HC55185 Intersil Corporation, HC55185 Datasheet - Page 11

• Manufacturer Part Number: HC55185
• Description: VoIP Ringing SLIC Family
• Manufacturer: Intersil Corporation

When R
constant current feed to constant voltage, resistive feed. The
line segment I
load characteristic.
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 25.
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 26.
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
I
V
V
B
SA
VTX
RING
=
TIP
V
----------------------- -
=
R
=
TR OC
FIGURE 7. AC SIGNAL TRANSMISSION MODEL
–
LOOP
KNEE
–
V
V
T
SA
–
V
B
is exceeded, the device will transition from
----------
8e3
20
20
R
R
represents the resistive feed portion of the
S
30
------
Z
L
R
R
+
+
-
-
RX
4-11
4R
4R
4R
4R
. The echo must be cancelled to
SA
, as a function of Tip and
1:1
-
+
3R
3R
V
T
SA
S
A
, determines the
8K
R
R
TX
(EQ. 24)
(EQ. 25)
(EQ. 26)
output
VRX
VTX
R
-IN
VFB
C
FB
S
FB
HC55185
maintain voice quality. Most applications will use a summing
amplifier in the CODEC front end as shown below to cancel
the echo signal.
The resistor ratio, R
the transmit gain, G
Equation 27.
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 28.
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 30.
If the loop length is greater than R
operating in the constant voltage, resistive feed region. The
power dissipated in this region is calculated using Equation 31.
Since the current relationships are different for constant
current versus constant voltage, the region of device
operation is critical to valid power dissipation calculations.
R
P
P
P
TX
FAQ
FA IA
FA IB
A
1:1
=
=
--------- -
G
R
=
FIGURE 8. TRANSHYBRID BALANCE INTERFACE
44
–
B
=
V
G
=
A
HC5518x
BH
T
P
24
, and the power dissipation is calculated using
A
P
FA Q
FA Q
R
------- -
R
I
R
F
B
BHQ
R
+
KNEE
+
TX
+
V
F
F
V
44
BL
/R
/R
V
VRX
-IN
VTX
BL
R
. The transmit gain is calculated using
BL
. R
S
F
B
A
xI
, the device is providing constant
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
+
A
LOOP
V
R
R
KNEE
is calculated using
CC
A
B
xI
xI
2
I
R
2
, the device is
B
CCQ
A
F
B
is greater than
+2.4V
-
+
(EQ. 27)
TX IN
RX OUT
CODEC
(EQ. 28)
(EQ. 29)
(EQ. 30)
(EQ. 31)