參數(shù)資料
型號(hào)： HC55171CB
廠(chǎng)商： INTERSIL CORP
元件分類(lèi)： 模擬傳輸電路
英文描述： 5 REN Ringing SLIC for ISDN Modem/TA and WLL
中文描述： TELECOM-SLIC, PDSO28
封裝： SOIC-28
文件頁(yè)數(shù)： 8/18頁(yè)
文件大?。?/td> 153K
代理商： HC55171CB
69
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
BAT
/2 reference, resulting in V
BAT
+4V
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
LOOP
. The 200 term is actually formed
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
S1
, R
S2
). The 200 term factors into the loop
current limit and loop detector functions of the SLIC.
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
loop resistance,
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
D
) as a function of the
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
IL1
and R
IL2
. The scaled voltage
is the input to a transconductance amplifier (GM) that com-
pares it to an internal reference level. When the scaled volt-
age
exceeds
the
internal
transconductance amplifier sources current. This current
charges C
IL
in the positive direction causing the ring feed
voltage (V
D
) to approach the tip feed voltage (V
C
). This
effectively reduces the tip feed to ring feed voltage (V
T-R
).
and holds the maximum loop current constant.
reference
level,
the
The maximum loop current is programed by resistors R
IL1
and R
IL2
as shown in Equation 7 (Note: R
IL1
is typically
100k
).
Figure 6 illustrates the relationship between V
T-R
and the
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
the loop current (obeying Ohm’s 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
programmed loop current limit (Equation 7).
V
TIPFEED
V
C
2V
–
-----------
4V
–
=
=
=
(EQ. 3)
V
RINGFEED
V
D
V
--------------
1
R
R
---
+
V
TIPFEED
–
=
=
(EQ. 4)
V
RINGFEED
V
D
V
BAT
4
+
=
=
(EQ. 5)
FIGURE 5. OPERATION OF THE TIP AND RING AMPLIFIERS
V
RX
OUT1
V
RING
V
BAT
2
-
+
+
-
V
C
V
D
R
S2
R
P2
R
P1
R
S1
90k
90k
R
R/20
R
R/2
TIP FEED
RING FEED
V
OUT1
, V
RX
GROUNDED FOR
DC ANALYSIS
TIP
RING
TRANSVERSAL
AMP
+
-
V
TX
R
IL1
R
IL2
RF2
GM
C
IL
90k
INTERNAL
+2V REF
+
-
+
-
R
TA
+
-
+
-
+
-
200
I
LOOP
×
2 R
S1
2 R
S2
+
(
)
I
LOOP
×
=
(EQ. 6)
I
LIMIT
0.6
-------------------------------------------------
(
)
R
IL2
R
+
(
)
=
(EQ. 7)
0
-25
-20
-15
-10
-5
0
LOOP RESISTANCE (
)
CONSTANT VOLTAGE
REGION
T
∞
FIGURE 6. V
T-R
vs R
L
(V
BAT
= -24V,
I
LIMIT
= 25mA
)
250
500
750
V
RING FEED
= -20V
V
TIP FEED
= -4V
CURRENT LIMIT
REGION I
LOOP
= 25mA
HC55171
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