hc55183. Intersil Corporation, hc55183. Datasheet - Page 11

• Manufacturer Part Number: hc55183.
• Description: Extended Reach Ringing Slic Family
• Manufacturer: Intersil Corporation

Voice Transmission
The feedback mechanism for monitoring the AC portion of
the loop current consists of two amplifiers, the sense
amplifier (SA) and the transmit amplifier (TA). The AC
feedback signal is used for impedance synthesis. A detailed
model of the AC feed back loop is provided below.
The gain of the transmit amplifier, set by R
programmed impedance of the device. The capacitor C
blocks the DC component of the loop current. The ground
symbols in the model represent AC grounds, not actual DC
potentials.
The sense amp output voltage, V
Ring voltage and load is calculated using Equation 23.
The transmit amplifier provides the programmable gain
required for impedance synthesis. In addition, the output of
this amplifier interfaces to the CODEC transmit input. The
output voltage is calculated using Equation 24.
Once the impedance matching components have been
selected using the design equations, the above equations
provide additional insight as to the expected AC node
voltages for a specific Tip and Ring load.
Transhybrid Balance
The final step in completing the impedance synthesis design
is calculating the necessary gains for transhybrid balance.
The AC feed back loop produces an echo at the V
of the signal injected at V
maintain voice quality. Most applications will use a summing
amplifier in the CODEC front end as shown below to cancel
the echo signal.
V
V
RING
SA
VTX
TIP
=
=
FIGURE 6. AC SIGNAL TRANSMISSION MODEL
–
–
V
V
T
SA
–
20
20
V
----------
8e3
R
R
S
10
------
Z
L
R
R
+
+
-
-
RX
3R
3R
3R
3R
. The echo must be cancelled to
38
HC55180, HC55181, HC55182, HC55183, HC55184
SA
0.75R
1:1
-
+
R/2
, as a function of Tip and
V
T
SA
A
S
, determines the
8K
R
R
TX
(EQ. 23)
(EQ. 24)
output
VRX
VTX
R
-IN
C
VFB
FB
S
FB
The resistor ratio, R
the transmit gain, G
Equation 25.
Most applications set R
4-wire equals the transmit gain. Typically R
20k to prevent loading of the device transmit output.
The resistor ratio, R
gain of the device, G
transmit gain requirement and R
Equation 26.
Power Dissipation
The power dissipated by the device during on hook
transmission is strictly a function of the quiescent currents
for each supply voltage during Forward Active operation.
Off hook power dissipation is increased above the quiescent
power dissipation by the DC load. If the loop length is less
than or equal to R
current, I
Equation 28.
If the loop length is greater than R
in the constant voltage, resistive feed region. The power
dissipated in this region is calculated using Equation 29.
Since the current relationships are different for constant
current versus constant voltage, the region of device
operation is critical to valid power dissipation calculations.
G
R
P
P
P
FAQ
FA IA
FA IB
A
TX
=
1:1
=
--------- -
G
R
=
FIGURE 7. TRANSHYBRID BALANCE INTERFACE
44
B
–
=
V
=
G
A
BH
HC5518x
T
P
24
, and the power dissipation is calculated using
A
P
FA Q
FA Q
R
------- -
R
I
R
BHQ
F
B
R
+
KNEE
+
TX
+
V
F
F
V
44
BL
/R
/R
V
VRX
-IN
BL
VTX
. The transmit gain is calculated using
BL
R
. R
F
B
A
xI
, the device is providing constant
S
xI
, provides the final adjustment for
, is determined by the transhybrid
= R
B
F
A
I
BLQ
is previously defined by the
–
–
B
R
, hence the device 2-wire to
R
LOOP
+
KNEE
A
LOOP
V
R
R
is calculated using
CC
B
A
xI
, the device is operating
xI
2
R
I
2
B
CCQ
F
A
B
is greater than
+2.4V
-
+
TX IN
RX OUT
CODEC
(EQ. 25)
(EQ. 26)
(EQ. 27)
(EQ. 28)
(EQ. 29)