CPC7582
16 www.clare.com R05
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 at
pin 3 (T
LINE
) and pin 14 (R
LINE
) through the break
switches will drop to zero. Once the device enters
thermal shutdown it will remain in the all-off state until
the temperature of the device drops below the de-
activation 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 at 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.
The thermal shutdown mechanism of the CPC7582
can be disabled by applying a logic high to pin 7 (T
SD
).
2.10 External Protection Elements
The CPC7582 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 line side. The secondary
protector limits voltage transients to levels that do not
exceed the breakdown voltage or input-output
isolation barrier of the CPC7582. A foldback or
crowbar type protector is recommended to minimize
stresses on the device.
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.
