HC5549_14 Datasheet, PDF (8/13 Pages) Intersil Corporation – Low Power SLIC with Battery Switch

HC5549
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 4.7µF.
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_VBL + 4V. The following diagram shows the DC
feed characteristic.
V_TR(OC)
m = (∆V_TR/∆I_L) = 10kΩ
I_LOOP (mA)
I_LIM
FIGURE 4. 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.
V_TR(OC) = V_BL – 8
(EQ. 18)
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
I_A
I_LIM
I_B
2R_{P RLOOP (Ω)}
R_KNEE
FIGURE 5. I_LOOP VERSUS R_LOOP LOAD CHARACTERISTIC
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.
I_SC
=
I_L
I
M
+
-^V----^T---^R-----⁽--^O----^C----⁾----–----²----^R----^P----^I--^L----^I-^M---
10 e 3
(EQ. 19)
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.
I_A=
I_L
IM
+
V_{-----T---R-----(--O----C----)----–----}R_{-----L---O----O-----P----}I_{--L---I--M---}
10e3
(EQ. 20)
The maximum loop impedance for a programmed loop
current is defined as R_KNEE.
R_KNEE
=
V_{-----T---R-----(--O----C----)}
I_LIM
(EQ. 21)
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.
I_B
=
V_{-----T---R-----(--O----C----)}
R_LOOP
(EQ. 22)
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.
TIP
RING
R
20
-
+
R
VRX
R
1:1
20
+_-
VTX
T_A
R_S
R _3R
3R
3R
3R
0.75R
-
8K
+
V_SA
R/2
-IN
C_FB
VFB
FIGURE 6. AC SIGNAL TRANSMISSION MODEL
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.
V_SA
=
–(V_T
–
V_R
)
-¹---⁰--
Z_L
(EQ. 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.
V_VTX
=
–V_S
A


8^--R--e^---S-3^{--}
(EQ. 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.
8
FN4539.3