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參數(shù)資料
| 型號(hào)： | HC5549 |
| 廠商： | Intersil Corporation |
| 英文描述： | Low Power SLIC with Battery Switch(用戶線接口電路) |
| 中文描述： | 低功耗用戶接口與電池開關(guān)（用戶線接口電路） |
| 文件頁數(shù)： | 8/13頁 |
| 文件大?。?/td> | 125K |
| 代理商： | HC5549 |
4-8
DC Loop Feed
The feedback mechanism for monitoring the DC portion of
the loop current is the loop detector. A low pass filter is used
in the feedback to block voice band signals from interfering
with the loop current limit function. The pole of the low pass
filter is set by the external capacitor C
DC
. The value of the
external capacitor should be 4.7
μ
F.
Most applications will operate the device from low battery
while off hook. The DC feed characteristic of the device will
drive Tip and Ring towards half battery to regulate the DC
loop current. For light loads, Tip will be near -4V and Ring
will be near V
VBL
+ 4V. The following diagram shows the DC
feed characteristic.
The point on the y-axis labeled V
TR(OC)
is the open circuit
Tip to Ring voltage and is defined by the feed battery
voltage.
The curve of Figure 5 determines the actual loop current for
a given set of loop conditions. The loop conditions are
determined by the low battery voltage and the DC loop
impedance. The DC loop impedance is the sum of the
protection resistance, copper resistance (ohms/foot) and the
telephone off hook DC resistance.
I
SC
The slope of the feed characteristic and the battery voltage
define the maximum loop current on the shortest possible
loop as the short circuit current I
SC
.
V
The term I
LIM
is the programmed current limit, 1760/R
IL
.
The line segment I
A
represents the constant current region
of the loop current limit function.
The maximum loop impedance for a programmed loop
current is defined as R
KNEE
.
V
)
LIM
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
FIGURE 4. DC FEED CHARACTERISTIC
m = (
V
TR
/
I
L
) = 10k
I
LOOP
(mA)
I
LIM
V
TR(OC)
V
T
,
V
TR OC
)
V
BL
8
–
=
(EQ. 18)
FIGURE 5. I
LOOP
VERSUS R
LOOP
LOAD CHARACTERISTIC
R
LOOP
(
)
R
KNEE
I
LIM
I
L
I
A
I
B
2R
P
I
SC
I
LIM
2R
I
–
-----------------------------------------------------
+
=
(EQ. 19)
I
A
I
LIM
V
--------------------------------------------------------------
R
I
–
+
=
(EQ. 20)
R
KNEE
-----------------------
=
(EQ. 21)
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)
HC5549
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