CPC7581
Battery Voltage Monitor
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
The CPC7581 also uses the voltage reference to moni-
tor battery voltage. If battery voltage is lost, the CPC7581
will immediately enter the “all off” state and remain in this
state until the battery voltage is restored. The device will
also enter the “all off” state if the battery voltage rises
above –10V and will remain there until the battery volt-
age drops below –15V. This battery monitor feature
draws a small current from the battery (< 1µA typ.) and
will add slightly to the device’s overall power dissipation.
to power cross fault, the measured current at pin 2 (TBAT
)
and pin 15 (RBAT) will decrease as the device tempera-
ture increases. If the device temperature rises sufficient-
ly, the temperature shutdown mechanism will activate
and the device will default to the “all off” state.
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 this thermal shutdown mode, pin 8 (TSD)
will read 0V. Normal output of TSD is +VDD
Protection
Diode Bridge/SCR
The CPC7581 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 and to ground. Voltage is
clamped to the diode drop above ground. During a neg-
ative transient of two to four volts more negative than
the battery, the SCR conducts and faults are shunted to
ground via the SCR and diode bridge.
If presented with a short duration transient such as a
lightning event, the thermal shutdown feature will not
typically activate. But in an extended power cross tran-
sient, the device temperature will rise and the thermal
shutdown will activate forcing the switches to an “all off”
state. At this point the current measured at pin 2 (TBAT
)
and pin 15 (RBAT) 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 activation level of the thermal shutdown circuit. This
will return the device to the state prior to thermal shut-
down. If the transient has not passed, current will flow
at the value allowed by the dynamic DC current limiting
of the switches and heating will begin again, reactivat-
ing 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.
Also, in order for the SCR to crowbar or foldback, the on
voltage (see Table 7) of the SCR must be less negative
than the battery reference voltage. If the battery voltage
is less negative the SCR on voltage, the SCR will not
crowbar, however it will conduct fault currents to ground.
For power induction or power cross fault conditions, the
positive cycle of the transient is clamped to the diode
drop above ground and the fault current directed to
ground. The negative cycle of the transient will cause
the SCR to conduct when the voltage exceeds the bat-
tery reference voltage by two to four volts, steering the
current to ground.
External Protection Elements
The CPC7581 requires only one overvoltage secondary
protector on the loop side of the device. The integrated
protection feature described above negates the need
for protection on the line side. The purpose of the sec-
ondary protector is to limit voltage transients to levels
that do not exceed the breakdown voltage or input-out-
put isolation barrier of the CPC7581. A foldback or
crowbar type protector is recommended to minimize
stresses on the device.
Current Limiting function
If a lightning strike transient occurs when the device in
the talk/idle state, the current is passed along the line to
the integrated protection circuitry and limited by the
dynamic current limit response of break switches SW1
and SW2. When a 1000V 10x1000 pulse (LSSGR light-
ning) is applied to the line though a properly clamped
external protector, the current seen at pins 2 (TBAT) and
pin 15 (RBAT) will be a pulse with a typical magnitude
and duration of 2.5A and < 0.5ms.
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.
If a power cross fault occurs with device in the talk/idle
state, the current is passed though the break switches
SW1 and SW2 on to the integrated protection circuit
and is limited by the dynamic DC current limit response
Rev. 2
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