HC55171_03 Datasheet, PDF (10/20 Pages) Intersil Corporation – 5 REN Ringing SLIC for ISDN Modem/TA and WLL

HC55171
Design Equations and Operational Theory
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 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 voltage and an internal V_BAT/2 reference, resulting in
V_BAT +4V at the output. (See Equation 3, Equation 4 and
Equation 5.)
V_TIPFEED
=
V_C
=
–2
V


R^----R--⁄^---2^{--}
=
–4V
V_RINGFEED
=
V_D
=
-^V----^B----^A----^T-
2


1
+
RR_{---}
–
V_TIPFE
E
D


RR_{---}
(EQ. 3)
(EQ. 4)
V_RINGFEED = V_D = V_BAT + 4
(EQ. 5)
TIP R_P1
R_S1
R
V_RX
R
R
OUT1
TIP FEED
R/20
-
+
R/2
-
+V^C
TRANSVERSAL
AMP
- ^TA
+
V_RING
+_-
INTERNAL
+2V REF
V_TX
90kΩ
R_IL1
R_IL2
90kΩ
GM
-
+
RF2
RING ^RP2 ^RS2
V_OUT1, V_RX
GROUNDED FOR
DC ANALYSIS
RING FEED
-
90kΩ
+
+
- ^VD
C_IL
+^{- 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 I_LOOP . The 200 term is actually formed
by the sum of twice the sense resistors and is shown in the
following equation.
200 × I_LOOP = (2 ⋅ R_S1 + 2 ⋅ R_S2) × I_LOOP
(EQ. 6)
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 600Ω 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 compares it to an internal reference level. When the
scaled voltage exceeds the internal reference level, the
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.
The maximum loop current is programed by resistors R_IL1 and
R_IL2 as shown in Equation 7 (Note: R_IL1 is typically 100kΩ).
I_LIMIT = -⁽--⁰---^.--⁶--₍--⁾₂--⁽--₀-^R--₀--^I--_x^L---_R¹-----+_I--_L---^R₂---₎-^I--^L---²----⁾
(EQ. 7)
0
V_{TIP FEED} = -4V
-5
-10
CONSTANT VOLTAGE
REGION
-15
-20
-25
0
V_{RING FEED} = -20V
CURRENT LIMIT
REGION I_LOOP = 25mA
250
500
750
∞
LOOP RESISTANCE (Ω)
FIGURE 6. V_T-R vs R_L (V_BAT = -24V, I_LIMIT = 25mA)
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 640Ω 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).
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