AD9854ASVZ datasheet(40/52 Pages) AD

40 / 52 page

AD9854

Rev. E | Page 40 of 52

POWER DISSIPATION AND THERMAL CONSIDERATIONS

The AD9854 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 AD9854. However, in most cases, disabling

the inverse sinc filter prevents exceeding the maximum die

temperature, because the inverse sinc filter consumes

approximately 30% of the total power.

The AD9854 is specified to operate within the industrial tem-

perature 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 temperatures, 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 AD9854 for a given set of operating

conditions.

The AD9854 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 16.2°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 dependent

on the ambient air conditions and the physical connection made

between the IC package and the PCB.

Adequate dissipation of heat from the AD9854 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 AD9854ASVZ has an exposed paddle on the

bottom of the package that must be soldered to a large copper

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

for either package style of the device are not recommended.

JUNCTION TEMPERATURE CONSIDERATIONS

The power dissipation (PDISS) of the AD9854 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

(Thermal Impedance × Power Consumption) +

Ambient Temperature

The maximum ambient temperature combined with the

maximum junction temperature establishes the following power

consumption limits for each package: 4.06 W for ASVZ models

and 1.71 W for ASTZ models.

Supply Voltage

The supply voltage affects power dissipation and junction

temperature because PDISS = 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 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, to minimize power dissipation, the user should select the

lowest possible internal clock speed to support a given application.

Typically, the usable frequency output bandwidth from a DDS is

limited to 40% of the clock rate to ensure that the requirements

of 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 of the AD9854 significantly

influences the total power consumption. Although the AD9854

offers many features targeting a wide variety of applications, the

device is designed to operate with only a few features enabled at

once for a given application. If multiple features are enabled at

higher clock speeds, the maximum junction temperature of the

die may be exceeded, severely limiting the long-term reliability of

the device. Figure 62 and Figure 63 show the power requirements

associated with each feature of the AD9854. These graphs should

be used as a guide in determining power consumption for

specific feature sets.

Figure 62 shows the supply current consumed by the AD9854

over a range of frequencies for two possible configurations. All

circuits enabled means that the output scaling multipliers, the

inverse sinc filter, the Q DAC, and the on-board comparator are

enabled. Basic configuration means that the output scaling

AD9854ASVZ датащи(PDF) 40 Page - Analog Devices

AD9854ASVZ датащи(HTML) 40 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52

датащи скачать

ссылки URL