HC55171_03 Datasheet, PDF (14/20 Pages) Intersil Corporation – 5 REN Ringing SLIC for ISDN Modem/TA and WLL

HC55171
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
differential 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
trapezoidal 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
operate for loop lengths ranging from 0Ω to 500Ω . In addition,
one set of component values will satisfy the entire ringing loop
range of the SLIC. A single resistor sets the open circuit RMS
ringing voltage, which will set the crest factor of the ringing
waveform. The crest factor of the HC55171 ringing waveform
is independent of the ringing load (REN) and the loop length.
Another robust feature of the HC55171 ringing SLIC is the
ring trip detector circuit. The suggested values for the ring trip
detector circuit cover quite a large range of applications.
The assumptions used to design the trapezoidal ringing
application circuit are listed below:
• Loop current limit set to 25mA.
• Impedance matching is set to 600Ω resistive.
• 2-wire surge protection is not required.
• System able to monitor RTD and SHD.
• Logic ringing signal is used to drive RC trapezoid network.
Crest Factor Programming
As previously mentioned, a single resistor is required to set
the crest factor of the trapezoidal waveform. The only design
variable in determining the crest factor is the battery voltage.
The battery voltage limits the peak signal swing and
therefore directly determines the crest factor.
A set of tables will be provided to allow selection of the crest
factor setting resistor. The tables will include crest factors
below the Bellcore minimum of 1.2 since many ringing SLIC
applications are not constrained by Bellcore requirements.
TABLE 2. CREST FACTOR PROGRAMMING RESISTOR FOR
V_BAT = -80V
R_TRAP
CF
0Ω 1.10
389Ω 1.15
640Ω 1.20
RMS
65.0
62.6
60.0
R_TRAP
CF
825Ω 1.25
964Ω 1.30
1095Ω 1.35
RMS
57.6
55.4
53.3
The RMS voltage listed in the table is the open circuit RMS
voltage generated by the SLIC.
TABLE 3. CREST FACTOR PROGRAMMING RESISTOR FOR
V_BAT = -75V
R_TRAP
CF
0Ω 1.10
500Ω 1.15
791Ω 1.20
RMS
60.9
58.3
55.9
R_TRAP
CF
1010Ω 1.25
1190Ω 1.30
1334Ω 1.35
RMS
53.7
51.6
49.7
TABLE 4. CREST FACTOR PROGRAMMING RESISTOR FOR
V_BAT = -65V
R_TRAP
CF
0Ω 1.10
660Ω 1.15
1040Ω 1.20
RMS
52.5
49.8
47.8
R_TRAP
CF
1330Ω 1.25
1600Ω 1.30
1800Ω 1.35
RMS
45.9
44.1
42.5
TABLE 5. CREST FACTOR PROGRAMMING RESISTOR FOR
V_BAT = -60V
R_TRAP
CF
0Ω 1.10
740Ω 1.15
RMS
48.2
45.6
R_TRAP
CF
1460Ω 1.25
1760Ω 1.30
RMS
42.0
40.4
1129Ω 1.20
43.7
2030Ω 1.35
38.8
Ringing Voltage Limiting Factors
As the load impedance decreases (increasing REN), the
feedback used for impedance synthesis slightly attenuates
the ringing signal. Another factor that attenuates the ringing
signal is the voltage divider formed by the sense resistors
and the impedance of the ringing load. As the load
impedance decreases, the 100Ω of sense resistors becomes
a larger percentage of the load impedance.
If surge protection resistance must be used with the
trapezoidal circuit, the loop length performance of the circuit
will decrease. The decrease in ringing loop length is caused
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