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ATTL7581AAE Datasheet(PDF) 9 Page - Agere Systems |
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ATTL7581AAE Datasheet(HTML) 9 Page - Agere Systems |
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9 / 16 page  Lucent Technologies Inc. 9 Data Sheet November 1999 L7581 Ringing Access Switch 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. 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. During a lightning event, due to the relatively short duration, the thermal shutdown will not typically acti- vate. 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. If the magnitude of the power cross is great enough, the external secondary protector may trigger shunting all current to ground. 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. 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: s |VBATmax| + |Vbreakovermax| < |Vbreakdownmin(break)| s |Vringingpeakmax| + |VBATmax| + |Vbreakovermax| < |Vbreakdownmin(ring)| s |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 L7581 break switch. Vbreakdownmin(ring)—Minimum magnitude breakdown voltage of L7581 ring access switch. Vringingpeakmax—Maximum magnitude peak voltage of ringing signal. 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. |
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