3222/3223-DB1 Silicon Laboratories Inc, 3222/3223-DB1 Datasheet - Page 34

• 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
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, 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 of several kV between the telephone line and the local
ground.
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 are 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  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 they have Zener voltage clamp characteristics. Voltage clamps (MOVs are the
common example) need 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.
Metallic (differential) surges arise from the longitudinal lightning surges causing either the asymmetric triggering of
the primary arrestors, or arcing of only one side of the line to ground (if only one primary arrestor is functioning). To
protect against metallic surges, a DAA uses a surge suppressor that clamps the differential voltage to prevent
damage. Good solutions provide surge immunity for both on-hook and off-hook DAA states. Protecting the off-hook
state requires some form of current limiting to protect the off-hook path during the surge. Breakover diodes
generally work better since they collapse the surge voltage, thus reducing the energy dissipated in the off-hook
circuit over 100 times. MOVs can be used for surges, but because of their nearly two-to-one spread between
minimum and maximum clamp voltages, the hook switch must be capable of withstanding much higher peak
voltages than with breakover diodes. In addition, the hook circuit must turn itself off (blanking) during the surge in
order to prevent excess dissipation.
Because the primary arrestors are not typically in a mutually triggered pair (unlike some gas tubes) during a
common mode high voltage transient, one arrestor will always fire before the other. Ironically, on a telephone
product with a breakover secondary surge protection diode between tip and ring, this can lead to overstress of the
diode especially if the primary arrestors have a breakover characteristic (carbon gap, gas tube, or semiconductor
breakover diode). The reason for this is that, once a primary arrestor triggers on one side of the line, the
longitudinal surge becomes metallic. This triggers the secondary breakover diode in the telephone product. At that
point, the other primary arrestor will not trigger at all, since there is now a low-voltage path around it through the
secondary protector and back through the first primary protector that fired. In this situation, the breakover diode
sees the same current as the primary arrestor. Typically, for this mechanism to occur, both primary and secondary
surge protectors need to have break-over characteristics.
There are several remedies to prevent this. One is to insert a resistance of about 5  in series with Tip and Ring
but before the breakover diode. The added resistance increases the voltage drop sufficiently to ensure that the
second primary arrestor triggers on large current transients. Small transients can be absorbed by the breakover
diode. If a resistor is used, it must be capable of withstanding the worse-case surge. If it has suitable fuse
characteristics and is flame proof, it can be used as an inexpensive slow-blow fuse for protection against line cross.
Another remedy is to use a larger secondary breakover diode.
Experience shows that a DAA that survives an FCC part 68 Type-B surge provides good field immunity against
most lightning surges over the life of the product. This surge specifies a 1 kV peak produced by discharging a
20 µF source capacitor with about 40  of resistance for limiting current. Into a break-over diode, this produces a
peak current of about 25 A. Several vendors produce breakover diodes rated to survive this test.
Although the designer can use more robust components to survive an FCC part 68 Type-A surge, (800 V, 100 A),
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