HC55171 Datasheet, PDF (12/18 Pages) Intersil Corporation – 5 REN Ringing SLIC for ISDN Modem/TA and WLL

HC55171
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 R_TRAP
CF
65.0
825Ω
1.25
62.6
964Ω
1.30
60.0 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 R_TRAP
CF
60.9 1010Ω 1.25
58.3 1190Ω 1.30
55.9 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 R_TRAP
CF
52.5 1330Ω 1.25
49.8 1600Ω 1.30
47.8 1800Ω 1.35
RMS
45.9
44.1
42.5
TABLE 5. CREST FACTOR PROGRAMMING RESISTOR FOR
V_BAT = -60V
R_TRAP
CF
RMS R_TRAP
CF
RMS
0Ω
1.10
48.2 1460Ω 1.25
42.0
740Ω
1.15
45.6 1760Ω 1.30
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 imped-
ance 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
by the addition of protection resistors in series with the Tip
and Ring outputs. The amount of protection resistance that
is added will subtract directly from the loop length. For exam-
ple if 30Ω protection resistors is used in each of the Tip and
Ring leads, the ringing loop length will decrease by a total of
60Ω. Therefore, subtracting 60Ω from the graphs will provide
the reduced loop length data.
Lab Measurements
The lab measurements of the trapezoidal ringing circuit were
made with the crest factor programming resistor set to 0Ω
and the battery voltage set to -80V. The Bellcore suggested
REN model was used to simulate the various ringing loads.
A resistor in series with the Tip terminal was used to emulate
loop length.
A logic gate is used to drive the RC shaping network. When
the crest factor programming resistor is set to 0Ω, the output
impedance of the logic gate results in a 0.8V/ms slewing
voltage on C_TRAP.
Each graph shows the RMS ringing voltage into a fixed REN
load versus loop length. The ringing voltage was measured
across the test load. Each test also verified proper operation
of the ring trip detector. Proper ring trip detector operation is
defined as a constant logic high while ringing and on hook
and a constant logic low when off hook is detected. The
component values in the application circuit provide a ring trip
response in the 100ms to 150ms range.
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