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Data Sheet

AD9523

Rev. D | Page 41 of 60

POWER DISSIPATION AND THERMAL CONSIDERATIONS

The AD9523 is a multifunctional, high speed device that targets

a wide variety of clock applications. The numerous innovative

features contained in the device each consume incremental power.

If all outputs are enabled in the maximum frequency and mode

that have the highest power, the safe thermal operating conditions

of the device may be exceeded. Careful analysis and consideration

of power dissipation and thermal management are critical

elements in the proper application of the AD9523 device.

The AD9523 device is specified to operate within the industrial

temperature range of –40°C to +85°C. This specification is

conditional, however, such that the absolute maximum junction

temperature is not exceeded (as specified in Table 17). At

high operating temperatures, extreme care must be taken when

operating the device to avoid exceeding the junction temperature

and potentially damaging the device.

A maximum junction temperature is listed in Table 1 with the

ambient operating range. The ambient range and maximum

junction temperature specifications ensure the performance of

the device, as guaranteed in the Specifications section.

Many variables contribute to the operating junction temperature

within the device, including

•

Selected driver mode of operation

•

Output clock speed

•

Supply voltage

•

Ambient temperature

The combination of these variables determines the junction

temperature within the AD9523 device for a given set of

operating conditions.

The AD9523 is specified for an ambient temperature (TA). To

ensure that TA is not exceeded, an airflow source can be used.

Use the following equation to determine the junction

temperature on the application PCB:

= TCASE + (ΨJT × PD)

where:

is the junction temperature (°C).

TCASE

is the case temperature (°C) measured by the user at the

top center of the package.

ΨJT is the value from Table 18.

PD is the power dissipation of the AD9523.

Values of θJA are provided for package comparison and PCB

design considerations. θJA can be used for a first-order

approximation of TJ by the equation

= TA + (θJA × PD)

where TA is the ambient temperature (°C).

Values of θJC are provided for package comparison and PCB

design considerations when an external heat sink is required.

Values of ΨJB are provided for package comparison and PCB

design considerations.

CLOCK SPEED AND DRIVER MODE

Clock speed directly and linearly influences the total power

dissipation of the device and, therefore, the junction temperature.

Two operating frequencies are listed under the incremental power

dissipation parameter in Table 3. Using linear interpretation is

a sufficient approximation for frequencies not listed in the table.

When calculating power dissipation for thermal consideration,

the amount of power dissipated in the 100 Ω resistor must be

removed. If using the data in Table 2, this power is already

removed. If using the current vs. frequency graphs provided in

the Typical Performance Characteristics section, the power into

the load must be subtracted, using the following equation:

Ω

100

Swing

Voltage

Output

Differenti

EVALUATION OF OPERATING CONDITIONS

The first step in evaluating the operating conditions is to

determine the maximum power consumption (PD) internal

to the AD9523. The maximum PD excludes power dissipated

in the load resistors of the drivers because such power is external

to the device. Use the power dissipation specifications listed in

Table 3 to calculate the total power dissipated for the desired

configuration. The base typical configuration parameter in

Table 3 lists a power of 428 mW, which includes one LVPECL

output at 122.88 MHz. If the frequency of operation is not listed

in Table 3, see the Typical Performance Characteristics section,

current vs. frequency and driver mode to calculate the power

dissipation; then add 20% for maximum current draw. Remove

the power dissipated in the load resistor to achieve the most

accurate power dissipation internal to the AD9523. See Table 30

for a summary of the incremental power dissipation from the base

power configuration for two different examples.

Table 30. Temperature Gradient Examples

Description

Mode

Frequency

(MHz)

Maximum

Power (mW)

Example 1

Base Typical

Configuration

428

Output Driver

6 × LVPECL

122.88

330

Output Driver

6 × LVDS

245.76

110

Total Power

868

Example 2

Base Typical

Configuration

428

Output Driver

13 × LVPECL

983.04

2066

Total Power

2500

The second step is to multiply the power dissipated by the thermal

impedance to determine the maximum power gradient. For this

example, a thermal impedance of θJA = 20.1°C/W was used.

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