HC55171_03 Datasheet, PDF (13/20 Pages) Intersil Corporation – 5 REN Ringing SLIC for ISDN Modem/TA and WLL

HC55171
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.
(KR_RF
=
40 k Ω )


K
R_Z
0
=
100 ( R e s i s t i v e
– 200)
+
R_-----e_----1a---0-c_--0-t_---i_-v_----e_{- }
(EQ. 29)
Refer to application note AN9607 (“Impedance Matching
Design Equations for the HC5509 Series of SLICs”) for the
values of KR_RF and KR_Z0 for many worldwide typical line
impedances.
Through SLIC Ringing
The HC55171 uses linear amplification to produce the ringing
signal. As a result the ringing SLIC can produce sinusoid,
trapezoid or square wave ringing signals. Regardless of the
wave shape, the ringing signal is balanced. The balanced
waveform is another way of saying that the tip and ring DC
potentials are the same during ringing. The following figure
shows the Tip and Ring waveforms for sinusoid and trapezoid
wave shapes as can be displayed using an oscilloscope.
GROUND
TIP
BATTERY
GROUND
(A) SINUSOID
RING
TIP
BATTERY
RING
(B) TRAPEZOID
FIGURE 9. BALANCED RINGING WAVESHAPES
Pertinent Bellcore Ringing Specifications
Bellcore has defined bounds around the existing unbalanced
ringing signal that is supplied by the central office. The
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
approximately 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
signal 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
instrument. 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
generate 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
outputs 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----^A----^T-
2
– 4_
⁄ 20
(EQ. 30)
13