ia3222 integration, ia3222 Datasheet - Page 36

• Manufacturer Part Number: ia3222
• Description: Ia3222/ia3223 Ez Daa? Chipset With Analog Interface
• Manufacturer: integration

IA3222/IA3223
lightning voltage drop down the power line. The net effect is that several kV of longitudinal transients can be put on the telephone
line for any lightning strike on the power line that runs above the same telephone lines. Since the phone line may run for several
miles, it may not be unlikely that a strike above it happens several times every lightning season.
Another coupling method for lightning is via ground-return bounce. Since the lightning strike must return to ground and especially if
the ground is resistive, the local voltage at the ground return may bounce by thousands of volts for several tens of microseconds. If
the local ground is at the switch end of the telephone line, of course this will induce a common-mode transient toward the CPE end
of several kV. Conversely, if the strike ground return is local to the CPE, it will make the local ground bounce by several kV relative to
the telephone switch end that may be miles away. Either mechanism generates a longitudinal transient between the telephone line
and the local ground of several kV.
Lightning is not the only source of such transients. The power-distribution system can also generate common-mode induction
transients through the same coupling mechanism. These transients may arise from power-distribution switching or heavy-duty loads
(industrial motors, etc.). Generally, these induction events are rare, and are mostly a source of common-mode noise, not producing
voltages high enough to cause damage.
These transients are the reason why telephone lines all have primary lightning arrestors to local ground at the PSTN (Public Switched
Telephone Network) network access port. Normally, there is one primary arrestor on each side of the telephone line to a local
ground, typically a clamp on a water pipe or ground stake. These arrestors trigger in the 300 to 600 V range. Common arrestors have
been 6-mil carbon gaps, gas tubes, MOVs (Metal Oxide Varistors), or semiconductor breakover diodes. The carbon gaps and gas-tube
arrestors are slow and may take several µs to trigger, allowing up to several kV for a few µs. Typically, the arrestors can withstand at
least a 100 A surge for a standard lightning surge pulse of several hundred µs. The resistance of the telephone line limits the
current. Typical 26-gauge twisted-pair cable has a resistance of 40 Ω per kft. Surge suppressors either have breakover
characteristics where their forward voltages drop to a few volts when triggered but need at least 100 mA to keep them conductive, or
have Zener voltage clamp characteristics. Voltage clamps (MOVs are the common example) need to be able to absorb many Joules
of energy without damage (1 kV x 100 A x 100 µs = 10 J). With the breakover diode, the peak current may be several times higher
because it provides little blocking voltage, but it dissipates less than 1/100th of the energy of voltage clamp because of its low
forward voltage. The bulk of the surge energy is dissipated down the series resistance of the telephone line.
If the primary arrestors always were in place and properly grounded, then much of the observed lightning damage to DAAs would not
occur. Unfortunately, over the life of telephone line, a number of ground connections at the network-access point may get
disconnected due to building construction. The ground is often not reconnected because the telephone line works fine without it. The
surge arrestors may also be damaged and not replaced, or the network-access port is removed and not replaced. Even a properly
installed ground stake in a desert climate may fail if the soil dries out, thus causing a high-impedance return path to ground.
Arcing across the isolation barrier is the more serious DAA failure that arises when the primary arrestor protection is defective.
Longitudinal voltages need to rise above 2 to 3 kV before arcing occurs. Lower voltages usually don’t arc since most DAAs are
designed to withstand transients of at least 1.5 kV and the continuous application of 1 kV
. Even though longitudinal transients
RMS
above 5 kV may be rare, electrostatic (ESD) transients can easily exceed 10 or 15 kV. A telephone product that includes a DAA might
be struck by an ESD event and not have an adequate ground return. The resulting ESD event may then arc across the isolation
barrier. For example, a user might be installing a fax machine at home and then plug the telephone line before plugging the power
cord. If an ESD transient strikes while the fax machine is unplugged, then the DAA might be damaged due to arcing across its
isolation.
There are several remedies for the ESD event. One is the use of common-mode, high voltage EMI capacitors between the chassis
ground and the phone line. These are typically installed for reduction of EMI radiation and susceptibility. If each of these is around
470 pF, they will divide the voltage of an ESD transient by as much as ten times. Since these capacitors must meet the telephone-
line isolation requirements, they will naturally withstand the divided voltage.
The IA3222/3223 chipset does not need these costly EMI capacitors because of the high RF impedance of the isolation capacitors.
These capacitors achieve an effective breakdown voltage in the tens of kV at only the cost of the PCB area they occupy. In practice,
excessive common-mode voltage will arc across the surface of the board. If the DAA designer doesn’t select a preferred path for
common-mode arcing, the surge will find its own path with consequent damage. A preferred arc path would normally be between
either Tip or Ring and the chassis ground of the telephone product. The desired arc gap should be both shorter and more pointed
than any other potential arc path. If part of the arc gap is on the circuit board it is important that the ends not have insulating
silkscreen over them. For most worldwide applications, the gap should be at least 2.5 mm. This means that the other creepage
(surface distance) distances should be at least 3 mm.
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