AD9852ASQZ datasheet(38/52 Pages) AD

38 / 52 page

AD9852

Rev. E | Page 38 of 52

POWER DISSIPATION AND THERMAL CONSIDERATIONS

The AD9852 is a multifunctional, high speed device that targets

a wide variety of synthesizer and agile clock applications. The

numerous innovative features contained in the device each

consume incremental power. If enabled in combination, the safe

thermal operating conditions of the device may be exceeded.

Careful analysis and consideration of power dissipation and

thermal management is a critical element in the successful

application of the AD9852 device.

The AD9852 device is specified to operate within the industrial

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

ditional, however, such that the absolute maximum junction

temperature of 150°C is not exceeded. At high operating tem-

peratures, extreme care must be taken when operating the device

to avoid exceeding the junction temperature and potentially

damaging the device.

Many variables contribute to the operating junction temperature

within the device, including

•

Package style

•

Selected mode of operation

•

Internal system clock speed

•

Supply voltage

•

Ambient temperature

The combination of these variables determines the junction

temperature within the AD9852 device for a given set of

operating conditions.

The AD9852 device is available in two package styles: a

thermally enhanced surface-mount package with an exposed

heat sink and a standard (nonthermally enhanced) surface-

mount package. The thermal impedance of these packages is

16°C/W and 38°C/W, respectively, measured under still air

conditions.

THERMAL IMPEDANCE

The thermal impedance of a package can be thought of as a

thermal resistor that exists between the semiconductor surface

and the ambient air. The thermal impedance is determined by

the package material and the physical dimensions of the package.

The dissipation of the heat from the package is directly depen-

dent on the ambient air conditions and the physical connection

made between the IC package and the PCB.

Adequate dissipation of power from the AD9852 relies on all

power and ground pins of the device being soldered directly to a

copper plane on a PCB. In addition, the thermally enhanced

package of the AD9852ASVZ has an exposed paddle on the

bottom that must be soldered to a large copper plane, which,

for convenience, can be the ground plane. Sockets for either

package style of the AD9852 device are not recommended.

JUNCTION TEMPERATURE CONSIDERATIONS

The power dissipation (PDISS) of the AD9852 device in a given

application is determined by many operating conditions. Some

of the conditions have a direct relationship with PDISS, such as

supply voltage and clock speed, but others are less deterministic.

The total power dissipation within the device and its effect on

the junction temperature must be considered when using the

device. The junction temperature of the device is given by

Junction Temperature = (Thermal Impedance ×

Power Consumption) + Ambient Temperature

The maximum ambient temperature combined with the

maximum junction temperature establish the following power

consumption limits for each package: 4.06 W for ASVZ models

and 1.71 W for ASTZ models.

Supply Voltage

Because PDISS = V × I, the supply voltage affects power dissipation

and junction temperature. Users should design for 3.3 V nominally;

however, the device is guaranteed to meet specifications over the

full temperature range and over the supply voltage range of

3.135 V to 3.465 V.

Clock Speed

Clock speed directly and linearly influences the total power

dissipation of the device and therefore the junction temperature.

As a rule, the user should select the lowest internal clock speed

possible to support a given application to minimize power

dissipation. Typically, the usable frequency output bandwidth

from a DDS is limited to 40% of the clock rate to ensure that the

requirements on the output low-pass filter are reasonable. For a

typical DDS application, the system clock frequency should be

2.5 times the highest desired output frequency.

Mode of Operation

The selected mode of operation for the AD9852 significantly

influences the total power consumption. The AD9852 offers

many features and modes, each of which imposes an additional

power requirement. The available features make the AD9852

suitable for a variety of applications, but the device is designed

to operate with only a few features enabled in a given application.

Enabling multiple features at high clock speeds may result in

exceeding the maximum junction temperature of the die and

therefore severely limit the long-term reliability of the device.

Figure 59 and Figure 60 show the power requirements associated

with each feature of the AD9852. These charts should be used

as a guide when determining how to optimize the AD9852 for

reliable operation in a specific application.

Figure 59 shows the supply current consumed by the AD9852

over a range of frequencies for two possible configurations. All

circuits enabled means that the output scaling multipliers, the

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