LUCL9500 AGERE [Agere Systems], LUCL9500 Datasheet - Page 21

• Manufacturer Part Number: LUCL9500
• Description: High-Voltage Ringing SLIC for VolP Applications
• Manufacturer: AGERE [Agere Systems]

September 2001
Applications
Power Control
Under normal device operating conditions, power dissi-
pation on the device must be controlled to prevent the
device temperature from rising above the thermal shut-
down and causing the device to shut down. Power dis-
sipation is highest with higher battery voltages, higher
current limit, and under shorter dc loop conditions.
Additionally, higher ambient temperature will also
reduce thermal margin.
To support required power ringing voltages, this device
is meant to operate with a high-voltage primary battery
(–65 V to –75 V typically). Thus, power control is nor-
mally achieved by use of the battery switch and an aux-
iliary lower absolute voltage battery. Operating
temperature range, maximum current limit, maximum
battery voltage, minimum dc loop length and protection
resistors values, airflow, and number of PC board lay-
ers will influence the overall thermal performance. The
following example illustrates typical thermal design
considerations.
The thermal resistance of the 28-pin PLCC package is
typically 35.5 C/W, which is representative of the natu-
ral airflow as seen in a typical switch cabinet with a
multilayer board.
The L9500 will enter thermal shutdown at a typical tem-
perature of 150 C. The thermal design should ensure
that the SLIC does not reach this temperature under
normal operating conditions.
For this example, assume a maximum ambient operat-
ing temperature of 85 C, a designed current limit of
30 mA, a maximum battery of –75 V, and an auxiliary
battery of –21 V. Assume a (worst-case) minimum dc
loop of 20
tors, and 200
the effects of parameter tolerance.
1. T
2. Allowed thermal rise = package thermal
Agere Systems Inc.
impedance
65 °C = 35.5°C/W
SLIC power dissipation (P
150°C – 85 °C = 65 °C.
TSD
– T
AMBIENT(max)
of wire resistance, 30
•
for the handset. Additionally, include
SLIC power dissipation.
•
= allowed thermal rise.
SLIC power dissipation
D
) = 1.83 W.
protection resis-
High-Voltage Ringing SLIC for VoIP Applications
Thus, if the total power dissipated in the SLIC is less
than 1.83 W, it will not enter the thermal shutdown
state. Total SLIC power is calculated as:
For the L9500A, the worst-case SLIC on-hook active
power is 64 mW. Thus,
The power dissipated in the SLIC is the total power dis-
sipation less the power that is dissipated in the loop.
Thus, under the worst-case normal operating condi-
tions of this example, the thermal design, using the
auxiliary, is adequate to ensure the device is not driven
into thermal shutdown under worst-case operating con-
ditions.
Total P
limit + SLIC quiescent power.
Total off-hook power = (I
tolerance) * (V
Total off-hook power = (0.030 A)(1.08) * (21) +
75 mW
Total off-hook power = 744.4 mW
SLIC P
Loop off-hook power = (I
min + 2R
Loop off-hook power = ((0.030 A)(1.08))
60
Loop off-hook power = 293.9 mW
SLIC off-hook power = Total off-hook power – loop
off-hook power
SLIC off-hook power = 744.4 mW – 293.9 mW
SLIC off-hook power = 450.5 mW < 1.83 W
+ 200
D
D
= maximum battery
= Total power – loop power
PROTECTION
)
BATAPPLIED
+ R
HANDSET
) + SLIC on-hook power
LOOP
LOOP
•
)(current-limit
maximum current
* 1.08)
)
2
•
2
(R
•
LOOP(dc)
(20
21
+