connector. If metal must be accessible on the outside of a device’s enclosure, then it
may be necessary to provide an earth ground connection to that metal. For example,
if metal toggle switches must be user-accessible, then it may be necessary to mount
the switches on a metal plate that is earth grounded. In this way, ESD hits to the
toggle switch handles will be diverted to the local earth ground. The device's logic
ground must still remain isolated from this earth-grounded “guard” plate, and the
leakage capacitance from logic ground to earth ground (Cleak,GND) should be held
below about 10-20pF to minimize damage from network ESD hits.
Surge Design Issues
Surge voltages encountered in industrial and residential environments as a result of
nearby AC mains switching transients and lightning can cause disturbances or
failures of electronic communications systems. Transient voltages and currents can
couple capacitively or magnetically to the twisted pair wiring of the link power
system. Physical construction of twisted pair wire causes transients to couple to the
cable in a common-mode fashion, i.e., both conductors of the wire pair see the same
transient. Since link power devices float with respect to earth and communication
occurs in a balanced (symmetric) fashion, common-mode transients have minimal
effect on transmission.
The single point of earth reference for a link power network segment is the LPI-10
module. When the LPI-10 module clamps an incident voltage transient, a residual
differential transient voltage may result across the twisted pair conductors. This
residual voltage is seen by the LPT-11 devices. For small transient voltages found in
benign environments no additional protection for LPT-11 devices is required.
However, additional protection is required to prevent higher energy transients from
damaging LPT-11 devices. This section describes the recommended protection
scheme necessary to achieve immunity for link power devices to various levels of
transient voltages defined by the EN61000-4-5 EMC Surge immunity requirements
specification.
Designing Systems for Surge Immunity
Adequate creepage and clearance distances must be built into each device's enclosure
to prevent surge discharges from the local LPT-11 transceiver logic ground to earth.
LPT-11 transceivers are immune to EN-61000-4-5 surge test level 1 (0.5kV). Level 2
(1.0kV) and level 3 (2.0kV) surge immunity can be attained with the addition of a bi-
directional transient voltage suppresser (TVS). The TVS must be placed directly
across Net_A and Net_B lines at each LPT-11 (not for use between either of the data
lines and earth).
TVS
Differential
Protection
Figure 6.2
TVS Schematic Symbol
6-6
Design
Issues