HC5517_00 Datasheet, PDF (6/19 Pages) Intersil Corporation – 3 REN Ringing SLIC For ISDN Modem/TA and WLL

HC5517
Circuit Operation and Design Information
The HC5517 is a voltage feed current sense Subscriber Line
Interface Circuit (SLIC). This means that for long loop
applications 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 voltage and an
internal V_BAT/2 reference, resulting in V_BAT +4V at the
output. (See Equation 1, Equation 2 and Equation 3.)
V_TIPFEED
=
V_C
=
–2
V


R^----R--⁄^---2^{--}
=
–4V
(EQ. 1)
V_RINGFEED = V_D = ^V-----^B--₂--^A----^T- _1 + ^RR---_ – V_{TIPF EED}^RR---_
(EQ. 2)
V_RINGFEED = V_D = V_BAT + 4
(EQ. 3)
TIP R₁₁
R₁₃
R
V_RX
R
R
OUT1
TIP FEED
R/20
-
+
R/2
-
+ ^VC
TRANSVERSAL
AMP
- ^TA
+
V_RING
+ INTERNAL
- _{+2V REF}
V_TX
90kΩ
R₁₀
R₂₈
90kΩ
GM
-
+
RF2
RING ^R12 ^R14
V_OUT1, V_RX
GROUNDED FOR
DC ANALYSIS
RING FEED
-
+
+
- ^VD
90kΩ
C₁₆
+^{- VBAT}
2
FIGURE 1. OPERATION OF THE TIP AND RING AMPLIFIERS
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 600Ω loop resistance,
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_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 R₁₀ and R₂₈. 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 C₁₆ 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 R₁₀ and
R₂₈ as shown in Equation 4 (Note: R₁₀ is typically 100kΩ).
I_LIMIT = ⁽---⁰---^.--⁶-₍--₂-⁾--⁽-₀--^R-₀---¹-_x--⁰-_R----+-₂---₈^R---₎--²---⁸----⁾
(EQ. 4)
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 2. V_T-R vs R_L (V_BAT = -24V, I_LIMIT = 25mA)
Figure 2 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 a
infinite loop resistance both 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
programed loop current limit (Equation 4).
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