HC55171CM INTERSIL [Intersil Corporation], HC55171CM Datasheet - Page 8

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

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 5 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 3, Equation 4 and Equation 5.)
Transmit Output Voltage
The transmit output voltage in terms of loop current is
expressed as 200x I
by the sum of twice the sense resistors and is shown in the
following equation.
This is a relationship that is critical when modifying the
sense resistor (R
current limit and loop detector functions of the SLIC.
V
V
V
200 I
FIGURE 5. OPERATION OF THE TIP AND RING AMPLIFIERS
TIPFEED
RINGFEED
RINGFEED
RING
V
GROUNDED FOR
DC ANALYSIS
TIP
OUT1
LOOP
, V
R
P1
R
RX
P2
=
=
=
=
V
C
V
V
R
R
2 R
D
D
S1
=
S1
S2
=
=
–
, R
S1
2V
LOOP
V
-------------- - 1
V
BAT
S2
BAT
BAT
+
2
---------- -
R 2
RING FEED
TIP FEED
2 R
+
). The 200 term factors into the loop
-
+
R
-
/2 reference, resulting in V
V
V
. The 200 term is actually formed
+
C
D
90k
4
S2
69
+
TRANSVERSAL
AMP
R
+
=
-
-
+
R
--- -
R
+
-
–
4V
–
TA
I
LOOP
V
90k
C
TIPFEED
IL
R/20
R/2
R
R
90k
R
R
IL1
IL2
V
OUT1
V
R
--- -
R
RING
+
-
RX
RF2
+
INTERNAL
+2V REF
-
+
-
V
BAT
BAT
2
GM
V
(EQ. 3)
(EQ. 4)
(EQ. 5)
(EQ. 6)
TX
+4V
HC55171
Current Limit
The tip feed to ring feed voltage (Equation 3 minus
Equation 5) 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 would be
66.6mA (26.6mA). On short loops the line resistance often
approaches zero and there is a need to control the maximum
DC loop current.
Current limiting is achieved by a feedback network (Figure 5)
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
is the input to a transconductance amplifier (GM) that com-
pares it to an internal reference level. When the scaled volt-
age
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 6 illustrates the relationship between V
loop resistance. The conditions are shown for a battery
voltage of -24V and the loop current limit set to 25mA. For an
open circuit loop the tip feed and ring feed are at -4V and
-20V respectively. When the loop resistance decreases from
infinity to about 640
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
programmed loop current limit (Equation 7).
I
LIMIT
-10
-15
-20
-25
-5
0
IL2
FIGURE 6. V
exceeds
=
0
------------------------------------------------- -
0.6 R
REGION I
as shown in Equation 7 (Note: R
IL
D
V
) to approach the tip feed voltage (V
TIP FEED
CURRENT LIMIT
in the positive direction causing the ring feed
200xR
IL1
LOOP
the
250
T-R
+
IL2
= -4V
R
the loop current (obeying Ohm’s Law)
vs R
IL2
= 25mA
LOOP RESISTANCE ( )
internal
L
IL1
(V
BAT
and R
500
= -24V,
reference
IL2
D
) as a function of the
CONSTANT VOLTAGE
. The scaled voltage
V
I
LIMIT
RING FEED
loop resistance,
750
REGION
IL1
= 25mA
T-R
level,
is typically
= -20V
C
and the
)
). This
(EQ. 7)
T-R
the
IL1
).