CPC7583MATR datasheet(17/21 Pages) CLARE

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CPC7583

R06

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2.9.2 Current Limiting function

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 TLINE or RLINE will be a pulse with a typical

magnitude of 2.5 A and a duration of less than 0.5

μs.

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 TLINE or RLINE 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.

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 TSD pin will read 0 V. Normal output

of TSD is VDD.

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

TLINE or RLINE 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 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.

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.

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.

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