HC55171_03 Datasheet, PDF (9/20 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
impedance 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
subscriber 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
9
LOAD IMPEDANCE
RSYNTH
RP
RS
TIP
RP
RS
RING
RSYNTH
-1
SLIC SOURCE IMPEDANCE
FIGURE 4. SLIC IMPEDANCE DIAGRAM
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 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 coincides
with a zero DC potential between Tip and Ring. Therefore 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.