Data Sheet
November 1999
L7581 Ringing Access Switch
In the L7581, the thermal shutdown mechanism
cannot be disabled by logic control at the TSD pin.
The functionality of TSD differs from the L7541, L7582,
and L7583. For the proper use of and understanding of
any caveats related to TSD, please refer to the appropri-
ate data sheet specifications.
Protection (continued)
Integrated SLIC Protection (continued)
During a power cross 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 dc current-limit response of the break
switches (assuming idle/talk state). The dc current limit
is specified over temperature between 100 mA and
250 mA.
Electrical specifications relating to the overvoltage
clamping circuit are outlined in Table 11.
External Secondary Protector
With the above integrated protection features, only one
overvoltage secondary protection device on the loop
side of the L7581 is required. The purpose of this
device is to limit fault voltages seen by the L7581 so as
not to exceed the breakdown voltage or input-output
isolation rating of the device. To minimize stress on the
L7581, use of a foldback- or crowbar-type device is rec-
ommended. A detailed explanation and design equa-
tions on the choice of the external secondary
protection device are given in the An Introduction to
L758X Series of Line Card Access Switches Applica-
tion Note. Basic design equations governing the choice
of external secondary protector are given below:
Note that the current-limit circuitry has a negative tem-
perature coefficient. Thus, if the device is subjected to
an extended power cross, the value of current seen at
TBAT/RBAT will decrease as the device heats due to the
fault current. If sufficient heating occurs, the tempera-
ture shutdown mechanism will activate and the device
will enter an all off mode.
Temperature Shutdown Mechanism
When the device temperature reaches a minimum of
110 °C, the thermal shutdown mechanism will activate
and force the device into an all OFF state, regardless of
the logic input pins. Pin TSD, when used as an output,
will read 0 V when the device is in the thermal shut-
down mode and +VDD during normal operation.
■
|VBATmax| + |Vbreakovermax| < |Vbreakdownmin(break)|
■
|Vringingpeakmax| + |VBATmax| + |Vbreakovermax| <
|Vbreakdownmin(ring)|
During a lightning event, due to the relatively short
duration, the thermal shutdown will not typically acti-
vate.
■
|Vringingpeakmax| + |VBATmax| < |Vbreakovermin|
where:
During an extended power cross, the device tempera-
ture will rise and cause the device to enter the thermal
shutdown mode. This forces an all off mode, and the
current seen at TBAT/RBAT drops to zero. Once in the
thermal shutdown mode, the device will cool and exit
the thermal shutdown mode, thus reentering the state it
was in prior to thermal shutdown. Current, limited to the
dc current-limit value, will again begin to flow and
device heating will begin again. This cycle of entering
and exiting thermal shutdown will last as long as the
power cross fault is present. The frequency of entering
and exiting thermal shutdown will depend on the mag-
nitude of the power cross.
VBATmax—Maximum magnitude of battery voltage.
Vbreakovermax—Maximum magnitude breakover voltage
of external secondary protector.
Vbreakovermin—Minimum magnitude breakover voltage
of external secondary protector.
Vbreakdownmin(break)—Minimum magnitude breakdown
voltage of L7581 break switch.
Vbreakdownmin(ring)—Minimum magnitude breakdown
voltage of L7581 ring access switch.
Vringingpeakmax—Maximum magnitude peak voltage of
ringing signal.
If the magnitude of the power cross is great enough,
the external secondary protector may trigger shunting
all current to ground.
Series current-limiting fused resistors or PTCs should
be chosen so as not to exceed the current rating of the
external secondary protector. Refer to the manufac-
turer’s data sheet for specifications.
Lucent Technologies Inc.
9
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