HC5520
Circuit Operation and Design Information
The HC5520 is a current feed voltage sense
Subscriber Line
Interface Circuit
(SLIC). It provides extensive digitally con-
trolled supervisory functions, DC loop feed functions, and
user selectable 2 wire impedance matching functions.
the specifications applicable to this mode of operation are
provided in the electrical specifications portion of the
HC5520 data sheet.
Battery Feed
Modes of Operation
The HC5520 has seven possible modes of operation. These
modes of operation are either controlled by the digital control
inputs to the SLIC or controlled by the loop status output of
the SLIC. The modes of operation and the function of the
digital control inputs are given in Table 1.
TABLE 1.
SLIC
FUNCTION
Normal
Normal
Loop
power down
Ring trip
detection only
Normal
Normal
Loop
Powerdown
CONTROL
INPUTS
PRI = High
PRI = Low
PDI = Low
RCI = Low
TAI = Low
TBI = Low
OPERATION
Normal
Loop Feed
Reverse
Loop Feed
Loop
Powerdown
Ringing
Test out
Test in
Thermal
Shut Down
MODE
Normal
Reverse
P’down
Ringing
Test-out
Test-in
TSD
The HC5520 is designed to provide a 300Ω resistive feed
(150Ω per wire) for long loop applications. It will supply a DC
loop feed current of 18mA into an 1800Ω loop at the nominal
battery supply of -48V. At shorter loop lengths or higher bat-
tery supply voltages, the DC feed is current-limited to nomi-
nally 26mA in order to conserve power. For internal chip
power management purposes, external power sharing resis-
tors are used to provide some of the DC loop current. This
allows a substantial amount of the power to be dissipated off
the chip, particularly in short loop applications. A typical loop
feeding characteristic for Normal and Reverse Loop Feed
Modes of operation is shown in Figure 1.
I
LOOP
30
mA
20
10
600 800 1.0K 1.2K 1.4K 1.6K 1.8K 2.0K 2.2K 2.4K
R
LOOP
Normal Loop Feed Mode
When PDI = 1, setting the PRI to a logic “1” places the SLIC
in the Normal Loop Feed mode. This is the normal opera-
tional mode of the SLIC. With a nominal battery supply of -
48V and an on-hook condition, the voltage at the Tip termi-
nal will be approximately 8% of the battery supply voltage. In
this case the Tip voltage is about -3.8V. Similarly, the voltage
at the Ring terminal will be approximately 92% of the battery
supply voltage or about -44.2V.
In the Normal mode the Tip voltage is more positive than
the Ring voltage; therefore, in an off-hook condition, the DC
loop current flows from Tip to Ring. The loop feeding
characteristics will be given in the battery feed section. All of
the specifications applicable to this mode of operation are
provided in the electrical specifications portion of the
HC5520 data sheet.
Reverse Loop Feed Mode
When PDI = 1, setting the PRI to a logic “0” places the SLIC
in the Reverse Loop Feed mode. In this mode, the Ring ter-
minal voltage is more positive than the Tip terminal voltage.
Thus, in an off-hook condition, the DC loop current flows
from Ring to Tip. The loop feeding characteristics in the
Reverse mode are the same as in the Normal mode. All of
FIGURE 1. BATTERY FEED CHARACTERISTICS
Loop Supervision - Switch Hook Detection
The Loop Supervision circuit operates in the Normal and
Reverse Loop Feed modes. The DC loop current is moni-
tored and the off-hook condition is indicated when the loop
resistance is less than 2.4kΩ. When this occurs, the SHDO
output will be set to a logic low in order to signal the system
that an off-hook condition exists. If the subscriber is using a
rotary dial telephone, the system can monitor the dial pulses
through the SHDO output.
Ringing - Ring Trip Detection Mode
The ringing voltage is cadenced to a subscriber loop by
applying a logic signal to the Rci input. When a logic “0” is
received at the RCI input, the HC5520 will set the RD output
to low and thus pull current through the ring relay coil and
energize the ring relay. This causes the subscriber’s tele-
phone to begin ringing. At this time the ringing current
through the ring ballast resistor is monitored to determine
whether an off-hook condition is present. Once the sub-
scriber goes off-hook, the ring trip circuit will turn off the ring
relay after the next occurrence of a zero net current flow
through the ring ballast resistor. At the same time, the SHDO
output will be set to a logic low to indicate the ring trip detec-
6
HARRIS [ HARRIS CORPORATION ]
