HC55171CM INTERSIL [Intersil Corporation], HC55171CM Datasheet - Page 10

• Manufacturer Part Number: HC55171CM
• Description: 5 REN Ringing SLIC for ISDN Modem/TA and WLL
• Manufacturer: INTERSIL [Intersil Corporation]

feedback circuitry becomes relatively easy to match the
impedance at points “A” and “B”.
Impedance Matching Design Equations
Matching the impedance of the SLIC to the load is
accomplished by writing a loop equation starting at V
going around the loop to V
The loop equation to match the impedance of any load is as
follows (note: V
–
Equation 22 can be separated into two terms, the feedback
(-8R
------------ -
4R
V
V
V
V
I
IN
TR
L
IN
IN
S
S
(R
=
=
=
I
R
V
L
– R
–
Z0
IN
L
-----------
R
8
R
8R
I
L
+
-
/R
RF
Z0
S
S
–
RF
8R
I R
-----------
R
R
+
I
+
-
L
I
RING
RX
)) and the loop impedance (+4R
+
-
RF
Z0
L
S
TIP
I
2R
=
L
B
A
-----------
R
R
-----------
R
R
–
----------------------------- -
= 0 for this analysis):
RF
S
Z0
4
+
RF
Z0
R S I L
-
+
I
4R
L
R
R
R
+
–
+
P1
P2
I
I
L
4R
L
S
4R
V
+
-
C
+
IN
S
.
S
R
------------ -
R
R Z0
71
+
-
+
L
RF
I
+
L
R
R
R
R
R
S1
S2
I
+
I
L
P1
L
L
L
FIGURE 7. AC VOLTAGE GAI
+
R
+
-
= R
V RX
------------ -
I
L
L
R
+
I
P2
L
2R
= R
+
+
-
-
V
V
S
C
D
S1
90k
I
I
L
R
R
+
+
= R
-
-
–
4R
S
S2
+R
V C
S
= R
90k
L
V
I
(EQ. 23)
(EQ. 20)
(EQ. 21)
(EQ. 22)
).
=
D
L
S
D
R/20
– R S
-----------
R
=
R
R/2
4
R
R
and
RF
Z0
4R
HC55171
S
The result is shown in Equation 23. Figure 8 is a schematic
representation of Equation 18. To match the impedance of
the SLIC to the impedance of the load, set:
If R
Therefore to match the HC5517, with R
600 load:
and
To prevent loading of the V
R
Since the impedance matching is a function of the voltage
gain, scaling of the resistors to achieve a standard value is
recommended.
For complex impedances the above analysis is the same.
Refer to application note AN9607 (“Impedance Matching
Design Equations for the HC5509 Series of SLICs”) for the
values of KR
impedances.
Through SLIC Ringing
The HC55171 uses linear amplification to produce the ringing
signal. As a result the ringing SLIC can produce sinusoid,
trapezoid or square wave ringing signals. Regardless of the
wave shape, the ringing signal is balanced. The balanced
waveform is another way of saying that the tip and ring DC
potentials are the same during ringing. The following figure
shows the Tip and Ring waveforms for sinusoid and trapezoid
wave shapes as can be displayed using an oscilloscope.
Pertinent Bellcore Ringing Specifications
Bellcore has defined bounds around the existing unbalanced
ringing signal that is supplied by the central office. The
8R
R
R
R
KR
KR
FIGURE 8. SCHEMATIC REPRESENTATION OF EQUATION 20
RF
Z0
RF
Z0
S
RF
Z0
RF
+
are typically scaled by a factor of 100:
=
=
-----------
R
R
4R
=
is made to equal 8R
=
RF
Z0
R
8R
40k
L
40k
S
– R
S
V
+
=
4
IN
4R
=
R
RF
S
L
8 50
+
S
-
=
KR
and KR
=
600
KR
R
Z0
L
RF
=
=
–
100 Resistive 200
LOAD
400
Z0
200
=
R
L
S
TX
40k
for many worldwide typical line
then:
output, the value of R
=
SLIC
400
–
8RS
S
equal to 50 , to a
------------ -
R
R Z0
+
RF
Reactive
------------------------- -
+
100
4R
(EQ. 29)
(EQ. 24)
(EQ. 25)
(EQ. 26)
(EQ. 27)
(EQ. 28)
S
Z0
and