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LTC2879X

Rev 0

For more information www.analog.com

APPLICATIONS INFORMATION

Figure 14 shows an unprotected LTC2879X struck repeat-

edly with 26kV from an ESD gun using air discharge to

illustrate the strike energy. The device continues to func-

tion normally after the strikes, without damage or cycling

the power.

EFT

Electrical fast transients can result from arcing contacts

in switches and relays, common when switching induc-

tive loads. The IEC standard for EFT is IEC61000-4-4 and

specifies a repetitive burst pattern lasting 60 seconds. The

LTC2879X is robust to EFT events and passes the highest

level recognized in the IEC standard: level 4, ±2kV on the

A and B pins, without any external protection.

PROFIBUS APPLICATIONS

The LTC2879X meets the requirements for PROFIBUS appli-

cations for all data rates up to 12Mbps over the full tem-

perature range. Unlike some RS485 transceivers that claim

PROFIBUS compliance but test only to RS485 standards, the

LTC2879X is tested to the specifications for PROFIBUS-DP

masters and PROFIBUS-DP slaves, fully compatible with IEC

61158-2, Type 3: medium attachment unit: asynchronous

transmission, wire medium. These tests are performed using

specific PROFIBUS loads to ensure compliance.

When using the LTC2879X in PROFIBUS applications,

beware that the naming convention for PROFIBUS wires

is opposite to convention commonly used for RS485. In

standard installations, connect the LTC2879X A pin to the

B wire (red) and the LTC2879X B pin to the A wire (green).

For more information on PROFIBUS applications, refer to

the LTC2876/LTC2877 data sheet.

3.3V OPERATION

The LTC2879X can be used with a supply voltage as low

as 3.0V in RS485 installations. Reducing the supply volt-

age reduces the driver output signal swing below what

is specified in the RS485 standard but still produces

signals much larger than the 200mV minimum signal

swing required at the receiver input. A plot in the Typical

Performance Characteristics section shows the driver

output signal for 3.3V and 5V supply voltages.

3.3V-powered LTC2879X devices can be mixed with other

RS485 transceivers running from 5V on the same network.

There is no concern for the higher voltage of a 5V node

overdriving the 3.3V node due to the overvoltage-tolerant

design of the LTC2879X, as illustrated in Figure 13.

One advantage to using a lower supply voltage is reduced

VCC current draw. VCC supply currents are roughly propor-

tional to the applied supply voltage when the LTC2879X

is driving loads. The Typical Performance Characteristics

section shows the typical power supply currents versus

transmission rates for 3.3V and 5V supplies.

PROFIBUS installations that use the LTC2879X with sup-

ply voltages less than 4.5V, may fall out of compliance to

the PROFIBUS specification.

HIGH SPEED CONSIDERATIONS

A ground plane layout with a 0.1µF bypass capacitor

placed less than 7mm away from VCC is recommended.

The PC board traces connected to signal A and B should

be symmetrical and as short as possible to maintain good

differential signal integrity. To minimize capacitive effects,

the differential signals should be separated by more than

the width of a trace and should not be routed on top of

each other if they are on different signal planes.

Care should be taken to route the outputs away from the

sensitive inputs to reduce feedback effects that might

cause noise, jitter, and even oscillations. For example, DI

and RO should not be routed next to each other or next

to A and B.

Logic inputs have a typical hysteresis of about 150mV to

provide noise immunity. Fast edges on the outputs can

cause glitches in the ground and power supplies which

are exacerbated by capacitive loading. If a logic input is

held near its threshold (typically VCC/2), a noise glitch

from a driver transition may exceed the hysteresis levels

on the logic and data input pins, causing an unintended

state change. This can be avoided by maintaining normal

logic levels on the pins and by slewing inputs faster than

1V/µs. Good supply decoupling and proper driver termi-

nation also reduces glitches caused by driver transitions.

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