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L7581 Datasheet(PDF) 8 Page - Agere Systems |
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L7581 Datasheet(HTML) 8 Page - Agere Systems |
8 / 16 page 8 8 Lucent Technologies Inc. Data Sheet November 1999 L7581 Ringing Access Switch Power Supplies Both the 5 V and battery supply are brought onto the L7581. The L7581 requires only the 5 V supply for switch operation; that is, state control is powered exclu- sively off of the 5 V supply. Because of this, the L7581 offers extremely low power dissipation, both in the idle and active states. The battery voltage is not used for switch state control. The battery is used as a reference voltage by the inte- grated secondary protection circuit. When the voltage at TBAT or RBAT drops 2 V to 4 V below the battery, the integrated SCR will trigger, thus preventing fault- induced overvoltage situations at the TBAT/RBAT nodes. Loss of Battery Voltage As an additional protection feature, the L7581 monitors the battery voltage. Upon loss of battery voltage, the L7581 will automatically enter an all OFF state and remain in that state until the battery voltage is restored. The L7581 is designed so that the device will enter the all OFF state if the battery rises above –10 V and will remain off until the battery drops below –15 V. Monitoring the battery for the automatic shutdown fea- ture will draw a small current from the battery, typically 4 µA. This will add slightly to the overall power dissipa- tion of the device. Impulse Noise Using the L7581 will minimize and possibly eliminate the contribution to the overall system impulse noise that is associated with ringing access switches. Because of this characteristic of the L7581, it may not be necessary to incorporate a zero cross switching scheme. This ultimately depends upon the characteris- tics of the individual system and is best evaluated at the board level. Protection Integrated SLIC Protection Diode Bridge/SCR In the L7581Axx version, protection to the SLIC device or other subsequent circuitry is provided by a combina- tion of current-limited break switches, a diode bridge/ SCR clamping circuit, and a thermal shutdown mecha- nism. In the L7581Bxx 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 version, 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 Table 10) of the SCR must be less nega- tive 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 ver- sions, the positive cycle of the fault is clamped a diode drop above ground and fault currents steered to ground. In the A version, the negative cycle will cause the SCR to trigger when the voltage exceeds the bat- tery reference voltage by 2 V to 4 V. When the SCR triggers, fault current is steered to ground. In the B ver- sion, 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 inte- grated protection circuit and subsequent circuitry is lim- ited 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 TBAT/RBAT nodes will typically be a pulse of magnitude 2.5 A and duration less than 0.5 µs. |
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