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HC55171 Datasheet, PDF (8/18 Pages) Intersil Corporation – 5 REN Ringing SLIC for ISDN Modem/TA and WLL | |||
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HC55171
DC Operation of Tip and Ring Ampliï¬ers
SLIC in the Active Mode
The tip and ring ampliï¬ers 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 ampliï¬ers. 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 VBAT/2 reference, resulting in VBAT +4V
at the output. (See Equation 3, Equation 4 and Equation 5.)
VTIPFEED = VC = â2Vï£ï£«R----R--â---2-- = â4V
VRINGFEED
=
VD
=
V-----B-2---A----T-

ï£
1
+
RR---
â
VTIPFE
E
D

ï£
RR---
(EQ. 3)
(EQ. 4)
VRINGFEED = VD = VBAT + 4
(EQ. 5)
TIP RP1
RS1
R
VRX
R
R
OUT1
TIP FEED
R/20
-
+
R/2
-
+ VC
TRANSVERSAL
AMP
- TA
+
VRING
+-
INTERNAL
+2V REF
VTX
90kâ¦
RIL1
RIL2
90kâ¦
GM
-
+
RF2
RING RP2 RS2
VOUT1, VRX
GROUNDED FOR
DC ANALYSIS
RING FEED
-
90kâ¦
+
+
- VD
CIL
+- VBAT
2
FIGURE 5. OPERATION OF THE TIP AND RING AMPLIFIERS
Transmit Output Voltage
The transmit output voltage in terms of loop current is
expressed as 200x ILOOP. The 200 term is actually formed
by the sum of twice the sense resistors and is shown in the
following equation.
200 Ã ILOOP = (2 â
RS1 + 2 â
RS2) Ã ILOOP
(EQ. 6)
This is a relationship that is critical when modifying the
sense resistor (RS1, RS2). 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 modiï¬es the ring feed voltage (VD) as a function of the
loop current. The output of the Transversal Ampliï¬er (TA) has
a DC voltage that is directly proportional to the loop current.
This voltage is scaled by RIL1 and RIL2. The scaled voltage
is the input to a transconductance ampliï¬er (GM) that com-
pares it to an internal reference level. When the scaled volt-
age exceeds the internal reference level, the
transconductance ampliï¬er sources current. This current
charges CIL in the positive direction causing the ring feed
voltage (VD) to approach the tip feed voltage (VC). This
effectively reduces the tip feed to ring feed voltage (VT-R).
and holds the maximum loop current constant.
The maximum loop current is programed by resistors RIL1
and RIL2 as shown in Equation 7 (Note: RIL1 is typically
100kâ¦).
ILIMIT = (---0---.--6--(--)2--(--0-R--0--I--xL---R1-----+I--L---R2---)-I--L----2---)
(EQ. 7)
0
VTIP FEED = -4V
-5
-10
CONSTANT VOLTAGE
REGION
-15
-20
-25
0
VRING FEED = -20V
CURRENT LIMIT
REGION ILOOP = 25mA
250
500
750
â
LOOP RESISTANCE (â¦)
FIGURE 6. VT-R vs RL (VBAT = -24V, ILIMIT = 25mA)
Figure 6 illustrates the relationship between VT-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
inï¬nity to about 640⦠the loop current (obeying Ohmâs 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).
69
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