HC5517 Intersil Corporation, HC5517 Datasheet - Page 8

• Manufacturer Part Number: HC5517
• Description: 3 REN Ringing SLIC For ISDN Modem/TA and WLL
• Manufacturer: Intersil Corporation

same amount to account for resistors R
The net effect cancels out the voltage drop across the feed
resistors. By nullifying the effects of the feed resistors the
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
impedance of any load is as follows (Note: V
analysis):
Equation 19 can be separated into two terms, the feedback
–
R
4R
V
V
V
L
TR
IN
IN
S
I
L
=
=
+
R
V
I
L
IN
2R
–
L
8R
I
R
------ -
R
+
L
-
S
8
9
S
–
8R
I
+
L
I R
+
I
-
I
RING
2R
–
L
+
-
L
TIP
S
I
4R
L
B
A
=
R
------ -
R
S
R
------ -
R
8
9
S
–
----------------------------- -
8
9
4
I
-
L
+
+
I
R
+
–
L
R
R
R
4R
S
11
12
I
4R
I
L
L
R
------ -
R
V
I
S
+
-
8
9
IN
L
C
S
. The loop equation to match the
I
+
67
=
L
R 8
-------
R 9
+
-
+
R
+
R
R
0
R
L
13
14
I
R
I
L
11
+
L
L
+
FIGURE 3. AC VOLTAGE GAIN AND IMPEDANCE MATCHING
V RX
------------ -
-
= R
R
I
L
12
12
= R
+
+
-
-
V
V
and R
13
C
D
90k
I
= R
R
R
-
+
+
-
RX
14
14
= 0 for this
= R
V
90k
.
C
V D
S
(EQ. 17)
(EQ. 18)
(EQ. 19)
=
D
R/20
R/2
– R
=
R
R
4
and
4R S I L
S
HC5517
I
V
L
OUT1
V
RING
RX
R
-------
R 9
R 8
-------
R 9
8
+
+
-
-
+
+R
-R
–
V
S
(-8R
The result is shown in Equation 20. Figure 4 is a schematic
representation of Equation 15.
To match the impedance of the SLIC to the impedance of the
load, set:
If R
------------ -
S
V RX
R
R
RX
FIGURE 4. SCHEMATIC REPRESENTATION OF EQUATION 20
1V
V
L
L
I
I
I
L
IN
L
L
P
9
=
=
S
is made to equal 8R
(R
=
8R
R
+
8
-
– R
8
2
S
8
/R
+
+
+
-
-
V
4R
R
------ -
R
9
IN
S
)) and the loop impedance (+4R
8
9
2V
V
S
R
------ -
R
BAT
2
4R
DC
+
+
V
-
8
9
TX
+
4R
-
S
+
†
†
I
S
L
4R
S
R
+
LOAD
†
9
S
R
R
GROUNDED FOR AC ANALYSIS
L
L
then:
I
OUT1
SLIC
-IN
=
1
4R
2R
S
I
8RS
L
+
-
R
R
R
8
4R
8
9
S
R 8
-------
R
S
+
+R
9
-
I
+
L
L
4R S
).
R
R
8
9
(EQ. 20)
(EQ. 21)
(EQ. 22)