Datasheet

LTC2875
17
Rev A
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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 ESD transients. The LTC2875 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 LTC2875 to extremely high levels.
In addition to providing surge, EFT and ESD protection,
an external network should preserve the ability of the
LTC2875 to operate over a wide common mode and com-
municate at high frequencies. In order to meet the first
requirement, protection components with suitably high
conduction voltages must be chosen. A means to limit
current must be provided to prevent damage in case a sec-
ondary protection device or the ESD cell on the LTC2875
fires and conducts. The capacitance of these components
must be kept low in order to permit high frequency com
-
munication over a network with multiple nodes.
The protection network shown in Figure14 in the Typical
Application section provides the following protection:
IEC 61000-4-2 Edition 2.0 2008-12 ESD Level 4: ±30kV
air, ±15kV contact (line to GND, direct discharge to bus
pins with transceiver and protection circuit mounted
on a ground referenced test card per Figure4 of the
standard)
IEC 61000-4-4 Second Edition 2004-07 EFT Level 4:
±5kV (line to GND,100kHz repetition rate, 0.75ms burst
duration, 60 second test duration, discharge coupled
to bus pins through 100pF capacitor per paragraph
7.3.2 of the standard)
IEC 61000-4-5 Second Edition 2005-11 Surge Level
4: ±5kV (line to GND, line to line, 8/20µs waveform,
each line coupled to generator through 80Ω resistor
per Figure14 of the standard)
This protection circuit adds only ~36pF of capacitance
per line (line to GND), thereby providing an extremely
high level of protection without significant impact to the
performance of the LTC2875 at high data rates.
The gas discharge tubes (GDTs) provide the primary pro
-
tection against electrical surges. These devices provide a
very low impedance and high current carrying capability
when they fire, safely discharging the surge current to
GND. The transient blocking units (TBUs) are solid state
devices that switch from a low impedance pass through
state to a high impedance current limiting state when a
specified current level is reached. These devices limit the
current and power that can pass through to the second
-
ary protection. The secondary protection consists of a
bidirectional TVS diode, which avalanches above 36V to
protect the bus pins of the LTC2875 transceiver. The high
avalanche voltage of the secondary protection maintains
a wide common mode range. The final component of the
network is the metal oxide varistors (MOVs) which are
used to clamp the voltage across the TBUs to protect
them against fast ESD and EFT transients which exceed
the turn-on time of the GDT.
The high performance of this network is attributable
to the low capacitance of the GDT primary protection
devices. The high capacitance MOV floats on the line and
is shunted by the TBU, so it contributes no appreciable
capacitive load on the signal.
Logic I/O Interface Voltages and Power Supply
Sequencing
Logic inputs RS and TXD are protected by ground ref-
erenced ESD devices. These inputs do not draw a high
current if driven by voltages exceeding V
CC
as long as
the absolute maximum ratings for these pins are not
exceeded. The V
CC
supply for the LTC2875 may be safely
brought up before or after the supplies powering the
logic driving the RXD and TXD inputs with no adverse
consequences.
APPLICATIONS INFORMATION
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