CPC7583
2.9.2 Current Limiting function
2.11 External Protection Elements
The CPC7583 requires only over-voltage secondary
protection on the loop side of the device. The
integrated protection feature described above negates
the need for additional protection on the SLIC side.
The secondary protector must limit voltage transients
to levels that do not exceed the breakdown voltage or
input-output isolation barrier of the CPC7583. A
foldback or crowbar type protector is recommended to
minimize stresses on the CPC7583.
If a lightning strike transient occurs when the device is
in the talk state, the current is passed along the line to
the integrated protection circuitry and restricted by the
dynamic current limit response of the active switches.
During the talk state when a 1000V 10x1000 μS pulse
(GR-1089-CORE lightning) is applied to the line
though a properly clamped external protector, the
current into T
or R
will be a pulse with a typical
LINE
LINE
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 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 at T
or R
will
LINE
LINE
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.
2.10 Temperature Shutdown
The thermal shutdown mechanism will activate when
the device temperature reaches a minimum of 110° C,
placing the device in the all-off state regardless of
logic input. During thermal shutdown mode, the
voltage out of the T pin will read 0 V. Normal output
SD
of T is V
.
SD
DD
If presented with a short duration transient such as a
lightning event, the thermal shutdown feature will
typically not activate. But in an extended power-cross
transient, the device temperature will rise and the
thermal shutdown will activate forcing the switches to
the all-off state. At this point the current measured into
T
or R
will drop to zero. Once the device
LINE
LINE
enters thermal shutdown it will remain in the all-off
state until the temperature of the device drops below
the deactivation level of the thermal shutdown circuit.
This will permit the device to return to normal
operation. If the transient has not passed, current will
flow up to the value allowed by the dynamic DC
current limiting of the switches and heating will begin
again, reactivating the thermal shutdown mechanism.
This cycle of entering and exiting the thermal
shutdown mode will continue as long as the fault
condition persists. If the magnitude of the fault
condition is great enough, the external secondary
protector could activate and shunt all current to
ground.
R06
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