CPC7581 Datasheet, PDF (12/15 Pages) Clare, Inc. – Line Card Access Switch

INTEGRATED CIRCUITS DIVISION
CPC7581
state. At this point the current measured through the
break switches (SW1 and SW2) 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 de-activation level of the
thermal shutdown circuit. This permits the device to
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.
The T_SD pin is a pull-up current source with a nominal
value of 300 A biased from V_DD. For applications
using low-voltage logic devices (lower than V_DD), IXYS
Integrated Circuits Division recommends the use of an
open-drain type output to control T_SD. This avoids
sinking the T_SD bias current to ground during normal
operation when the all-off state is not required.
2.5 Ringing Switch Zero-Cross Current
Turn Off
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 IXYS Integrated Circuits
Division’s 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 in series with the ring generator is
recommended.
2.6 Power Supplies
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.
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
trigger when pin 2 (T_BAT) or pin 15 (R_BAT) drops 2 to
4 V below the voltage on pin 16 (V_BAT). This trigger
prevents a fault-induced overvoltage event at the T_BAT
or R_BAT nodes.
2.7 Battery Voltage Monitor
The CPC7581 also uses the V_BAT voltage to monitor
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 A typical) and adds slightly to the device’s
overall power dissipation.
Due to the nature of the internal protection circuitry,
the V_BAT pin can be biased via potentials applied to
T_BAT or R_BAT. This allows the CPC7581 switches to
operate, but offers no transient protection. The supply
voltage applied to V_BAT should therefor be the same
supply voltage applied to the line driver device.
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_GND. Voltage is clamped to a diode drop above
ground. During a negative transient of 2 to 4 V more
negative than the battery, the SCR conducts and faults
are shunted to F_GND via the SCR or the diode bridge.
In order for the SCR to crowbar (or foldback), the on
voltage (see “Protection Circuitry Electrical
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