24
Lucent Technologies Inc.
Data Sheet
April 2000
L8560 Low-Power SLIC with Ringing
Applications
(continued)
dc Applications
(continued)
Adjusting Overhead Voltage and dc Feed
Resistance Simultaneously
The above paragraphs describe the independent set-
ting of the overhead voltage and the dc feed resis-
tance. If both need to be set to customized values,
combine the two circuits as shown in Figure 26.
12-2561.d (F)
Figure 26. Adjusting Both Overhead Voltage and dc
Feed Resistance
This is an equivalent circuit for adjusting both the dc
feed resistance and overhead voltage together.
The adjustments can be made by simple superposition
of the overhead and dc feed equations:
V
OH
= 7.1 + 40
R
dc
= 55 k
+ 705
Lower-value resistors can be used; the only disadvan-
tage is the power consumption of the external resistors.
Loop Range
The equation below can be rearranged to provide the
loop range for a required loop current:
Off-Hook Detection
The loop closure comparator has built-in longitudinal
rejection, eliminating the need for an external 60 Hz fil-
ter. This applies in both powerup and low-power scan
states. The loop closure detection threshold is set by
resistor R
LCTH
. Referring to Figure 27, NLC is high in an
on-hook condition (I
TR
= 0, V
DCOUT
= 0) and
V
LCTH
= 0.05 mA x R
LCTH
. The off-hook comparator
goes low when V
LCTH
crosses zero and then goes neg-
ative:
V
LCTH
=
0.05 mA x R
LCTH
+ V
DCOUT
=
0.05 x R
LCTH
– 0.04167 V/mA x I
TR
R
LTCH
(k
) =
0.833 I
TR
(mA)
Testing data shows that:
R
LTCH
(k
) =
0.899 I
TR
(mA)
12-2553.d (F)
Figure 27. Off-Hook Detection Circuit
Note that NLC is internally wired-OR with the output of
the ring trip detector (NRDET). The wired-OR, NSTAT,
is a package output pin.
Note that if NSTAT is used to directly control logic input
B2, connect a 0.01 μF capacitor from node LCTH to
ground for filtering purposes. In this mode of operation,
the L8560 will automatically switch to the lower-voltage
battery under off-hook conditions.
Also note that NSTAT will toggle low with a ring ground
in the ground start application. Under a ring ground,
one-half of the current appears as differential. This total
ring ground current is approximately two times the cur-
rent limit; thus, the differential current is approximately
equal to the current limit, which typically exceeds the
loop closure threshold. Thus, in the ground start appli-
cation, if RGDET trips, NSTAT will also trip. Under this
condition, via software, ignore the NSTAT transition.
DCR
R
1
R
3
DCOUT
R
2
–5 V
R
2
+
R
1
R
3
||
----------------------------------
+
R
2
R
1
---------------------
R
L
----------------------------
I
L
2R
P
–
R
dc
–
=
R
L
ITR
R
P
R
P
RING
–
+
–
+
DCOUT
R
LCTH
LCTH
NLC
TIP
–0.04167 V/mA
0.05 mA