HC55180 Datasheet, PDF (18/19 Pages) Intersil Corporation – Extended Reach Ringing SLIC Family

HC55180, HC55181, HC55182, HC55183, HC55184
Additional Application Diagrams
Reducing Overhead Voltages
The transmission overhead voltage of the device is internally
set to 4V per side. The overhead voltage may be reduced by
injecting a negative DC voltage on the receive input using a
voltage divider (Fig. 19). Accordingly, the 2-wire port overload
level will decrease the same amount as the injected offset.
160 kΩ
R_{2 CRX}
VRX
V_D
1:1
HC5518X
R₁
VBL
FROM
CODEC
FIGURE 19. EXTERNAL OVERHEAD CONTROL
The divider shunt resistance is the parallel combination of
the internal 160kΩ resistor and the external R₂. The sum of
R₁ and R₂ should be greater than 500kΩ to minimize the
additional power dissipation of the divider. The DC gain
relationship from the divider voltage, V_D, to the Tip and Ring
outputs is shown below.
V_{T – R}= V_BL – 8 – (2 × V_D)
(EQ. 38)
With a low battery voltage -24V and a divider voltage of
-0.5V, the Tip to Ring voltage is 17V. As a result, the
overhead voltage is reduced from 8V to 7V and the overload
level will decrease from 3.5V_PEAK to 3.0V_PEAK.
CODEC Ringing Generation
Maximum ringing amplitudes of the device are achieved with
signal levels approximately 2.4V_P-P. Therefore the low pass
receive output of the CODEC may serve as the low level ring
generator. The ringing input impedance of 480kΩ minimum
should not interfere with CODEC drive capability. A single
external capacitor is required to AC coupled the ringing
signal from the CODEC. The circuit diagram for CODEC
ringing is shown below.
1:1
HC5518X
160 kΩ
VRX
-
+
480K
VRS
RX OUT
CODEC
FIGURE 20. CODEC RINGING INTERFACE
Implementing Teletax Signalling
A resistor, R_T, is required at the -IN input of the device for
injecting the teletax signal (Figure 20). For most
applications the synthesized device impedance (i.e., 600Ω)
will not match the 200Ω teletax impedance. The gain set by
R_T cancels the impedance matching feedback with respect
to the teletax injection point. Therefore the device appears
as a low impedance source for teletax. The resistor R_T is
calculated using the following equation.
R_T = ₂----₀---₀-----+-----₂-----_×-----_R--²--_P--⁰---⁰_+-----(---_R----_S-----_⁄---₄---₀---₀-----₎ × R_S
(EQ. 39)
The signal level across a 200Ω load will be twice the injected
teletax signal level. As the teletax level at VTX will equal the
injection level, set R_C = R_B for cancellation. The value of R_B
is based on the voice band transhybrid balance
requirements. The connection of the teletax source to the
transhybrid amplifier should be AC coupled to allow proper
biasing of the transhybrid amplifier input
TA
CFB
VFB
RT
RS
-IN
VTX
TELETAX
SOURCE
R_B R_F
R_C
-
+
TX IN
+2.4V
CODEC
FIGURE 21. TELETAX SIGNALLING
Ringing With DC Offsets
The balanced ringing waveform consists of zero DC offset
between the Tip and Ring terminals. However, the linear
amplifier architecture provides control of the DC offset
during ringing. The DC gain is the same as the AC gain,
40V/V per amplifier. Positive DC offsets applied directly to
the ringing input will shift both Tip and Ring away from half
battery towards ground and battery respectively. A voltage
divider on the ringing input may be used to generate the
offset (Figure 22). The reference voltage, V_REF, can be
either the CODEC 2.4V reference voltage or the 5V supply.
-
+
480K
VRS
R_{2 CRS}
V_D
R₁
FROM
RING GEN.
HC5518X
V_REF
FIGURE 22. EXTERNAL OVERHEAD CONTROL
An offset during ringing of 30V, would require a DC shift of
15V at Tip and 15V at Ring. The DC offset would be created
by a +0.375V (V_D) at the VRS input. The divider resistors
should be selected to minimize the value of the AC coupling
capacitor C_RS and the loading of the ring generator and
voltage reference. The ringing input impedance should also
be accounted for in divider resistor calculations.
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