AD9523-1SLASHPCBZ datasheet(38/60 Pages) AD

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AD9523-1

Rev. B | Page 38 of 60

POWER DISSIPATION AND THERMAL CONSIDERATIONS

CLOCK SPEED AND DRIVER MODE

The AD9523-1 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 AD9523-1 device.

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:

The AD9523-1 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 16). At high operating temperatures, extreme care must

be taken when operating the device to avoid exceeding the

junction temperature and potentially damaging the device.

100

Swing

Voltage

Output

Differenti

EVALUATION OF OPERATING CONDITIONS

Many variables contribute to the operating junction temperature

within the device, including

The first step in evaluating the operating conditions is to

determine the maximum power consumption (PD) internal

to the AD9523-1. 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 maximum power of 434.7 mW, which includes

one LVPECL output at 122.88 MHz. For one LVDS output that

is operating at 122.88 MHz, the power is 35 mW; for operation

at 983.04 MHz, the power is 106 mW. Using linear interpolation,

the power for operation at 245.76 MHz is 45 mW. Table 29

summarizes the incremental power dissipation from the base

power configuration for two different examples.

Table 29. Temperature Gradient Examples

Description

Mode

•

Selected driver mode of operation

•

Output clock speed

•

Supply voltage

•

Ambient temperature

The combination of these variables determines the junction

temperature within the AD9523-1 device for a given set of

operating conditions.

The AD9523-1 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)

Frequency

(MHz)

Maximum

Power (mW)

Example 1

Base Typical

Configuration

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 17.

PD is the power dissipation of the AD9523-1.

434.7

Output Driver

6 × LVPECL

122.88

306

Output Driver

3 × LVDS

61.44

Output Driver

3 × LVDS

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

245.76

135

Total Power

966

Example 2

Base Typical

Configuration

TJ = TA + (θJA × PD)

434.7

Output Driver

13 × LVPECL

where TA is the ambient temperature (°C).

983.04

2066

Total Power

Values of θJC are provided for package comparison and PCB

design considerations when an external heat sink is required.

2500

Values of ΨJB are provided for package comparison and PCB

design considerations.

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