AD9524SLASHPCBZ datasheet(37/56 Pages) AD

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Data Sheet

AD9524

Rev. E | Page 37 of 56

POWER DISSIPATION AND THERMAL CONSIDERATIONS

The AD9524 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 successful application of the AD9524 device.

The AD9524 device is specified to operate within the industrial

ambient 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 AD9524 device for a given set of

operating conditions.

The AD9524 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:

TJ = TCASE + (ΨJT × PD)

where:

TJ 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 AD9524.

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

TJ = 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 frequency not listed in the table.

When calculating power dissipation for thermal consideration,

the amount of power dissipated in the 100 Ω resistor should 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 AD9524.

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 AD9524. 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

5 × LVPECL

122.88

275

Total Power

703

Example 2

Base Typical

Configuration

428

Output Driver

5 × LVPECL

983.04

795

Total Power

1223

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