LUCL9500AGF-DT datasheet(21/32 Pages) AGERE

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Preliminary Data Sheet

September 2001

High-Voltage Ringing SLIC for VoIP Applications

L9500A

Agere Systems Inc.

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

Ω of wire resistance, 30 Ω protection resis-

tors, and 200

Ω for the handset. Additionally, include

the effects of parameter tolerance.

1. TTSD – TAMBIENT(max) = allowed thermal rise.

150°C – 85 °C = 65 °C.

2. Allowed thermal rise = package thermal

impedance

• SLIC power dissipation.

65 °C = 35.5°C/W

• SLIC power dissipation

SLIC power dissipation (PD) = 1.83 W.

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:

Total PD = maximum battery

• maximum current

limit + SLIC quiescent power.

For the L9500A, the worst-case SLIC on-hook active

power is 64 mW. Thus,

Total off-hook power = (ILOOP)(current-limit

tolerance) * (VBATAPPLIED) + SLIC on-hook power

Total off-hook power = (0.030 A)(1.08) * (21) +

75 mW

Total off-hook power = 744.4 mW

The power dissipated in the SLIC is the total power dis-

sipation less the power that is dissipated in the loop.

SLIC PD = Total power – loop power

Loop off-hook power = (ILOOP * 1.08)2

• (RLOOP(dc)

min + 2RPROTECTION + RHANDSET)

Loop off-hook power = ((0.030 A)(1.08))2

• (20

Ω +

Ω + 200 Ω)

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

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.

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