HC5517 Intersil Corporation, HC5517 Datasheet - Page 6

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

The HC5517 is a voltage feed current sense Subscriber Line
Interface Circuit (SLIC). This means that for long loop appli-
cations the SLIC provides a constant voltage to the tip and
ring terminals while sensing the tip to ring current. For short
loops, where the loop current limit is exceeded, the tip to ring
voltage decreases as a function of loop resistance.
The following discussion separates the SLIC’s operation into
its DC and AC path, then follows up with additional circuit
design and application information.
DC Operation of Tip and Ring Amplifiers
SLIC in the Active Mode
The tip and ring amplifiers are voltage feedback op amps
that are connected to generate a differential output (e.g. if tip
sources 20mA then ring sinks 20mA). Figure 1 shows the
connection of the tip and ring amplifiers. The tip DC voltage
is set by an internal +2V reference, resulting in -4V at the
output. The ring DC voltage is set by the tip DC output volt-
age and an internal V
at the output. (See Equation 1, Equation 2 and Equation 3.)
V
V
V
FIGURE 1. OPERATION OF THE TIP AND RING AMPLIFIERS
TIPFEED
RINGFEED
RINGFEED
RING
V
GROUNDED FOR
DC ANALYSIS
TIP
OUT1
, V
R
11
R
RX
12
=
=
=
V
C
V
V
R
R
D
D
13
=
14
=
=
–
2V
Circuit Operation and Design Information
V
-------------- - 1
V
BAT
BAT
BAT
2
---------- -
R 2
RING FEED
TIP FEED
+
-
+
R
-
/2 reference, resulting in V
V
V
+
C
D
90k
4
65
+
TRANSVERSAL
AMP
R
+
=
-
-
+
R
--- -
R
+
-
–
4V
–
TA
V
90k
C
TIPFEED
16
R/20
R/2
R
R
90k
R
R
10
28
V
OUT1
V
R
--- -
R
RING
+
-
RX
RF2
+
INTERNAL
+2V REF
-
+
-
V
BAT
BAT
2
GM
V
(EQ. 1)
(EQ. 2)
(EQ. 3)
TX
+4V
HC5517
Current Limit
The tip feed to ring feed voltage (Equation 1 minus
Equation 3) is equal to the battery voltage minus 8V. Thus,
with a 48 (24) volt battery and a 600
including the feed resistors, the loop current is 66.6mA
(26.6mA). On short loops the line resistance often
approaches zero and the need exists to control the
maximum DC loop current.
Current limiting is achieved by a feedback network (Figure 1)
that modifies the ring feed voltage (V
loop current. The output of the Transversal Amplifier (TA) has
a DC voltage that is directly proportional to the loop current.
This voltage is scaled by R
the input to a transconductance amplifier (GM) that
compares it to an internal reference level. When the scaled
voltage
transconductance amplifier sources current. This current
charges C
voltage (V
effectively reduces the tip feed to ring feed voltage (V
and holds the maximum loop current constant.
The maximum loop current is programed by resistors R
and R
100k ).
Figure 2 illustrates the relationship between V
loop resistance. The conditions are shown for a battery volt-
age of -24V and the loop current limit set to 25mA. For a infi-
nite loop resistance both tip feed and ring feed are at -4V
and -20V respectively. When the loop resistance decreases
from infinity to about 640
Law) increases from 0mA to the set loop current limit. As the
loop resistance continues to decrease, the ring feed voltage
approaches the tip feed voltage as a function of the pro-
gramed loop current limit (Equation 4).
I
LIMIT
-10
-15
-20
-25
-5
0
28
FIGURE 2. V
=
0
-------------------------------------------- -
exceeds
as shown in Equation 4 (Note: R
0.6 R
REGION I
16
D
V
) to approach the tip feed voltage (V
200xR
TIP FEED
CURRENT LIMIT
in the positive direction causing the ring feed
10
LOOP
+
250
28
T-R
R
= -4V
the
28
vs R
= 25mA
LOOP RESISTANCE ( )
10
L
internal
the loop current (obeying Ohm’s
(V
and R
BAT
500
= -24V,
28
reference
D
. The scaled voltage is
) as a function of the
CONSTANT VOLTAGE
V
I
LIMIT
RING FEED
loop resistance,
750
REGION
10
= 25mA
T-R
level,
is typically
= -20V
C
and the
)
). This
(EQ. 4)
T-R
the
10
),