HC55171 Datasheet, PDF (7/18 Pages) Intersil Corporation – 5 REN Ringing SLIC for ISDN Modem/TA and WLL

HC55171
SLIC
TIP
V_RX
CODEC
RX
OUT
PCM
IN
OUT1
RING
TX
IN
ANALOG
PCM
OUT
DIGITAL
FIGURE 3. COMPLETE VOICE SIGNAL PATH
Impedance Matching
Impedance matching is used to match the AC source imped-
ance of the SLIC to the AC source impedance of the load.
When the impedance is matched, the voltage level at the
receive input of the SLIC will be the same voltage level that is at
the 2-wire differential output (i.e., Tip and Ring). Impedance
matching applies only to the 2-wire interface, not the 4-wire
interface.
Slic AC signal power levels are most commonly assigned the
units dBmO. The term dBmO refers to milliwatts in a 600Ω
load. The typical AC power level is 0dBmO which is 1mW
referenced to a 600Ω load. The relationship between dBmO
and V_RMS is provided in Equation 2.
dBmO
=
10
⋅
log



1000
⋅
-⁽--^V----^R-₆---₀^M----₀-^S----⁾--²--_{}
(EQ. 2)
Substituting 0dBmO into the equation should result in
0.7746 V_RMS. For sinusoidal signals, multiply the RMS
voltage by 1.414 to obtain the peak sinusoidal voltage.
The SLIC impedance matching is achieved by applying a feed
back loop from the transmit output of the SLIC to the receive
input of the SLIC. The transmit output voltage of the HC55171
is proportional to the loop current (DC + AC) flowing in the sub-
scriber loop. The impedance matching feedback only uses the
AC portion of the transmit output voltage. Applying a voltage
gain to the feedback term and injecting it into the receive signal
path, will cause the SLIC to “synthesize” a source impedance
that is nonzero. Recall that the impedance matching sets the
SLIC source impedance equal to the load impedance.
The SLIC application circuit requires external sense resistors
in the Tip and Ring signal paths to achieve the differential
receive function. The sense resistors contribute to the source
impedance of the SLIC and are accounted for in the design
equations. Specifically, if the load impedance is 600Ω and
each sense resistor is 50Ω, the SLIC must synthesize an
additional source impedance of 500Ω (i.e., 600Ω - 2(50Ω)).
In addition to the sense resistors, some applications may use a
protection resistor in each of the Tip and Ring leads as part of a
surge protection network. These resistors also contribute to the
SLIC source impedance and can be easily accounted for in the
design equations. If 50Ω protection resistors are added to the
prior example, the SLIC would then have to synthesize 400Ω to
match the load (i.e., 600Ω - 2(50Ω) - 2(50Ω)). A diagram
68
LOAD IMPEDANCE
RSYNTH
RP
RS
TIP
RING
RP
RS
RSYNTH
-1
SLIC SOURCE IMPEDANCE
FIGURE 4. SLIC IMPEDANCE DIAGRAM
showing the impedance terms is shown in Figure 4.
Loop Supervision
The SLIC must detect when the subscriber picks up the
handset when the SLIC is not ringing the phone and when
the SLIC is ringing the phone. The HC55171 uses a switch
hook detector output to indicate loop closure when the SLIC
is not ringing the phone. When the SLIC is ringing the
phone, loop closure is indicated by the ring trip detector.
(Recall from earlier discussions that the subscriber loop is
open when the handset is on hook and closed when off
hook. The DC impedance of the handset when off hook is
typically 400Ω.)
When the handset is off hook, DC loop current flows from
Tip to Ring and the transmit output voltage increases to a
negative value. In addition to interfacing to the CODEC and
providing the feedback for impedance matching, the transmit
output also drives the input to a voltage comparator. When
the comparator threshold is exceeded, the SHD output goes
to a logic low, indicating the handset is off hook. When the
call is terminated and the handset is returned on hook, the
transmit voltage decreases to zero, crossing the comparator
threshold and setting SHD to a logic high.
Loop closure must also be detected when the SLIC is ringing
the handset. The balanced ringing output of the SLIC coin-
cides with a zero DC potential between Tip and Ring. There-
fore the ring trip must be designed around an AC only
waveform at the transmit output. When the SLIC is ringing
and the handset is on hook, the echo of the ringing signal is
at the transmit output. When the handset goes off hook, the
amplitude of the ringing echo increases. The increase in
amplitude is detected by an envelope detector. When the
echo increases, the envelope detector output increases and
exceeds the ring trip comparator threshold. Then RTD goes
to a logic low, indicating the handset is off hook. When the
system controller detects a logic low on RTD, the ringing is
turned off and the Tip and Ring terminals return to their
typical negative DC potentials.
Design Equations and Operational Theory
The following discussion separates the SLICS’s operation
into its DC and AC path, then follows up with additional cir-
cuit design and application information.