LTC2862A
17
2862af
For more information www.linear.com/LTC2862A
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
following discussion applies to the PROFIBUS Type A cables
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 necessar
y 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 V
CC
, 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 V
OD
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 V
OD
tolerances are met.
Auxiliary Protection for 5kV Surge, 5kV EFT, and 30kV
IEC ESD
An interface transceiver used in an industrial setting may
be exposed to extremely high levels of electrical overstress
due to phenomena such as lightning surge, electrical fast
transient (EFT) from switching high current inductive loads,
and electrostatic discharge (ESD) from the discharge of
electrically charged personnel or equipment. Test methods
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. The transients produced by the EFT and
particularly the surge tests contain much more energy than
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
provided to prevent damage in case a secondary protection
device or the ESD cell on the LTC2862A fires and conducts.
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.
