CPC7695 Datasheet, PDF (20/23 Pages) Clare, Inc. – Line Card Access Switch

CPC7695
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
V_BAT reference voltage by two to four volts, steering
the fault current to ground.
Note: The CPC7695xB does not contain a protection
SCR but instead utilizes a diode bridge to clamp both
polarities of a fault transient. These diodes pass the
charge of negative fault potentials to the V_BAT pin.
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 seen at T_LINE or R_LINE 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 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,
the measured current into T_LINE or R_LINE 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 Thermal Shutdown
The thermal shutdown mechanism will activate when
the device die temperature reaches a minimum of
110° C, placing the device in the all-off state
regardless of IN_RINGING, IN_TESTin and IN_TESTout logic
inputs. During thermal shutdown events the T_SD pin
will output a logic low with a nominal 0 V level. A logic
high is output from the T_SD pin during normal
operation with a typical output level equal to V_DD.
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
into T_LINE or R_LINE will drop to zero. Once the device
enters thermal shutdown it will remain in the all-off
state until the temperature of the die drops below the
deactivation level of the thermal shutdown circuit. This
permits the device to 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 CPC7695 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
CPC7695. A foldback or crowbar type protector is
recommended to minimize stresses on the CPC7695.
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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PRELIMINARY
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