3222/3223-DB1 Silicon Laboratories Inc, 3222/3223-DB1 Datasheet - Page 36

• Manufacturer Part Number: 3222/3223-DB1
• Description: EVAL BOARD FOR IA3222/3223
• Manufacturer: Silicon Laboratories Inc
• Series: EZ DAA™r

Specifications of 3222/3223-DB1

• Main Purpose: Telecom, Data Acquisition Arrangement (DAA)
• Embedded: Yes, FPGA / CPLD
• Utilized Ic / Part: IA3222, IA3223
• Primary Attributes: -
• Secondary Attributes: -
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

IA3222/23
6.17. Power-Line Cross
A power-line cross is a fault condition that occurs when a power line is connected to the phone line. This is usually
due to a downed power line. All DAAs provide isolation protection against common-mode power line cross, but
may not provide protection against differential line cross (full power applied between Tip and Ring). Most regulatory
standards only require protection against common-mode power line ac voltages and not differential power line
voltages. Line crosses are much rarer events than lightning surges.
A line cross can occur from the user side or from the telephone system side. If the chassis of a telephone product
somehow gets shorted to one side of the ac power line, the DAA isolation protects telephone-company technicians
and equipment from excessive voltages and power. From the telephone system side, a line cross might occur if a
power line falls across the telephone line shorting to one side of the line. Power-line cross is different than lightning
surges because of its longer duration. This makes it much more dangerous even though it is less likely to occur
than a lightning surge. Failure can occur either from isolation breakdown or from excessive voltage between Tip
and Ring, which can create a fire hazard in the DAA.
A power-line cross may start out as a longitudinal high-voltage event but may quickly turn into a metallic event.
When a power line gets connected to one side or the other of the telephone line, it may cause the primary surge
suppressor to trigger, which, in turn, burns open, leaving the ac mains on one side of the phone line. Then, if the
telephone device goes off hook, or, if the breakover diode triggers, the other primary arrestor may trigger and
create a path directly through the DAA for the ac power line. In this scenario, there may be very little telephone line
resistance in the current loop to limit the current. The result is a destructive failure of the DAA. It may burst into
flames due to the continuous flow of energy into the DAA surge suppressor, which is normally not capable of
continuous currents above 1 A.
Safe differential line-crossing failure, when required, only means that the product needs to fail safely on a line
cross, i.e. not burst into flames during the test. Designing a DAA to survive a line cross is possible but at a
significant cost. The simple method is to use an expensive 600 VAC PTC (Positive Temperature Coefficient)
resetable fuse or a slow fusible link. Fast-blow fuses will likely get blown by lightning transients and are, therefore,
not recommended. There also exist special (and costly) telecom fuses that will survive a 25 A peak Type B surge
but will blow on a differential line cross event. Another solution is to use a 5 to 10 , 1 to 2 W, flame-proof metal-
oxide resistor for a fusible link. With some testing and care, this cheaper solution will withstand the Type B surge
but safely blow on a line-crossing event. This also has the advantage of limiting the surge current that results from
asymmetrical firing of the primary lightning arrestors. Any fusible link needs to be flame proof and physically
separate from the PC board since the UL 1459 test ramps the ac voltage slowly up to 600 VAC to allow
components to generate heat and possibly start a fire, rather than just blow apart. If a component, such as a metal
oxide resistor, begins to glow and is lying flat on the PC board, it will carbonize the PC board material, which may
lead to conductive tracking (carbonized insulator becoming conductive) and possibly fire.
6.18. Common-Mode Noise from the Mains Supply
A hidden common-mode noise issue arises from the absence of the third (green) wire safety ground in home ac
power wiring. In the U.S., third-wire grounds and three-prong ac outlets were not installed extensively until the mid-
1950s and were not required by code until the early 1960s. Europe and other countries have similar histories.
Thus, in older homes, third-wire grounds are missing in some or all rooms, even if three-prong sockets are present.
When computer equipment with switching supplies is plugged into such an outlet, up to half of the ac mains voltage
can be measured on the chassis ground relative to real earth ground (or the telephone) line. The reason is that
most computer switching supplies have pi network power-line EMI filters that have RF decoupling capacitors in the
nF range tied between live, neutral, and ground. If the third-wire ground is not actually grounded, the capacitors in
the filter create a divider between live and neutral with the third-wire ground. It is possible to get a slight electrical
shock from a computer chassis just from this effect. More significantly, it creates a very large common-mode noise
voltage between the phone line (in effect a ground connection) and the local, ungrounded ground wiring.
This large ac common-mode voltage sometimes causes overload problems on resistor-capacitor isolated Caller ID
circuits. The IA3222 does not have this issue since it is completely isolated. Even with otherwise isolated DAAs,
EMI immunity capacitors, if mismatched, can introduce noise on the telephone line, especially if large ac line
transients are present. For example, if two 470 pF EMI capacitors are mismatched by 5%, the 23.5 pF unbalance
has an impedance of 2.3 M at 3 kHz. Against a typical line impedance of 600 , this represents 72 dB of
36
Rev. 5.0