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 (P
DISS
) of the AD9854 in a given
application is determined by many operating conditions. Some
of the conditions have a direct relationship with P
DISS
, 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
P
DISS
=
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
