HC5517_00 Datasheet, PDF (9/19 Pages) Intersil Corporation – 3 REN Ringing SLIC For ISDN Modem/TA and WLL

HC5517
To prevent loading of the V_TX output, the value of R₈ and R₉
are typically scaled by a factor of 100:
KR₈ = 40kΩ
KR₉ = 40k Ω
(EQ. 25)
Since the impedance matching is a function of the voltage
gain, scaling of the resistors to achieve a standard value is
recommended.
For complex impedances the above analysis is the same.
KR₉ = 40kΩ
KR₈ = 100(Resistive – 200) + ^R-----^e----_1a---₀-^c--₀-^t--ⁱ--^v----^e--
(EQ. 26)
Reference application note AN9607 (“Impedance Matching
Design Equations for the HC5509 Series of SLICs”) for the
values of KR₉ and KR₈ for several worldwide Typical line
impedances.
Tip-to-Ring Open-Circuit Voltage
The tip-to-ring open-circuit voltage, V_OC , of the HC5517 is
programmable to meet a variety of applications. The design
of the HC5517 defaults the value of V_OC to:
V_OC ≅ V_BAT – 8
The HC5517 application circuit overrides the default V_OC
operation when operating from a -80V battery. While
operating from a -80V battery, the SLIC will be in either the
ringing mode or on-hook standby mode. In the ringing mode,
V_OC is designed to switch from 0V (centering voltage) to -
47V (Maintenance Termination Unit voltage). The centering
voltage is active during the ringing portion of the ringing
waveform and the Maintenance Termination Unit (MTU)
voltage is active during the silent portion of the ringing
signal. In the on-hook standby mode, the application circuit
is designed to maintain V_OC at the MTU voltage.
Centering Voltage Application Circuit Overview
The centering voltage is used during ringing to center the DC
outputs of the tip feed and ring feed amplifiers. Centering the
amplifier outputs allows for the maximum undistorted voltage
swing of the ringing signal. Without centering, the output of
each amplifier would saturate at ground or V_BAT, minimizing
the ringing capability of the HC5517. The required centering
voltage, V_C, is +1.8V_DC when operating from a -80V battery.
Centering Voltage Application Circuit Operation
The circuit used to generate the centering voltage is shown
in Figure 5.
+5V
R₂₄
R₁₉
RC
T₂
TO ZENER
DIODE D₁₁
D₁₃
R
R/20
V_RING
R/2
-
D₆
TF
+
TIP FEED
+2V ⁺_-
V_C
R₁₈
AMPLIFIER
90kΩ
90kΩ
-
RF
+
RING FEED
AMPLIFIER
90kΩ
V-----B---2-A-----T--
FIGURE 5. CENTERING VOLTAGE APPLICATION CIRCUIT
The circuitry within the dotted lines is internal to the HC5517.
The value of the resistor designated as R is 108kΩ and the
resistor R/20 is 5.4kΩ. The tip amplifier gain of 20V/V amplifies
the +1.8V_DC at V_C to +36V_DC and adds it to the internal 4V_DC
offset, generating -40V_DC at the tip amplifier output. The -
40V_DC offset also sums into the ring amplifier, adding to the
battery voltage, achieving -40V at the ring amplifier output.
Centering Voltage Design Equations
The centering voltage (V_C) is dependent on the battery
voltage. A battery voltage of -80V requires a +1.8V_DC
centering voltage. The equation used to calculate the
centering voltage is shown below.
V_C
=


V_{-----B--2--A----T-}
– 4_
⁄ 20
(EQ. 27)
The DC voltage at the outputs of the centered tip and ring
amplifiers can be calculated from Equation 28 and
Equation 29.
V_TC = –(20V_C + 4)
(EQ. 28)
V_RC = V_BAT + (20V_C + 4)
(EQ. 29)
The shunt resistor of the divider network, R₁₈, is not
determined from a design equation. It is selected based on
the trade-off of power dissipation in the voltage divider (low
value of R₁₈) and loading affects of the internal R/20 resistor
(high value of R₁₈). The suggested range of R₁₈ is between
1.0kΩ and 2.0kΩ. The application circuit design equation
used to calculate the value of R₁₉ of the divider network is
as follows:
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