Le75181
PROTECTION
Integrated SLIC Device Protection
Diode Bridge/SCR
Data Sheet
In the Le75181A/C version, protection to the SLIC device or other subsequent circuitry is provided by a combination of current-
limited break switches, a diode bridge/SCR clamping circuit, and a thermal shutdown mechanism. In the Le75181B 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/C versions, 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
of the SCR must be less negative 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 versions, the positive cycle of the fault is clamped a diode drop above ground
and fault currents steered to ground. In the A/C versions, the negative cycle will cause the SCR to trigger when the voltage
exceeds the battery reference voltage by 2 V to 4 V. When the SCR triggers, fault current is steered to ground. In the B version,
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 integrated protection circuit
and subsequent circuitry is limited by the dynamic current-limit response of the break switches (assuming idle/talk state). When
the voltage seen at the TLINE/RLINE 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.
During a power cross event, the current that is passed through the break switches and presented to the integrated protection
circuit and subsequent circuitry is limited by the dc current-limit response of the break switches (assuming idle/talk state). The
DC current limit is specified over temperature between 80 mA and 250 mA.
Note that the current-limit circuitry has a negative temperature coefficient. Thus, if the device is subjected to an extended power
cross, the value of current seen at T
BAT
/R
BAT
will decrease as the device heats due to the fault current. If sufficient heating occurs,
the temperature 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 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 Le75181 device, the thermal shutdown mechanism cannot be disabled by logic control at the TSD pin. The functionality of
TSD differs from the Le75282 and Le75183 devices. For the proper use of and understanding of any caveats related to TSD,
please refer to the appropriate data sheet specifications.
Electrical specifications relating to the overvoltage clamping circuit are outlined in
External Secondary Protector
With the above integrated protection features, only one overvoltage secondary protection device on the loop side of the Le75181
device is required. The purpose of this device is to limit fault voltages seen by the Le75181 device so as not to exceed the
breakdown voltage or input-output isolation rating of the device. To minimize stress on the Le75181 device, use of a foldback- or
crowbar-type device is recommended. A detailed explanation and design equations on the choice of the external secondary
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Zarlink Semiconductor Inc.
ZARLINK [ ZARLINK SEMICONDUCTOR INC ]
