HC5517_00 Datasheet, PDF (11/19 Pages) Intersil Corporation – 3 REN Ringing SLIC For ISDN Modem/TA and WLL

HC5517
Mode Control Signals
The Battery Switch selects between the -80V and -24V
supplies. The Battery Switch circuitry is described in the
“Operation of the Battery Switch” section. A system alternative
to the battery switch signal is to use a buffered version of the
SHD output to select the battery voltage. Another alternative
is to control the output of a programmable battery supply,
removing the battery switch entirely from the application
circuit. F1 is used to put the SLIC in the power denial mode.
RC drives the base of T₂, which is the transistor used to
control the centering voltage and MTU voltage. The three
control signals can be driven from a TTL logic source or an
open collector output.
Ringing Mode
The ringing state, as the name indicates, is used to ring the
telephone with a -80V battery supply. The SLIC is designed
for balanced ringing with a differential gain of 40V/V across tip
and ring. Voltage feed amplifiers operating in the linear mode
are used to amplify the ringing signal. The linear amplifier
approach allows the system designer to define the shape and
amplitude of the ringing waveform. Both supervisory function
outputs, SHD and RTD, are active during ringing.
Spectral Content of the Ringing Signal
The shape of the waveform can range from sinusoidal to
trapezoidal. Sinusoidal waveforms are spectrally cleaner
than trapezoidal waveforms, although the latter does result
in lower power dissipation across the SLIC for a given RMS
amplitude. Systems where the ringing signal will be in
proximity to digital data lines will benefit from the sinusoidal
ringing capability of the HC5517. The slow edge rates of a
sinusoid will minimize coupling of the large amplitude ringing
signal. The linear amplifier architecture of the HC5517
allows the system designer to optimize the design for power
dissipation and spectral purity.
Amplitude of the Ringing Signal
Amplitude control is another benefit of the linear amplifier
architecture. Systems that require less ringing amplitude are
able to do so by driving the HC5517 with a lower level
ringing waveform. Solutions that use saturated amplifiers
can only vary the amplitude of the ringing signal by changing
the negative battery voltage to the SLIC.
HC5517 Through SLIC Ringing
The HC5517 is designed with a high gain input, V_RING , that
the system drives while ringing the phone. V_RING is one of
many signals summed at the inverting input to the tip feed
amplifier. The gain of the V_RING signal through the tip feed
amplifier is set to 20V/V. The output of the tip feed amplifier
is summed at the inverting input of the ring feed amplifier,
configured for unity gain. The result is a differential gain of
40V/V across tip and ring of the ringing signal.
The ringing function requires an input ringing waveform and
a centering voltage. The ringing waveform is the signal from
the 4-wire side that is amplified by the SLIC to ring the
telephone. The centering voltage, as previously discussed,
is a positive DC offset that is applied to the V_RING input
along with the ringing waveform. The HC5517 application
circuit provides the centering voltage, simplifying the system
interface to an AC coupled ringing waveform.
Ringer Equivalence Number
Before any further discussion, the Ringer Equivalence
Number or REN must be discussed. Based on FCC Part
68.313 a single REN can be defined as 5kΩ, 7kΩ or 8kΩ of
AC impedance at the ringing frequency. The ringing
frequency is based on the ringing types listed in Table 1 of
the FCC specification. The impedance of multiple REN is the
paralleling of a single REN. Therefore 5 REN can either be
1kΩ, 1.4kΩ or 1.6kΩ. The 7kΩ model of a single REN will be
used throughout the remainder of the data sheet.
Ringing Waveform
An amplitude of 1.2V_RMS will deliver approximately 46V_RMS
to a 1 REN load, and 42V_RMS to a 3 REN load. The
amplitude is REN dependent and is slightly attenuated by
the feedback scheme used for impedance matching. The
ringing waveform is cadenced, alternating between a 20Hz
burst and a silent portion between bursts. Bellcore
specification TR-NWT-000057 defines seven distinct ringing
waveforms or alerting (ringing) patterns. The following table
lists each type.
TABLE 2. DISTINCTIVE ALERTING PATTERNS
INTERVAL DURATION IN SECONDS
PATTERN RINGING SILENT RINGING SILENT RINGING SILENT
A
0.4
0.2
0.4
0.2
0.8
4.0
B
0.2
0.1
0.2
0.1
0.6
4.0
C
0.8
0.4
0.8
0.4
D
0.4
0.2
0.6
4.0
E
1.2
4.0
F
1 ± 0.2 3 ± 0.3
G
0.3
0.2
1.0
0.2
0.3
4.0
Figure 8 shows the relationship of the cadenced ringing
waveform and the Battery Switch and RC control signals.
Also shown are the states of the MTU voltage and the
centering voltage.
The state of Battery Switch is indicated by the desired
battery voltage to the SLIC. The RC signal is used to enable
and disable the centering voltage and MTU voltage. RC
follows the ring signal in that it is high during the 20Hz burst
and low during the static part of the waveform.
Open Circuit Voltage During the Ringing Mode
The mutually exclusive relationship of the centering voltage
and MTU implies that both functions will not exist at the
same time. During the silent portion of the ringing waveform
the HC5517 application circuit meets the hazardous voltage
requirements of FCC Part 68.306 by forcing the MTU
voltage. Without the zener clamping solution, a
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