CPC7581
return to normal operation. 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, 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 could activate and shunt all
current to ground.
prevents a fault-induced overvoltage event at the T
BAT
or R
nodes.
BAT
2.7 Battery Voltage Monitor
The CPC7581 also uses the V
voltage to monitor
BAT
battery voltage. If system battery voltage is lost, the
CPC7581 immediately enters the all-off state. It
remains in this state until the battery voltage is
restored. The device also enters the all-off state if the
system battery voltage goes more positive than –10 V,
and remains in the all-off state until the battery voltage
goes more negative than –15 V. This battery monitor
feature draws a small current from the battery (less
than 1 μA typical) and adds slightly to the device’s
overall power dissipation.
The T pin is a pull-up current source with a nominal
SD
value of 300 μA biased from V . For applications
DD
using low-voltage logic devices (lower than V ),
DD
Clare recommends the use of an open-drain type
output to control T . This avoids sinking the T bias
SD
SD
current to ground during normal operation when the
all-off state is not required.
Due to the nature of the internal protection circuitry,
the V
pin can be biased via potentials applied to
BAT
T
or R . This allows the CPC7581 switches to
BAT
BAT
2.5 Ringing Switch Zero-Cross
Current Turn Off
operate, but offers no transient protection. The supply
voltage applied to V should therefor be the same
supply voltage applied to the line driver device.
BAT
After the application of a logic input to turn SW4 off,
the ringing switch is designed to delay the change in
state until the next zero-crossing. Once on, the switch
requires a zero-current cross to turn off, and therefore
should not be used to switch a pure DC signal. The
switch will remain in the on state no matter the logic
input until the next zero crossing. These switching
characteristics will reduce and possibly eliminate
overall system impulse noise normally associated with
ringing switches. See Clare application note AN-144,
Impulse Noise Benefits of Line Card Access Switches for
more information. The attributes of ringing switch SW4
may make it possible to eliminate the need for a
zero-cross switching scheme. A minimum impedance
of 300 Ω in series with the ring generator is
2.8 Protection
2.8.1 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 to ground via
F
. Voltage is clamped to a diode drop above
GND
recommended.
ground. During a negative transient of 2 to 4 V more
negative than the battery, the SCR conducts and faults
are shunted to F
via the SCR or the diode bridge.
GND
2.6 Power Supplies
In order for the SCR to crowbar (or foldback), the on
voltage (see “Protection Circuitry Electrical
Specifications” on page 9) of the SCR must be less
negative than the battery reference voltage. If the
battery voltage is less negative than the SCR on
Both a +5 V supply and battery voltage are connected
to the CPC7581. CPC7581 switch state control is
powered exclusively by the +5 V supply. As a result,
the CPC7581 exhibits extremely low power dissipation
during both active and all-off states.
voltage, or if the V
supply is unable to source the
BAT
trigger current, the SCR will not crowbar.
The battery voltage is not used for switch control but
rather as a supply for the integrated secondary
protection circuitry. The integrated SCR is designed to
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 directed to
ground. The negative cycle of the transient will cause
the SCR to conduct when the voltage exceeds the
trigger when pin 2 (T ) or pin 15 (R ) drops 2 to
BAT
BAT
4 V below the voltage on pin 16 (V ). This trigger
BAT
12
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