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參數(shù)資料
| 型號： | HC5549IM |
| 廠商： | HARRIS SEMICONDUCTOR |
| 元件分類： | 模擬傳輸電路 |
| 英文描述： | THERMISTOR PTC1KOHM 70 DEG 0805 |
| 中文描述： | TELECOM-SLIC, PQCC28 |
| 文件頁數(shù)： | 8/13頁 |
| 文件大?。?/td> | 87K |
| 代理商： | HC5549IM |
4-87
When R
KNEE
is exceeded, the device will transition from
constant current feed to constant voltage, resistive feed. The
line segment I
B
represents the resistive feed portion of the
load characteristic.
V
)
LOOP
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
S
, determines the
programmed impedance of the device. The capacitor C
FB
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
SA
, as a function of Tip and
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
TX
output
of the signal injected at V
RX
. The echo must be cancelled to
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
F
/R
B
, provides the final adjustment for
the transmit gain, G
TX
. The transmit gain is calculated using
Equation 25.
Most applications set R
F
= R
B
, hence the device 2-wire to
4-wire equals the transmit gain. Typically R
B
is greater than
20k
to prevent loading of the device transmit output.
The resistor ratio, R
F
/R
A
, is determined by the transhybrid
gain of the device, G
44
. R
F
is previously defined by the
transmit gain requirement and R
A
is calculated using
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
KNEE
, the device is providing constant
current, I
A
, and the power dissipation is calculated using
Equation 28.
If the loop lengthis greater than R
KNEE
, the device is operating
in the constant voltage, resistive feed region. The power
dissipated in this region is calculated using Equation 29.
I
B
-----------------------
=
(EQ. 22)
FIGURE 6. AC SIGNAL TRANSMISSION MODEL
TIP
RING
+
-
-IN
VFB
VRX
VTX
R
R
R
R
+
-
+
-
+
-
1:1
20
20
0.75R
3R
3R
3R
3R
R/2
8K
R
S
C
FB
T
A
V
SA
V
SA
V
T
V
R
–
(
)
–
L
------
=
(EQ. 23)
V
VTX
V
SA
R
S
----------
–
=
(EQ. 24)
FIGURE 7. TRANSHYBRID BALANCE INTERFACE
+
-
R
F
R
B
R
A
HC5518x
CODEC
+2.4V
RX OUT
TX IN
-IN
VRX
VTX
R
R
+
-
1:1
R
S
T
A
G
TX
G
–
24
R
F
R
B
-------
=
(EQ. 25)
R
A
R
B
44
----------
=
(EQ. 26)
P
FAQ
V
BH
I
×
BHQ
V
BL
I
BLQ
×
V
CC
I
CCQ
×
+
+
=
(EQ. 27)
P
FA IA
)
P
FA Q
)
V
BL
xI
A
(
)
R
LOOP
xI
2A
(
)
–
+
=
(EQ. 28)
P
FA IB
)
P
FA Q
)
V
BL
xI
B
(
)
R
LOOP
xI
2B
(
)
–
+
=
(EQ. 29)
HC5549
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