DS21354/DS21554 3.3V/5V E1 Single-Chip Transceivers
15.4. Protected Interfaces
In certain applications, such as connecting to the PSTN, it is required that the network interface be
protected from and resistant to certain electrical conditions. These conditions are divided into two
categories, surge and power line cross. A typical cause of surge is lightening strike. Power-line cross
refers to accidental contact with high-voltage power wiring. For protection against surges, additional
components and PC board layout considerations are required to reroute and dissipate this energy. In a
surge event, the network interface must not be damaged and continue to work after the event. In the event
of a power line contact, components such as fuses or PTCs that can “open” the circuit are required to
prevent the possibility of a fire caused by overheating the transformer. The circuit examples in this data
sheet are for “Secondary Over Voltage Protection” schemes for the line terminating equipment. Primary
protection is typically provided by the network service provide and is external to the equipment.
Figure 15-6 shows an example circuit for the 5V device and Figure 15-7 is an example for the 3.3V
device. In both examples, fuses are used to provide protection against power-line cross. Surge protection
is provided by 470ꢁ input resistors on the receive pair, a transient suppresser, and a diode bridge on the
transmit pair. Resistors R1 to R4 provide surge protection for the fuse. Careful selection of the
transformer allows the use of a fuse that requires no additional surge protection such as the circuit shown
in Figure 15-7. The circuit shown in Figure 15-7 is required for 3.3V operation since additional resistance
in the transmit pair cannot be tolerated. For more information on line interface design, consult the E1 Line
Interface Design Criteria and Secondary Overvoltage Protection application notes available on our
website at www.maxim-ic.com/appnoteindex.
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