Le79555
PRODUCT DESCRIPTION
Data Sheet
The Le79555 device is designed for short loop and long loop high-density POTS applications requiring a power saving, small
footprint SLIC. The Le79555 device boasts increased power savings over
Zarlink's
other POTS solutions by using an internal
switching regulator for each channel to enhance system power management. The switching regulator eliminates the need for a
second voltage supply, commonly required for SLIC devices in POTS applications. Such elimination passes on board space and
cost savings to the designers. Thus, the smaller footprint and added features of the Le79555 device allows customers to amortize
the cost of common hardware across more channels and increase the line density per board. Additionally, the Le79555 device
gives line card designers a simple control interface that supports six control states: Active, Ringing, Standby, Disconnect, Reverse
Polarity, and Tip Open. The Le79555 device is a low cost, high performance device providing key features required for POTS
markets worldwide, including: low power dissipation and ground key detection, as well as all of the features offered currently by
Zarlink's
Transformer SLIC family, Le7920/22.
BLOCK DESCRIPTIONS
Two-Wire Interface
The two-wire interface provides DC current and sends voice signals to a telephone apparatus connected to the line card with a
two-wire line. The two-wire interface also receives the returning voice signals from the telephone.
Ground Detector
The ground detector block performs ground start and ground key detection, as well as automatically detects a ring-ground fault.
Therefore, when the longitudinal current is greater than the ground key detector threshold, IGK, in either Active, Standby, or Tip
Open, the DET will go low. Note that when the device is in Active or Standby, DET may be an indication of off-hook, ground fault,
or both.
Signal Transmission
The RSN input current controls the receive current sent to the two-wire interface. The AC line voltage is sensed by differential
amplifiers between the A and HPA leads, and between HPB and B leads. The outputs of these amplifiers are equal to the AC
metallic components of the line voltages. The transmission circuit also contains a longitudinal feedback circuit to shunt
longitudinal signals to a DC bias voltage. The longitudinal feedback does not affect metallic signals.
Power Feed Controller
The power feed controller has three sections: (1) the battery feed circuit, (2) the reverse polarity circuit, and (3) the bias circuit.
The battery feed circuit regulates the amount of DC current and voltage supplied to the telephone over a wide range of loop
resistance. The reverse polarity circuit provides the capability to reverse the loop current for pay telephone key pad disable and
other applications. The bias circuit provides a reference voltage, which is offset from the subscriber line voltage. The reference
voltage controls the switched mode regulator, which minimizes SLIC power consumption by providing the minimum supply
voltage needed by the line drivers for proper operation.
Switching Regulator
A switching regulator function is implemented on the chip with a few external components. The power feed controller generates
a reference voltage which is the minimum voltage required to feed the output line amplifiers. The efficiency of the switching
regulator (>80%) minimizes both the on-chip power dissipation and the system power dissipation. This is particularly important
for short loops operating at high currents which otherwise cause high power dissipation.
Input Decoder and Control
The input decoder and control block provides a means for a microprocessor or SLAC IC to control such system functions as line
activate, on-hook transmission, ringing, and reverse polarity. The input decoder and control block has TTL-compatible inputs,
which set the operating states of the SLIC. It also provides the supervision signal sent back to the controller.
Off-Hook Detector
The most important loop monitoring function is off-hook detection. The two-wire interface produces a current equal in magnitude
to the loop current divided by a constant, and sends it out on the RD pin. An external resistor and capacitor (RD and CD) connect
the RD pin to ground. The value of the voltage across resistor R
D
is proportional to the current leaving the RD pin times the value
of R
D
. The DET pin will show a logic Low when this voltage rises above a threshold.
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Zarlink Semiconductor Inc.
ZARLINK [ ZARLINK SEMICONDUCTOR INC ]
