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Le77D11Data Sheet Fault Detection

Each channel of the Le77D11 Dual VoSLIC device has a fault detection pin, F
1 or F
2 . These pins are driven low when a longitudinal current fault or foreign voltage fault occurs (see Figure4, DC Feed Block Diagram, Active and Standby Modes , on page5 ). When not in Disconnect state, there are three conditions that will cause the F
i pin to indicate a fault condition:•|I
A I
B | > I
LONG •In Normal Active and Standby state, a foreign voltage fault occurs in which V
A is above ground or V
B is close to V
REG .•In Reverse Polarity state, a foreign voltage fault occurs in which V
B is above ground or V
A is close to V
REG .In the Disconnect state, fault detection is not supported; however, fault conditions can be monitored by the Le78D11 device.For more details on AC, DC fault detection, loss of power, or clock-failure alarm, please refer to the Legerity Le77D11/Le78D11 Chip Set User’s Guide (document ID# 080716).

Signal Conditioning

The RDC
i pin is used to set the DC feed current limit, as described in the DC feed section.The IMT
i pin provides K
DC times the loop current to the Le78D11 VoSLAC device. The Le78D11 VoSLAC device implements all loop supervision and ring trip processing on this signal.
= ---------------- I + I I A B IMT 2 K DC

Thermal Overload

When the die temperature around the power amplifier of an Le77D11 Dual VoSLIC device channel reaches approximately 160°C, the IMT pin of that channel is pulled High. At the same time, all the blocks controlling that channel of the device are shut off, except for the logic interface block. The VoSLIC channel goes into a state similar to Disconnect, making the line current zero. When the temperature drops below 145°C, the VoSLIC channel returns to its previous state. It is important to recognize that even while a channel experiences thermal overload, the state of the device can be modified. At TSD, the switcher is turned off.

Control Logic

Each channel of the Le77D11 VoSLIC device has three input pins from the Le78D11 VoSLAC device (C3, C2, and C1). The inputs set the operational state of each channel. There are six operational VoSLIC device states (See Table 1 ): Low Power Standby, Disconnect, Normal Active, Reverse Polarity, Ringing and Line Test. This leaves two reserved logic states. Table 1.Device Operating States
C3C2C1Operating ModeDescription 000Low Power StandbyVoice transmission disabled. Maximum loop current capability and loop current sensing range are reduced.001DisconnectLe77D11 Dual VoSLIC device channel is shut down and switching power supply is shut off.*010Normal ActiveLe77D11 Dual VoSLIC device channel fully operational. A i (TIP) is more positive than B i (RING). Also used for on-hook transmission. 011Reverse PolaritySimilar to normal active, but DC polarity is reversed so that the B i (RING) lead is more positive than the A i (TIP) lead. Also used for on-hook transmission.100RingingRinging state with V AB set to K R • V IN . The switching supply maintains minimum headroom for the sourcing and sinking amplifiers in order to maximize power efficiency.101Line Test StateSimilar to ringing state with reduced bias currents for lower noise. Loop current sensing range is limited. See IMT pin specifications.110ReservedNot used.111ReservedNot used. Note: * When in Disconnect state, the DC-DC converter is disabled and the V REG voltage will decay to 0V. The A i and B i outputs are disabled; however, they still have ESD protection diodes to BGND and VREG which will provide a low impedance clamp to any line voltages >± 0.5V. *
When transitioning from any state to Disconnect, the Le77D11 device momentarily passes through Reverse Polarity, pulling the A-lead towards Vreg. During line testing, when the SLIC device is placed in the Disconnect state, wait >3 seconds before proceeding with line measurements.

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