Le75183
Data Sheet
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 LOW when
the device is in the thermal shutdown mode and HIGH during normal operation.
During a lightning event, due to the relatively short duration, the thermal shutdown will not typically activate.
During an extended power cross, the device temperature 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
magnitude of the power cross. If the magnitude of the power cross is great enough, the external secondary protector may trigger
shunting all current to ground.
In the Le75183, the thermal shutdown mechanism can be disabled by forcing the TSD pin to HIGH. This functionality is different
from the Le75181, whose thermal shutdown mechanism cannot be disabled.
Electrical specifications relating to the integrated overvoltage clamping circuit are outlined in Table 15.
External Secondary Protector
With the above integrated protection features, only one overvoltage secondary protection device on the loop side of the Le75183
is required. The purpose of this device is to limit fault voltages seen by the Le75183 so as not to exceed the breakdown voltage
or input-output isolation rating of the device. To minimize stress on the Le75183, use of a foldback- or crowbar-type device is
recommended. A detailed explanation and design equations on the choice of the external secondary protection device are given
in the An Introduction to Le758X Series of Line Card Access Switches application note. Basic design equations governing the
choice of external secondary protector are given below.
•
•
•
|VBATmax| + |Vbreakovermax| < |Vbreakdownmin(break)|
|Vringingpeakmax| + |VBATmax| + |Vbreakovermax| < |Vbreakdownmin(ring)|
|Vringingpeakmax| + |VBATmax| < |Vbreakovermin|
where:
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 Le75183 break switch.
Vbreakdownmin(ring)—Minimum magnitude breakdown voltage of Le75183 ring access switch.
Vringingpeakmax—Maximum magnitude peak voltage of ringing signal.
Series current-limiting fused resistors or PTC resistors should be chosen so as not to exceed the current rating of the external
secondary protector. Refer to the manufacturer’s data sheet for specifications.
16
Zarlink Semiconductor Inc.
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