HC5520
tion. The ring relay can not be reenergized until the system
acknowledges that a ring trip has occurred. Acknowledg-
ment is achieved by setting the RCI to a logic high.
R
• Z
SENSE
KZO
ZO= ------------------------------------------------
400 • R
N
If the subscriber goes off-hook during the silent portion of
the ringing cadence, the off-hook condition is detected in the
same manner as a switch hook detection. The SHDO output
will be set to a logic low in order to indicate that the sub-
scriber has answered the call and that ringing of the line
should cease.
where R
SENSE
is constrained to be 100kΩ.
4W to 2W Gain
The signal level voltage gain from the 4-wire analog input
(R ) to the 2-wire ∆V voltage is user programmable using
TR
X
the following equation:
Loop Power Down Mode
–R
SENSE
R
X
Under any condition when PDI is set to a logic “0”, the SLIC
will power down the two wire loop. During loop power down,
the voltages at Tip and Ring are both collapsed to one-half
of the battery voltage and the outputs of the Tip and Ring
feed amplifiers are in a high impedance state. Therefore all
of the supervisory functions and transmission functions are
disabled. The HC5520 will resume normal operation once
the loop power down command is removed.
A
= ----------------------------
4 – 2
where R
is constrained to be 100kΩ. The SLIC has a
SENSE
built-in +6.02dB gain to compensate for the divider effect of
matching the load impedance, making it transparent to the
user.
2W to 4W Gain
Thermal Shutdown Mode
The signal level voltage gain from the Tip and Ring terminals
The SLIC will power down the loop by itself once the
temperature of the SLIC die reaches 150oC. During this
thermal shutdown condition, both TSDO and SHDO outputs
will be set to a default logic low to indicate the condition. The
supervisory functions and transmission functions are
disabled. Once the SLIC die temperature drops 10oC lower
than the thermal shutdown temperature, the SLIC will
resume operation.
(∆V ) to the output of the 4-wire signal amplifier (R ) is
user programmable using the following equation:
TR
4W
R
4W
A
= -------------------------
2 – 4
R
SENSE
Transhybrid Balance
Test-Out and Test-In Modes
Functionally, when a voice signal is received at V
a cur-
RX
rent which is proportional to the voice signal will pass
Two additional relay drivers are provided for test-out and
test-in functions. Unlike the ring relay driver circuit, these
relay drivers are operated independently of the rest of the
HC5520 circuitry. The designation of test-out and test-in is
purely arbitrary. When desired, the subscriber’s loop condi-
tion can be interrogated through the test-out relay. Likewise,
through the test-in relay, the various SLIC functions and sig-
nal integrity can be examined.
through the SLIC 4 wire input R pin. This voice input cur-
X
rent will be amplified and inverted to drive the load across
the Tip and Ring. The AC voltages at Tip and Ring are fed
back to the SLIC and reproduced as the transmit signal at
the T pin. This received voice signal returned from 2 wire
X
side of the SLIC will have the same amplitude as the
received AC signal but will be 180 degrees out of phase.
This signal needs to be eliminated from transmission to pre-
vent far end echo.
Hybrid Transmission Model
The most common way of implementing the transhybrid bal-
ance function is to use the analog voice input amplifier in the
Combo as a summing amplifier. The circuit connections are
as shown in Figure 3. Notice that the input impedance net-
works for both received signal and returned signal are bas-
cally the same, if the 62pF capacitor were not added. The
addition of the 62pF capacitor to ground is to compensate for
the phase shift of the returned signal to achieve 15dB or
more improvement in the 2k to 4kHz frequency band as
compared to the data collected from the test circuit.
Figure 2 shows a simplified model for bidirectional signal
transmission and 2-wire impedance synthesis. The term
R
used in the equations below refers to the pair of
SENSE
external 100kΩ sense resistors R
and R
. The
TPS
RGS
HC5520 architecture gives the user the flexibility to set the
gains and 2-wire impedance with external resistors and
resistor ratios. However, to prevent adversely affecting other
SLIC control functions, the value of R
should always
SENSE
be selected to be 100kΩ.
2W Impedance
Sensitive Pins
The 2W impedance is the AC input impedance synthesized
by the SLIC between the Tip and Ring terminals and will be
referred to as ZO. The value of ZO is user programmable by
Tipsen, Ringsen Pins - These pins are very low imped-
ance virtual grounds used for providing feedback current to
the HC5520 DC, AC, and Longitudinal control loops. Para-
sitic capacitance on these pins from the PC board layout
and external components should be minimized to prevent
oscillation.
varying the value R and Z
. R is recommended to be
N
KZO
N
less than 7kΩ. Z
can be either a real resistance or a
KZO
complex impedance network. ZO is determined by the fol-
lowing equation:
7
HARRIS [ HARRIS CORPORATION ]
