ISL5585_04 Datasheet, PDF (10/24 Pages) Intersil Corporation – 3.3V Ringing SLIC Family for Voice Over Broadband (VOB)

ISL5585
detection of ring trip or programmed off hook loop current,
whichever is greater.
To allow for proper ring trip operation, the transient current
limit setting should be set at least 25% higher than the peak
ring trip current setting. Setting the transient current 25%
higher should account for programming tolerances of both
the ring trip threshold and the transient current limit.
If loop current is larger than ring trip current (low REN
applications) then the transient current limit should be set at
least 35% higher than the loop current setting. The slightly
higher offset accounts for the slope of the loop current limit
function.
Attention to detail should be exercised when programming
the transient current limit setting. If ring trip detect does not
occur while ringing, then re-examine the transient current
limit and ring trip threshold settings.
DC Loop Feed
The feedback mechanism for monitoring the DC portion of
the loop current is the loop detector. A low pass filter is used
in the feedback to block voice band signals from interfering
with the loop current limit function. The pole of the low pass
filter is set by the external capacitor C_DC. The value of the
external capacitor should be 4.7µF, 6.3V rated polarized or
non-polarized capacitor.
Most applications will operate the device from low battery
while off hook. The DC feed characteristic of the device will
drive Tip and Ring towards half battery to regulate the DC
loop current. For light loads or Long Loops, Tip will be near
-4V and Ring will be near V_VBL + 5V. Figure 2 shows the DC
feed characteristic in terms of tip to ring voltage and loop
current.
V_TR(OC)
LONG LOOP
SHORT LOOP
m = (∆V_TR/∆I_L) = 11.1kΩ
I_LOOP (mA)
I_LIM
FIGURE 2. DC FEED CHARACTERISTIC
The point on the y-axis labeled V_TR(OC) is the open circuit
Tip to Ring voltage and is defined by the feed battery voltage
in Equation 6.
V_TR(OC) = V_BL –9
(EQ. 6)
Figure 3 illustrates the actual loop current for a given set of
loop conditions. The loop conditions are determined by the
low battery voltage and the DC loop impedance. The DC
loop impedance is the sum of the protection resistance,
copper resistance (ohms/foot) and the telephone off hook
DC resistance.
I_SC
I_LIM
SHORT LOOP _IA
m=11.11k
LONG LOOP
I_B
m=Vtr(oc)/Rloop
CONSTANT VOLTAGE
CONSTANT CURRENT
OR
RESISTIVE FEED
2R_{P RLOOP (Ω)}
R_KNEE
FIGURE 3. I_LOOP vs. R_LOOP LOAD CHARACTERISTIC
The slope of the feed characteristic and the battery voltage
define the maximum loop current on the shortest possible
loop as the short circuit current I_SC.
I_SC
=
I_LIM
+
-^V----^T---^R-----⁽--^O----^C----⁾----^–----²----^R----^P----^I--^L----^I-^M---
1 1.11 k
(EQ. 7)
The term I_LIM is the programmed current limit, 1760/R_IL. The
line segment I_A represents the constant current region of the
loop current limit function.
I_A=
I_L
IM
+
-^V----^T---^R-----⁽--^O----^C----⁾----^–----^R-----^L---^O----^O-----^P----^I--^L---^I--^M---
11.11 k
(EQ. 8)
Process variations in the ISL5585 effect the I_LIM and
11.11kΩ slope in Equation 8. All units are tested with: a
300Ω load across tip and ring, V_BAT=-24V and I_LIM set to
25mA. Equation 8 can be used to predict the ideal current at
this setting (25.76mA). All units are tested to be within
±8.5% of this ideal value (23.57mA to 27.95mA).
The maximum loop impedance for a programmed loop
current is defined as R_KNEE.
R_KNEE
=
V_{-----T---R-----(--O----C----)}
I_LIM
(EQ. 9)
When R_KNEE is exceeded, the device will transition from
constant current feed to constant voltage or resistive feed.
The line segment I_B represents the resistive feed portion of
the load characteristic
I_B
=
V_{-----T---R-----(--O----C----)}
R_LOOP
(EQ. 10)
Impedance Matching
The impedance of the device is programmed with the
external component R_S. R_S is the gain setting resistor for
the Transmit Amplifier that provides impedance matching. If
complex impedance matching is required, then a complex
network can be substituted for R_S.
The feedback mechanism for monitoring the AC portion of
the loop current consists of two amplifiers, the Sense
Amplifier (SA) and the Transmit Amplifier (TA). The AC
feedback signal is used for impedance synthesis. A detailed
model of the AC feed back loop is shown in Figure 4.
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