hc5549 Intersil Corporation, hc5549 Datasheet - Page 8

• Manufacturer Part Number: hc5549
• Description: Low Power Slic With Battery Switch
• Manufacturer: Intersil Corporation

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
external capacitor should be 4.7µF.
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
feed characteristic.
The point on the y-axis labeled V
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.
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
The term I
line segment I
loop current limit function.
V
I
I
SC
A
FIGURE 5. I
TR OC
=
(
I
=
LIM
I
V
LIM
)
TR(OC)
+
=
LIM
V
--------------------------------------------------------------
FIGURE 4. DC FEED CHARACTERISTIC
+
TR OC
V
V
----------------------------------------------------- -
LOOP
VBL
BL
is the programmed current limit, 1760/R
A
TR OC
(
I
represents the constant current region of the
SC
(
–
I
+ 4V. The following diagram shows the DC
LIM
)
8
VERSUS R
10e3
I
–
LOOP
10e3
)
2R
R
–
LOOP
2R
P
(mA)
P
R
I
LOOP
LIM
I
LIM
LOOP
8
SC
I
TR(OC)
(Ω)
LIM
LOAD CHARACTERISTIC
.
DC
m = (∆V
I
A
R
is the open circuit
. The value of the
KNEE
TR
/∆I
L
I
B
) = 10kΩ
(EQ. 18)
(EQ. 19)
(EQ. 20)
IL
. The
HC5549
The maximum loop impedance for a programmed loop
current is defined as R
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 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.
R
I
V
V
RING
B
SA
VTX
KNEE
TIP
=
=
V
----------------------- -
R
=
TR OC
FIGURE 6. AC SIGNAL TRANSMISSION MODEL
–
LOOP
=
KNEE
(
–
(
V
V
V
----------------------- -
T
SA
TR OC
–
I
)
LIM
20
20


V
B
is exceeded, the device will transition from
(
----------
8e3
R
R
represents the resistive feed portion of the
)
S
10
------
Z
)


L
R
KNEE
R
+
+
-
-
3R
3R
3R
3R
.
SA
0.75R
1:1
-
+
R/2
, as a function of Tip and
V
T
SA
A
S
, determines the
8K
R
R
(EQ. 21)
(EQ. 22)
(EQ. 23)
(EQ. 24)
FN4539.3
VRX
VTX
R
-IN
C
VFB
FB
S
FB