Datasheet

Data Sheet AD9524
Rev. D | 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 (T
A
). To
ensure that T
A
is not exceeded, an airflow source can be used.
Use the following equation to determine the junction
temperature on the application PCB:
T
J
= T
CASE
+ (Ψ
JT
× PD)
where:
T
J
is the junction temperature (°C).
T
CASE
is the case temperature (°C) measured by the user at the
top center of the package.
Ψ
JT
is the value from Table 18.
PD is the power dissipation of the AD9524.
Values of θ
JA
are provided for package comparison and PCB
design considerations. θ
JA
can be used for a first-order
approximation of T
J
by the equation
T
J
= T
A
+ (θ
JA
× PD)
where T
A
is the ambient temperature (°C).
Values of θ
JC
are provided for package comparison and PCB design
considerations when an external heat sink is required.
Values of Ψ
JB
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:
Ω100
2
SwingVoltageOutputalDifferenti
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