CPC7592_12 Datasheet, PDF (16/19 Pages) IXYS Corporation – Line Card Access Switch

INTEGRATED CIRCUITS DIVISION
CPC7592
the device to the all-off state and a high impedance (Z)
state for normal operation. This requires the use of an
open-collector or open-drain type buffer.
2.6 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 IC 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 ringing
generator is recommended.
2.7 Power Supplies
Both a +5 V supply and battery voltage are connected
to the CPC7592. Switch state control is powered
exclusively by the +5 V supply. As a result, the
CPC7592 exhibits extremely low power consumption
during active and idle states.
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 2
to 4 V 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.
2.8 Battery Voltage Monitor
The CPC7592 also uses the V_BAT voltage to monitor
battery voltage. If system battery voltage is lost, the
CPC7592 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
battery voltage rises more positive than about –10 V
with respect to ground and remains in the all-off state
until the battery voltage drops below approximately
–15 V with respect to ground. This battery monitor
feature draws a small current from the battery (less
than 1 A typical) and will add slightly to the device’s
overall power dissipation.
This monitor function performs properly if the
CPC7592 and SLIC share a common battery supply
origin. Otherwise, if battery is lost to the CPC7592 but
not to the SLIC, then the V_BAT pin will be internally
biased by the potential applied at the T_BAT or R_BAT
pins via the internal protection circuitry SCR trigger
current path.
2.9 Protection
2.9.1 Diode Bridge/SCR
The CPC7592 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 voltage source at V_BAT, 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
SCR’s on-voltage (see “Protection Circuitry Electrical
Specifications” on page 9) 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 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 CPC7592xB does not contain the
protection SCR but instead uses diodes to clamp both
polarities of a transient fault. These diodes direct the
negative potential’s fault current to the V_BAT pin.
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