L7581 Ringing Access Switch
Data Sheet
November 1999
Power Supplies
Both the 5 V and battery supply are brought onto the
L7581. The L7581 requires only the 5 V supply for
switch operation; that is, state control is powered exclu-
sively off of the 5 V supply. Because of this, the L7581
offers extremely low power dissipation, both in the idle
and active states.
The battery voltage is not used for switch state control.
The battery is used as a reference voltage by the inte-
grated secondary protection circuit. When the voltage
at T
BAT
or R
BAT
drops 2 V to 4 V below the battery, the
integrated SCR will trigger, thus preventing fault-
induced overvoltage situations at the T
BAT
/R
BAT
nodes.
Protection
Integrated SLIC Protection
Diode Bridge/SCR
In the L7581Axx version, protection to the SLIC device
or other subsequent circuitry is provided by a combina-
tion of current-limited break switches, a diode bridge/
SCR clamping circuit, and a thermal shutdown mecha-
nism. In the L7581Bxx version, protection to the SLIC
device or other subsequent circuitry is provided by a
combination of current-limited break switches, a diode
bridge, and a thermal shutdown mechanism.
In both versions, during a positive lightning event, fault
current is directed to ground via steering diodes in the
diode bridge. Voltage is clamped to a diode drop above
ground. In the A version, negative lightning causes the
SCR to conduct when the voltage goes 2 V to 4 V more
negative than the battery, and fault currents are
directed to ground via the SCR and steering diodes in
the diode bridge.
Note that for the SCR to foldback or crowbar, the ON
voltage (see Table 10) of the SCR must be less nega-
tive than the battery reference voltage. If the battery
voltage is less negative than the SCR ON voltage, the
SCR will conduct fault currents to ground; however, it
will not crowbar.
In the B version, negative lightning is directed to battery
via steering diodes in the diode bridge.
For power cross and power induction faults, in both ver-
sions, the positive cycle of the fault is clamped a diode
drop above ground and fault currents steered to
ground. In the A version, the negative cycle will cause
the SCR to trigger when the voltage exceeds the bat-
tery reference voltage by 2 V to 4 V. When the SCR
triggers, fault current is steered to ground. In the B ver-
sion, the negative cycle of the power cross is steered to
battery.
Current Limiting
During a lightning event, the current that is passed
through the break switches and presented to the inte-
grated protection circuit and subsequent circuitry is lim-
ited by the dynamic current-limit response of the break
switches (assuming idle/talk state). When the voltage
seen at the T
LINE
/R
LINE
nodes is properly clamped by
an external secondary protector, upon application of a
1000 V 10 x 1000 pulse (LSSGR lightning), the current
seen at the T
BAT
/R
BAT
nodes will typically be a pulse of
magnitude 2.5 A and duration less than 0.5 µs.
Loss of Battery Voltage
As an additional protection feature, the L7581 monitors
the battery voltage. Upon loss of battery voltage, the
L7581 will automatically enter an all OFF state and
remain in that state until the battery voltage is restored.
The L7581 is designed so that the device will enter the
all OFF state if the battery rises above –10 V and will
remain off until the battery drops below –15 V.
Monitoring the battery for the automatic shutdown fea-
ture will draw a small current from the battery, typically
4 µA. This will add slightly to the overall power dissipa-
tion of the device.
Impulse Noise
Using the L7581 will minimize and possibly eliminate
the contribution to the overall system impulse noise
that is associated with ringing access switches.
Because of this characteristic of the L7581, it may not
be necessary to incorporate a zero cross switching
scheme. This ultimately depends upon the characteris-
tics of the individual system and is best evaluated at
the board level.
8
Lucent Technologies Inc.
AGERE [ AGERE SYSTEMS ]
