CPC7581
2.9 External Protection Elements
The CPC7581 requires only overvoltage secondary
protection on the loop side of the device. The
integrated protection feature described above negates
the need for protection on the other (usually SLIC)
side. The secondary protector limits voltage transients
to levels that do not exceed the breakdown voltage or
input-output isolation barrier of the CPC7581. A
foldback or crowbar type protector is recommended to
minimize stresses on the device.
battery reference voltage by two to four volts, steering
the current to ground.
Note: The CPC7581xB does not contain the
protection SCR.
2.8.2 Current Limiting function
If a lightning strike transient occurs when the device in
the talk state, the current is passed along the line to
the integrated protection circuitry and limited by the
dynamic current limit response of break switches SW1
and SW2. When a 1000V 10x1000 μs pulse
(GR-1089-CORE lightning) is applied to the line
though a properly clamped external protector, the
current seen through the break switches will be a
pulse with a typical magnitude of 2.5 A and a duration
of less than 0.5 μs.
Consult Clare’s application note, AN-100, “Designing
Surge and Power Fault Protection Circuits for Solid
State Subscriber Line Interfaces” for equations related
to the specifications of external secondary protectors,
fused resistors and PTCs.
If a power-cross fault occurs with the device in the talk
state, the current is passed though the break switches
SW1 and SW2 on to the integrated protection circuit
and is limited by the dynamic DC current limit
response of the two break switches. The DC current
limit, specified over temperature, is between 80 mA
and 425 mA, and the circuitry has a negative
temperature coefficient. As a result, if the device is
subjected to extended heating due to power cross
fault, the measured current through the break switches
(SW1 and SW2) will decrease as the device
temperature increases. If the device temperature rises
sufficiently, the temperature shutdown mechanism will
activate and the device will enter the all-off state.
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