CPC7695 Datasheet, PDF (21/25 Pages) Clare, Inc. – Line Card Access Switch

INTEGRATED CIRCUITS DIVISION
CPC7695
Although battery power is not used for switch control, it
is required to supply trigger current for the integrated
internal protection circuitry SCR during fault
conditions. This integrated SCR is designed to
activate whenever the voltage at T_BAT or R_BAT drops
2V to 4V below the applied voltage on the V_BAT pin.
Because the battery supply at this pin is required to
source trigger current during negative overvoltage
fault conditions at tip and ring, it is important that the
net supplying this current be a low impedance path for
high speed transients such as lightning. This will
permit trigger currents to flow enabling the SCR to
activate and thereby prevent a fault induced negative
overvoltage event at the T_BAT or R_BAT nodes.
2.10 Protection
2.10.1 Current Limiting Function
If a lightning strike transient occurs when the device is
in theTalk 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 theTalk state when a 1000V 10x1000 S 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 2.5A and a duration of less than
0.5 s.
If a power-cross fault occurs with the device in theTalk
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 80mA and
425mA, 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 thermal shutdown
mechanism will activate and the device will enter the
All-Off state.
2.10.2 Diode Bridge/SCR
The CPC7695 uses a combination of current limited
break switches, a diode bridge/SCR clamping circuit,
and a thermal shutdown mechanism to protect the
SLIC device and 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
F_GND. Voltage is clamped to a diode drop above
ground. During a negative transient of 2V to 4V more
negative than the voltage source at V_BAT, the SCR
conducts and faults are shunted to F_GND via the SCR.
Devices utilizing the diode bridge will steer negative
transients to V_BAT.
In order for the SCR to crowbar or foldback, the SCR’s
on-voltage (see “Protection Circuitry Electrical
Specifications” on page 14) must be less than the
applied voltage at the V_BAT pin. If the V_BAT voltage is
less negative than the SCR on-voltage, or if the V_BAT
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
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.
2.10.3 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 0V 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
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