CPC7582

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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

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

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.