AD9854 Analog Devices, AD9854 Datasheet - Page 32

• Manufacturer Part Number: AD9854
• Description: CMOS 300 MHz Quadrature Complete-DDS
• Manufacturer: Analog Devices

AD9854
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:
1. Package Style
2.
3.
4.
5.
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.
Selected Mode of Operation
Internal System Clock Speed
Supply Voltage
Ambient Temperature.
–32–
JUNCTION TEMPERATURE CONSIDERATIONS
The power dissipation (P
application is determined by many operating conditions. Some
of the conditions have a direct relationship with P
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:
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 P
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.
Junction Temperature = (Thermal Impedance
Power Consumption) + Ambient Temperature
DISS
DISS
) of the AD9854 device in a given
equals V I. Users should design
DISS
, such as
REV. 0