HC5549_14 Datasheet, PDF (9/13 Pages) Intersil Corporation – Low Power SLIC with Battery Switch

HC5549
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_F
R_B
RX OUT
-
+
TX IN
+2.4V
CODEC
FIGURE 7. 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 25.
G_TX
=
–G₂₄



R^-R----_BF^--_{}
(EQ. 25)
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 26.
R_A=
--^R----^B----
G₄₄
(EQ. 26)
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. 27)
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 28.
P_FA(IA) = P_FA(Q) + (V_BLxI_A) – (R_LOOPxI^2A)
(EQ. 28)
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 29.
P_FA(IB)= P_FA(Q) + (V_BLxI_B) – (R_LOOPxI^2B)
(EQ. 29)
Since the current relationships are different for constant
current versus constant voltage, the region of device
operation is critical to valid power dissipation calculations.
Reverse Active
Overview
The reverse active mode (RA, 011) provides the same
functionality as the forward active mode. On hook
transmission, DC loop feed and voice transmission are
supported. 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.
During reverse active the Tip and Ring DC voltage
characteristics exchange roles. That is, Ring is typically 4V
below ground and Tip is typically 4V more positive than
battery. Otherwise, all feed and voice transmission
characteristics are identical to forward active.
Silent Polarity Reversal
Changing from forward active to reverse active or vice versa
is referred to as polarity reversal. Many applications require
slew rate control of the polarity reversal event. Requirements
range from minimizing cross talk to protocol signalling.
The device uses an external low voltage capacitor, C_POL, to
set the reversal time. Once programmed, the reversal time
will remain nearly constant over various load conditions. In
addition, the reversal timing capacitor is isolated from the AC
loop, therefore loop stability is not impacted.
The internal circuitry used to set the polarity reversal time is
shown below.
I₁
POL
75kΩ
C_POL
I₂
FIGURE 8. REVERSAL TIMING CONTROL
During forward active, the current from source I1 charges
the external timing capacitor C_POL and the switch is open.
The internal resistor provides a clamping function for
voltages on the POL node. During reverse active, the switch
closes and I2 (roughly twice I1) pulls current from I1 and the
timing capacitor. The current at the POL node provides the
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FN4539.3