LTC2855 datasheet(18/24 Pages) AD

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LTC2862A

Rev. A

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APPLICATIONS INFORMATION

PROFIBUS Compatible Interface

PROFIBUS is an RS485-based field bus. In addition

to the specifications of TIA/EIA-485-A, the PROFIBUS

specification contains additional requirements for cables,

interconnects, line termination, and signal levels. The

followingdiscussionappliestothePROFIBUSTypeAcables

with associated connectors and termination. The Type A

cable is a twisted pair shielded cable with a characteristic

impedance of 135Ω to 165Ω and a loop resistance of

< 110Ω/km.

The LTC2862A RS485 transceiver may be used in

PROFIBUS compatible equipment if the following

considerations are implemented. (Please refer to the

schematic of the PROFIBUS Compatible Interface in the

Typical Applications Section.)

1. The polarity of the PROFIBUS signal is opposite to the

polarity convention used in this data sheet. The PRO-

FIBUS B wire is driven by a non-inverted signal, while

the A wire is driven by an inverted signal. Therefore,

it is necessary to swap the output connections from

the transceiver. Pin A is connected to the PROFIBUS

B wire, and Pin B is connected to the PROFIBUS A

wire.

2. Each end of the PROFIBUS line is terminated with

a 220Ω resistor between B and A, a 390Ω pull-up

resistor between B and VCC, and a 390Ω pull-down

resistor between A and GND. This provides suitable

termination for the 150Ω twisted pair transmission

cable.

3. The peak to peak differential voltage VOD received at

the end of a 100m cable with the cable and termina-

tions described above must be greater than 4V and

less than 7V. The LTC2862A produces signal levels

in excess of 7V when driving this network directly.

8.2Ω resistors may be inserted between the A and

B pins of the transceiver and the B and A pins of the

PROFIBUS cable to attenuate the transmitted signal

to meet the PROFIBUS upper limit of 7V while still

providing enough drive strength to meet the lower

limit of 4V.

4. The LTC2862A transceiver should be powered by a

5% tolerance 5V supply (4.75V to 5.25V) to ensure

that the PROFIBUS VOD tolerances are met.

Auxiliary Protection for 5kV Surge, 5kV EFT, and 30kV

IEC ESD

An interface transceiver used in an industrial setting may

beexposedtoextremelyhighlevelsofelectricaloverstress

due to phenomena such as lightning surge, electrical fast

transient(EFT)fromswitchinghighcurrentinductiveloads,

and electrostatic discharge (ESD) from the discharge of

electricallychargedpersonnelorequipment.Testmethods

to evaluate immunity of electronic equipment to these

phenomena are defined in the IEC standards 61000-4-2,

61000-4-4, and 61000-4-5, which address ESD, EFT, and

surge,respectively.ThetransientsproducedbytheEFTand

particularlythesurgetestscontainmuchmoreenergythan

the ESD transients. The LTC2862A is designed for high

robustness against ESD, but the on-chip protection is not

able to absorb the energy associated with the 61000-4-5

surge transients. Therefore, a properly designed external

protection network is necessary to achieve a high level of

surge protection, and can also extend the ESD and EFT

performance of the LTC2862A to extremely high levels.

In addition to providing surge, EFT and ESD protection,

an external network should preserve or extend the ability

of the LTC2862A to withstand overvoltage faults, operate

over a wide common mode, and communicate at high

frequencies. In order to meet the first two requirements,

protection components with suitably high conduction

voltages must be chosen. A means to limit current must be

providedtopreventdamageincaseasecondaryprotection

deviceortheESDcellontheLTC2862Afiresandconducts.

The capacitance of these components must be kept low

in order to permit high frequency communication over a

network with multiple nodes. Meeting the requirements

for conducting very high energy electrical transients while

maintaining high hold-off voltages and low capacitance is

a considerable challenge.

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