CPC7591

14

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R05

2.9 Protection

2.9.1 Diode Bridge/SCR

The CPC7591 uses a combination of current limited

break switches, a diode bridge/SCR clamping circuit,

and a thermal shutdown mechanism to protect the

SLIC device or other associated circuitry from damage

during line transient events such as lightning. During a

positive transient condition, the fault current is

conducted through the diode bridge to ground via

ground. During a negative transient of 2 to 4 V more

or the diode bridge.

In order for the SCR to crowbar (or foldback), the

SCR’s on-voltage (see “Protection Circuitry Electrical

Specifications” on page 9) must be less than the applied

supply is unable to source the trigger current, the SCR

will not crowbar.

For power induction or power-cross fault conditions,

the positive cycle of the transient is clamped to a diode

drop above ground and the fault current is directed to

ground. The negative cycle of the transient will cause

the SCR to conduct when the voltage exceeds the

the fault current to ground.

Note: The CPC7591xB does not contain the

protection SCR but instead uses diodes to clamp both

polarities of a transient fault. These diodes direct the

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

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

μs.

If a power-cross fault occurs with the device in the talk

state, the current is passed though the 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.