HC55171 Datasheet, PDF (11/18 Pages) Intersil Corporation – 5 REN Ringing SLIC for ISDN Modem/TA and WLL

HC55171
GROUND
TIP
BATTERY
GROUND
(A) SINUSOID
RING
TIP
BATTERY
RING
(B) TRAPEZOID
FIGURE 9. BALANCED RINGING WAVESHAPES
HC55171 ringing SLIC meets the REN drive requirement, the
crest factor limitations and the minimum RMS ringing voltage.
The foremost requirement is that the ringing source must be
able to drive 5 REN. A REN is a ringer equivalence number
modeled by a 6.93kΩ resistor in series with a 8µF capacitor
(see Figure 10). The impedance of 1 REN at 20Hz is approx-
imately 7kΩ. 5 REN is equivalent to five of the networks in
parallel. Figure 10 provides the Bellcore REN models.
The crest factor of the ringing waveform is the ratio of the
peak voltage to the RMS voltage. For reference, the crest
factor of a sinusoid is 1.414 and of a square wave is 1.0.
Bellcore defines the crest factor range from 1.2 to 1.6. A sig-
nal with a crest factor between 1.2 and 1.414 resembles the
trapezoid of Figure 9. A signal with a crest factor between
1.414 and 1.6 resembles a “rounded triangular” wave shape
and is an inefficient waveform for the ringing SLIC.
40µF
1386Ω
5 REN
8µF
6930Ω
1 REN
FIGURE 10. BELLCORE RINGER EQUIVALENCE MODELS
The third pertinent Bellcore requirement is the that RMS ringing
voltage must be greater than 40V_RMS at the telephone instru-
ment. The HC5517 is able to deliver 40V_RMS at the end of
500Ω loops. The 500Ω loop drive capability of the HC5517 is
achieved with trapezoidal ringing.
Sinusoidal Ringing
The HC55171 uses the same sinusoidal application circuit
as the HC5517. The only difference being the values of three
components in the ring trip filter. The following table lists the
components and the different values required by each
device. All reference designators refer to the application
circuit published in the HC5517 and HC55171 data sheet.
TABLE 1. RING TRIP COMPONENT DIFFERENCES
COMPONENT HC5517 COMPONENT HC55171
R₁₅
47kΩ
R_RT3
51.1kΩ
R₁₇
56.2kΩ
R_RT1
49.9kΩ
C₁₀
1.0µF
C_RT
0.47µF
The sinusoidal circuit published in the HC5517 can be used
as an additional reference circuit for the HC55171. To gener-
ate a sinusoid ringing signal, two conditions must be met on
the ringing (V_RING) input of the SLIC.
The first condition is that a positive DC voltage, which is directly
related to the battery voltage, must be present at the ringing
input. The DC voltage is used to force the Tip and Ring DC out-
puts to half the battery voltage. Having both the Tip and Ring
amplifiers biased at the same DC voltage during ringing is one
characteristic of balanced ringing. The centering voltage (V_C)
can be calculated from the following equation.
V_C
=


V_{-----B-2---A----T-}
– 4_
⁄ 20
(EQ. 30)
Substituting values of battery voltage, the centering voltage
is +1.8V for a -80V battery and +1.3V for a -60V battery.
The second condition that must be met for sinusoidal ringing
is a low level ringing signal must be applied to the ringing
input of the SLIC. The AC signal that is present at V_RING will
be amplified by a gain of 20 through the Tip amplifier and a
then inverted through the ring amplifier, resulting in a differ-
ential gain of 40. The maximum low level amplitude that can
be injected for a given battery voltage can be determined
from the following equation.
V_RING(Max) = (V_BAT – 8) ⁄ 20
(EQ. 31)
The maximum output swing may be increased by driving the
V_RING negative by 200mV. Equation 31 can then by
rewritten as:
V_RING(Max) = (V_BAT – 5) ⁄ 20
(EQ. 32)
Exceeding the maximum signal calculated from the above
equation will cause the peaks of the sinusoid to clip at
ground and battery. The compression will reduce the crest
factor of the waveform, producing a trapezoidal waveform.
This is just one method, though inefficient, for achieving trap-
ezoidal ringing. The application circuit provided with the
HC55171 has been specifically developed for trapezoidal
ringing and may also be used with the HC5517.
Trapezoidal Ringing
The trapezoidal ringing waveform provides a larger RMS
voltage to the handset. Larger RMS voltages to the handset
provide more power for ringing and also increase the loop
length supported by the ringing SLIC.
The HC55171 trapezoidal ringing application circuit will oper-
ate for loop lengths ranging from 0Ω to 500Ω. In addition, one
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