AD9854SLASHPCB datasheet(32/44 Pages) AD

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AD9854

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POWER DISSIPATION AND THERMAL

CONSIDERATIONS

The AD9854 is a multifunctional, very high-speed device that

targets a wide variety of synthesizer and agile clock applications.

The set of numerous innovative features contained in the device

each consume incremental power, the sum of which, if enabled

in combination, may exceed the safe thermal operating condi-

tions of the device. Careful analysis and consideration of power

dissipation and thermal management is a critical element in the

successful application of the AD9854 device.

The AD9854 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 of 150

°C is not exceeded. At high operating tempera-

tures, extreme care must be taken in the operation of the device

to avoid exceeding the junction temperature which results in a

potentially damaging thermal condition.

Many variables contribute to the operating junction tempera-

ture 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 AD9854 device for a given set of operating

conditions.

The AD9854 device is available in two package styles: a thermally-

enhanced surface-mount package with an exposed heat sink,

and a 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 of a package is

determined by package material and its physical dimensions. The

dissipation of the heat from the package is directly dependent upon

the ambient air conditions and the physical connection made

between the IC package and the PCB. Adequate dissipation of

power from the AD9854 relies upon 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 AD9854ASQ

contains a heat sink on the bottom of the package that must be

soldered to a ground pad on the PCB surface. This pad must be

connected to a large copper plane which, for convenience, may be

ground plane. Sockets for either package style of the AD9854

device are not recommended.

JUNCTION TEMPERATURE CONSIDERATIONS

The power dissipation (PDISS) of the AD9854 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

Given that the junction temperature should never exceed 150

°C

for the AD9854, and that the ambient temperature can be 85

°C,

the maximum power consumption for the AD9854AST is 1.7 W

and the AD9854ASQ (thermally-enhanced package) is 4.1 W.

Factors affecting the power dissipation are:

Supply Voltage—this obviously affects power dissipation and

junction temperature since PDISS equals V

× I. Users should design

for 3.3 V nominal; however, the device is guaranteed to meet

speciﬁcations, over the full temperature range and over the sup-

ply voltage range of 3.135 V to 3.465 V.

Clock Speed—this directly and linearly influences the total

power dissipation of the device, and, therefore, junction tem-

perature. As a rule, the user should always select the lowest

internal clock speed possible to support a given application, to

minimize power dissipation. Normally the usable frequency out-

put bandwidth from a DDS is limited to 40% of the clock rate

to keep reasonable requirements on the output low-pass ﬁlter.

For the 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

AD9854 has a great influence on total power consumption. The

AD9854 offers many features and modes, each of which imposes

an additional power requirement. The collection of features

contained in the AD9854 target a wide variety of applications

and the device was designed under the assumption that only a

few would be enabled for any given application. In fact, the user

must understand that enabling multiple features at higher clock

speeds may cause the junction temperature of the die to be

exceeded. This can severely limit the long-term reliability of the

device. Figure 57 provides a summary of the power requirements

associated with the individual features of the AD9854. This

table should be used as a guide in determining the optimum

application of the AD9854 for reliable operation.

As can be seen in the Figure 57, the Inverse Sinc ﬁlter function

requires a signiﬁcant amount of power, and much forethought

and scrutiny should be given to its use. As an alternate approach

to maintaining flatness across the output bandwidth, the digital

Multiplier function may be used to adjust the output signal

level, at a dramatic savings in power consumption. Careful plan-

ning and management in the use of the feature set will minimize

power dissipation and avoid exceeding junction temperature

requirements within the IC.

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