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.
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 INRING 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.
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).
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
Break
Switches 1 & 2
Ring Return Ring Access
INPUT
TSD
State
Timing
Switch 3
Switch 4
5 V
5 V/Float
Power
—
Open
Closed
Closed
Ringing
0 V
5 V/Float
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-
Closed
Open
Closed
rent-limit value will be sourced
from the ring node of the SLIC.
0 V
5 V/Float Idle/Talk Zero cross current has occurred.
Closed
Open
Open
Table 10. Break-Before-Make Operation
Break
Switches 1 & 2
Ring Return Ring Access
INPUT
TSD
State
Timing
Switch 3
Switch 4
5 V
5 V/Float
Power
Ringing
—
Open
Open
Open
Closed
Closed
5 V
0 V
0 V
0 V
All Off Hold this state for ≤25 ms. SW4
Open
Open
Closed
Open
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.
0 V
5 V/Float Idle/Talk Release break switch pair.
Closed
Open
Open
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