CPC7592

R04

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17

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

s

lightning pulse (GR-1089-CORE) is applied to the line

though a properly clamped external protector, the

typical magnitude of 2.5 A and a duration less than

0.5

s.

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

but 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 a power cross fault condition,

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 Thermal Shutdown

The thermal shutdown mechanism activates when the

device die temperature reaches a minimum of 110

 C,

placing the device in the all-off state regardless of

pin will output a logic low with a nominal 0 V level. A

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

event, the device temperature will rise and the thermal

shutdown mechanism will activate forcing the switches

to the all-off state. At this point the current measured

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 permits the device to autonomously return to

normal operation. If the transient has not passed,

current will again flow up to the value allowed by the

dynamic DC current limiting of the switches and

heating will resume, 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 will activate shunting the fault current to

ground.

2.11 External Protection Elements

The CPC7592 requires only over voltage secondary

protection on the loop side of the device. The

integrated protection feature described above negates

the need for additional external 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

CPC7592. A foldback or crowbar type protector is

recommended to minimize stresses on the CPC7592.

Consult IXYS IC Division’s application note, AN-100,

“Designing Surge and Power Fault Protection Circuits for

related to the specifications of external secondary

protectors, fused resistors and PTCs.