Lucent Technologies Inc.
25
Data Sheet
April 2000
L8560 Low-Power SLIC with Ringing
Applications
(continued)
dc Applications
(continued)
Ring Ground Detection
Pin ICM sinks a current proportional to the longitudinal
loop current. It is also connected to an internal compar-
ator whose output is pin RGDET. In a ground start
application where tip is open, the ring ground current is
half differential and half common mode. In this case, to
set the ring ground current threshold, connect a resis-
tor R
ICM
from pin ICM to V
CC
. Select the resistor
according to the following relation:
R
ICM
(k
) =
The above equation is shown graphically in Figure 17.
It applies for the case of tip open. The more general
equation can be used in ground key applications to
detect a common-mode current ICM:
R
ICM
(k
) =
Longitudinal Balance
The SLICs are graded with different codes to represent
different longitudinal balance specifications. The num-
bers are guaranteed by testing (Figures 5 and 8). How-
ever, for specific applications, the longitudinal balance
may also be determined by termination impedance,
protection resistance, and especially by the mismatch
between protection resistors at tip and ring. This can
be illustrated by:
LB = 20 x log
where:
LB: longitudinal balance
RP: protection resistor value in
ZT: magnitude of the termination impedance in
: protection resistor mismatch in
: SLIC internal tip/ring sensing mismatch
The
can be calculated using the above equation with
these exceptions:
=
0, ZT = 600
,
RP = 100
, and
the longitudinal balance specification on a specific
code.
Now with
available, the equation will predict the
actual longitudinal balance for RP, ZT, and .
Be aware that ZT may vary with frequency for complex
impedance applications.
Power Derating
Thermal considerations can affect the choice of a
32-pin PLCC or a 44-pin PLCC package. Operating
temperature range, maximum current limit, maximum
battery voltage, minimum dc loop, and protection resis-
tor values will influence the overall thermal perfor-
mance. This section shows the relevant design
equations and considerations in evaluating the SLIC
thermal performance.
First, consider the L8560 SLIC in a 44-pin PLCC pack-
age. The still-air thermal resistance is 47 °C/W; how-
ever, this number implies zero airflow as if the L8560
were totally enclosed in a box. A more realistic number
would be 43 °C/W. This is an experimental number that
represents a thermal impedance with no forced airflow
(i.e., from a muffin fan) but from the natural airflow as
seen in a typical switch cabinet.
The SLIC will enter the thermal shutdown state at typi-
cally 165 °C. The thermal shutdown design should
ensure that the SLIC temperature does not reach
165 °C under normal operating conditions.
Assume a maximum ambient operating temperature of
85 °C, a maximum current limit of 45 mA, and a maxi-
mum battery of –52 V. Further, assume a (worst case)
minimum dc loop of 100
and that 100
protection
resistors are used at both tip and ring.
1. T
TSD
– T
AMBIENT(max)
= allowed thermal rise.
165 °C – 85 °C = 80 °C
2. Allowed thermal rise = package thermal
impedance
SLIC power dissipation.
80 °C = 43 °C/W
SLIC power dissipation
SLIC power dissipation (P
D
) = 1.9 W
Thus, if the total power dissipated in the SLIC is less
than 1.9 W, it will not enter the thermal shutdown state.
Total SLIC power is calculated as:
Total P
D
= maximum battery
maximum
current limit + SLIC quiescent power.
For the L8560, SLIC quiescent power (P
Q
) is approxi-
mated at 0.167 W. Thus,
Total P
D
= (–52 V
45 mA) + 0.167 W
Total P
D
= 2.34 W + 0.167 W
Total P
D
= 2.507 W
I
RG
mA
(
)
---------------------
ICM mA
)
----------------------
368
2
×
(
ZT 2
–
×
RP
ε
×
[
]
+
)
×
------------------------------------------------------------------------------------------
ε
ε