circuits to any voltage within the absolute maximum

range of –60V to +60V, with a maximum differential volt-

age of –120V to +120V. The maximum short-circuit cur-

rent to any voltage within this range is ±250mA. The driver

includes a progressive foldback current limiting circuit

APPLICATIONS INFORMATION

that continuously reduces the driver current limit with

increasing output short circuit voltage to better manage

power dissipation and heating effects (see plot in Typical

Performance Characteristics section).

RECEIVER

The receiver provides full RS485/RS422 and PROFIBUS

compatibility. When enabled, the state of RO reflects the

polarity of (A–B). When the receiver is disabled, the out-

put is high impedance and RO weakly pulled high to VCC

through an internal 250k pull-up resistor.

High Receiver Input Resistance Permits 96 Nodes

The RS485 and PROFIBUS specifications allows for up

to 32 devices, each contributing one unit load, to be con-

nected together in one network. The A/B input current

of the LTC2879X is guaranteed to be 3× lower than the

standard, over the entire temperature range, permitting

a total of 96 LTC2879X devices in a contiguous network.

±25V Extended Common Mode Range

The LTC2879X receiver features an extended operating

common mode range of –25V to +25V. The wide com-

mon mode increases the reliability of operation in environ-

ments with high common mode voltages created by elec-

trical noise or local ground potential differences due to

ground loops. This extended common mode range allows

the LTC2879X to transmit and receive under conditions

that would cause data errors or possible device damage

in competing products.

Balanced Signal Threshold

The LTC2879X differential threshold is 120mV for rising

input signals and –120mV for falling signals. This consti-

tutes 240mV of hysteresis, which offers a high rejection

to signal noise that can otherwise falsely trip the receiver.

Since these thresholds are centered around zero volts

(i.e. “balanced”), the duty cycle is preserved for small

amplitude signals with slewed edges—typical of what is

observed at the end of a long cable. Figure 10 illustrates

this point.

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