HC5503 Intersil Corporation, HC5503 Datasheet - Page 8

• Manufacturer Part Number: HC5503
• Description: Low Cost 24V SLIC For PABX/Key Systems
• Manufacturer: Intersil Corporation

The Transversal Amplifier (TA)
Whereas the feed amplifiers perform the 4-wire to 2-wire
transmission function, the transversal amplifier acts as the
2-wire to 4-wire hybrid. The TA is a summing amplifier
configured to reject common mode signals. It will reject 2-
wire common mode signals. R
current sense resistors. The voice signal output of the
amplifier is a function of the differential voltages appearing
across R
The transversal amplifier also has a DC output proportional
to the metallic current in the loop. The output voltage is
given by:
V
This DC level is used as an input to a comparator whose
output feeds into the logic circuitry as SH. This signal is used
to gate SHD output.
Voice signals on the loop are transformed by the TA into
ground referenced signals. Since the TA output has a DC
offset it is necessary to AC couple the output to any external
circuitry. Note, that during 4-wire to 2-wire transmission, the
transversal amplifier will have an audio signal at its output
proportional to the 4-wire audio receive signal and the loop’s
equivalent AC impedance. This is called the transhybrid
return, and must be cancelled (or balanced) out to prevent
an echo effect. Reference the Transhybrid Circuit section for
more information.
Loop Current Limiting
The maximum loop length for this application is a 533 load
across the feed amplifiers (24V
30mA
resistance often approaches zero. Thus, a need exists to
control the maximum DC loop current that can flow around
the loop to prevent an excessive current drain from the
system battery. This limit is internally set to 30mA on the
HC5503. Figure 3 depicts the feedback network that
modifies the V
Figure 4 illustrates the loop current characteristics as a
function of line resistance.
As indicated above, the TA has a DC voltage output directly
proportional to the loop current. This voltage level is scaled
by R
to one side of a Transconductance Amplifier.
TX
= 2(I
19
MAX
and R
B1
TIP
loop current). However, on a short loop the line
and R
+ I
18
RF
RING
. The scaled level forms the ‘Metallic’ input
voltage as a function of metallic current.
B2
.
) (R
B1
+ R
8
B1
SUPPLY
B2
and R
)
B2
- 8V
act as loop
OVERHEAD
)/
HC5503
The reference input to this amplifier is generated in the bias
network, and is equivalent to 30mA. When the metallic input
exceeds the set reference level, the transconductance
amplifier sources current. This current will charge C
positive direction causing the V
approach the V
feed across the loop which will limit the DC loop current. C
will continue to charge until an equilibrium level is attained at
I
loop is set by R
The V
loop current during ring line faults (e.g., ground or power line
crosses), and thermal overload. Figure 8 illustrates this. The
thermal and fault current circuitry works in parallel with the
transconductance amplifier.
Longitudinal Amplifier
The longitudinal amplifier is an operational amplifier
configured as a closed loop differential amplifier with a
nominal gain of 0.1. The output is a measure of any
imbalance between I
this amplifier is given by:
V
The gain factor is much less than one since ring voltage (up
to 150V
terminals and are attenuated to avoid exceeding the
common mode range of the longitudinal amplifier’s input.
LOOP
V
LONG
V
RING
RF
V
RF
R
FIGURE 3. DC LOOP CURRENT CHARACTERISTICS
RF
B2
= I
= 0.1(I
PEAK
V
V
LOOP
voltage level is also modified to reduce or control
TF
TIP
R
B1
) can appear at the Ring or Ring Feed Sense
RING
FEED
TIP
mA (Max). The time constant of this feedback
TF
21
(90k ) and C
(Tip Feed), effectively reducing the battery
- I
RING
TIP
+
-
TRANSVERSAL
and I
) 150.
A
AMP
VCL
+
-
RING
RF
1
C
= 2
1
which is nominally 0.33 F.
(Ring Feed) voltage to
. The transfer function of
90K
R
21
R
1.8K
R
90K
V
19
18
TX
FOR KV
= -600 I
KV
V
TX
B5
TX
LOOP
I
1
GM
-4V
+
-
V
< V
in
B
> 0,
/2
B5
V
TX
1