HC55185_06 Datasheet, PDF (11/20 Pages) Intersil Corporation – VoIP Ringing SLIC Family

HC55185
Forward Active
Overview
The forward active mode (FA, 001) is the primary AC
transmission mode of the device. On hook transmission, DC
loop feed and voice transmission are supported during forward
active. Loop supervision is provided by either the switch hook
detector (E0 = 1) or the ground key detector (E0 = 0). The
device may be operated from either high or low battery for on-
hook transmission and low battery for loop feed.
On-Hook Transmission
The primary purpose of on hook transmission will be to
support caller ID and other advanced signalling features.
The transmission over load level while on hook is 3.5V_PEAK.
When operating from the high battery, the DC voltages at Tip
and Ring are MTU compliant. The typical Tip voltage is -4V
and the Ring voltage is a function of the battery voltage for
battery voltages less than -60V as shown in Equation 19.
V_RING = V_BH + 4
(EQ. 19)
Loop supervision is provided by the switch hook detector at
the DET output. When DET goes low, the low battery should
be selected for DC loop feed and voice transmission.
Feed Architecture
The design implements a voltage feed current sense
architecture. The device controls the voltage across Tip and
Ring based on the sensing of load current. Resistors are
placed in series with Tip and Ring outputs to provide the
current sensing. The diagram below illustrates the concept.
V_OUT
R_B
R_CS
-
+
R_L
R_A
V_IN
R_C
-
+
K_S
FIGURE 4. VOLTAGE FEED CURRENT SENSE DIAGRAM
By monitoring the current at the amplifier output, a negative
feedback mechanism sets the output voltage for a defined
load. The amplifier gains are set by resistor ratios (R_A, R_B,
R_C) providing all the performance benefits of matched
resistors. The internal sense resistor, R_CS, is much smaller
than the gain resistors and is typically 20Ω for this device.
The feedback mechanism, K_S, represents the amplifier
configuration providing the negative feedback.
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 = (ΔV_TR/ΔI_L) = 11.1kΩ
I_LOOP (mA)
I_LIM
FIGURE 5. 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. 20)
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 (Ω/foot) and the
telephone off hook DC resistance.
I_SC
I_A
I_LIM
I_B
2R_{P RLOOP (Ω)}
R_KNEE
FIGURE 6. I_LOOP vs 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_LIM
+
-^V----^T---^R-----⁽--^O----^C----⁾----–----²----^R----^P----^I--^L----^I-^M---
1.1 e 4
(EQ. 21)
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---
1.1 e 4
(EQ. 22)
The maximum loop impedance for a programmed loop
current is defined as R_KNEE.
R_KNEE
=
V_{-----T---R-----(--O----C----)}
I_LIM
(EQ. 23)
11
FN4831.14
December 18, 2006