HC55185_06 Datasheet, PDF (12/20 Pages) Intersil Corporation – VoIP Ringing SLIC Family

HC55185
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. 24)
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
-
+
20
+
-
R
R
1:1
T_A
VRX
VTX
R_S
R _4R
3R
-IN
4R
C_FB
4R
-
+
8K
VFB
V_SA
4R
3R
FIGURE 7. 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 25.
V_SA
=
–(V_T
–
V_R
)
-³---⁰--
Z_L
(EQ. 25)
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 26.
V_VTX
=
–V_S
A
⎛
⎝
8^--R--e^---S-3^{--⎠⎞}
(EQ. 26)
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.
R
R
1:1
T_A
HC5518x
VRX
VTX
R_S
-IN
R_A
R_B ^RF
RX OUT
-
+
TX IN
+2.4V
CODEC
FIGURE 8. TRANSHYBRID BALANCE INTERFACE
The resistor ratio, R_F/R_B, provides the final adjustment for
the transmit gain, G_TX. The transmit gain is calculated using
Equation 27.
G_TX
=
–G₂₄
⎛
⎜
⎝
R^-R----_BF^--_{⎠⎟⎞}
(EQ. 27)
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
20kΩ to prevent loading of the device transmit output.
The resistor ratio, R_F/R_A, is determined by the transhybrid
gain of the device, G₄₄. R_F is previously defined by the
transmit gain requirement and R_A is calculated using
Equation 28.
R_A=
--^R----^B----
G₄₄
(EQ. 28)
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.
P_FAQ= V_BH × I_BHQ + V_BL × I_BLQ + V_CC × I_CCQ
(EQ. 29)
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 30.
P_FA(IA) = P_FA(Q) + (V_BLxI_A) – (R_LOOPxI^2A)
(EQ. 30)
If the loop length is 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 31.
P_FA(IB)=
P_FA
(Q)
+
(
V_B
L
x
I_B
)
–
(R_LO
O
P
x
I
2
B
)
(EQ. 31)
12
FN4831.14
December 18, 2006