Data Sheet
November 1999
L7581 Ringing Access Switch
and if current in excess of the SCR’s turn-on current is
also available, the SCR may turn on. Once the SCR is
triggered on, if the SLIC is capable of supplying current
in excess of the holding current, the SCR may be
latched on by the SLIC.
The probability of this event depends on the character-
istics of the given SLIC and of the holding current of the
L7581A device. The SCR hold current distribution is
designed to be safely away from the test limit of 80 mA.
The higher the distribution, the lower the probability of
the latch.
If this situation is of concern for a given board design,
either use the A series device in the break-before-make
mode (eliminates the original 25 ms current pulse) or
use a B series device (eliminates the SCR).
Switching Behavior
(continued)
When using an L7581 in the make-before-break mode,
during the ring-to-idle transition, for a period of up to
one-half the ringing frequency, the ring break switch
and the pnpn-type ring access switch can both be in
the ON state. This is the maximum time after the logic
signal at IN
RING
has transitioned that the ring access
switch is waiting for the next zero current cross so it can
close. During this interval, current that is limited to the
dc break switch current-limit value will be sourced from
the ring node of the SLIC.
This current is presented to the internal protection cir-
cuit. If the SCR-type protector is used (A code), if by
random probability the ring-to-idle transition occurs
during a portion of the ring cycle when the ringing volt-
age exceeds the protection circuit SCR turn-on voltage,
Table 9. Make-Before-Break Operation
INPUT
5V
0V
T
SD
5 V/Float
5 V/Float
State
Timing
Break
Ring Return
Switches 1 & 2
Switch 3
Open
Closed
Closed
Open
Ring Access
Switch 4
Closed
Closed
0V
5 V/Float
Power
—
Ringing
Make- SW4 waiting for next zero current
before- crossing to turn off, maximum
break time—one-half of ringing. In this
transition state, current that is lim-
ited to the dc break switch cur-
rent-limit value will be sourced
from the ring node of the SLIC.
Idle/Talk Zero cross current has occurred.
Closed
Open
Open
Table 10. Break-Before-Make Operation
INPUT
5V
5V
0V
T
SD
5 V/Float
0V
0V
State
Power
Ringing
All Off
Timing
—
Break
Ring Return
Switches 1 & 2
Switch 3
Open
Open
Open
Closed
Open
Open
Ring Access
Switch 4
Closed
Closed
Open
0V
5 V/Float
Hold this state for
≤25
ms. SW4
waiting for zero current to turn off.
All Off Zero current has occurred and
SW4 has opened. Transition on
INPUT should occur during 25 ms
hold.
Idle/Talk Release break switch pair.
Closed
Open
Open
Lucent Technologies Inc.
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