LTC2876 datasheet(24/32 Pages) LINER

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LTC2876/LTC2877

28767fa

For more information www.linear.com/LTC2876

Figure 18. Using the LTC2876/LTC2877 in an RS485 Network

(Not PROFIBUS) with Optional Bus Bias Resistors

620

28767 F17

130

120

LTC2876

LTC2877

LTC2876

LTC2877

BUS BIAS

RESISTORS

AT ONE END

applicaTions inForMaTion

Unlike PROFIBUS, the biasing network is not part of the

RS485 standard. Although the LTC2876 and LTC2877

are compatible with this biasing arrangement, the inter-

nal failsafe feature eliminates the need for it, since an

undriven bus triggers a failsafe condition. In extremely

noisy environments the resistor biasing helps reinforce

the failsafe condition.

VL LOGIC SUPPLY

A separate logic supply pin VL allows the LTC2877 to

interface with any logic signal from 1.65V to 5.5V. All

logic I/Os use VL as their high supply. It is recommended

that VL does not exceed VCC during operation. If VL does

exceedVCC,nodamagewilloccurbuttheVLsupplycurrent

could increase about 300µA, depending on the operating

configuration and the state of the device. If VL is not con-

nected to VCC, bypass VL with a 0.1µF capacitor to GND.

The driver is disabled and pins PB and PA are undriven

when VL or VCC is grounded or disconnected.

3.3V OPERATION

The LTC2876 and LTC2877 can be used with a supply

voltage as low as 3.0V in RS485 installations. Reducing

the supply voltage 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 Characteristics section shows the driver output

signal for 3.3V and 5V supply voltages.

3.3V-powered LTC2876/LTC2877 devices can be mixed

withotherRS485transceiversrunningfrom5Vonthesame

network as shown in Figure 20. 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 LTC2876/

LTC2877, as illustrated in Figure 15.

One advantage to using a lower supply voltage is reduced

VCC current draw. VCC supply currents are roughly pro-

portional to the applied supply voltage when the LTC2876/

LTC2877 is driving loads. The Typical Characteristics

section shows the typical power supply currents versus

transmission rates for 3.3V and 5V supplies.

PROFIBUS installations that use the LTC2876/LTC2877

with supply voltages less than 4.5V, may fall out of com-

pliance to the PROFIBUS specification.

HIGH SPEED CONSIDERATIONS

A ground plane layout with a 1µF bypass capacitor placed

less than 7mm away from VCC is recommended. The PC

board traces connected to signal PB and PA 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 PB and PA.

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 or VL/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.

REFERENCES

1 “Application Guidelines for TIA/EIA-485-A”: TSB-89-A,

TIA Telecommunications System Bulletin, January 2006.

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