AD9854ASQ Analog Devices Inc, AD9854ASQ Datasheet - Page 39

• Manufacturer Part Number: AD9854ASQ
• Manufacturer: Analog Devices Inc

Specifications of AD9854ASQ

• Lead Free Status / RoHS Status: Not Compliant

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 can resolve exceeding the maximum die
temperature. The inverse sinc filter consumes approximately
30% of the total power.
The AD9854 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
temperatures, 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
temperature within the device, including
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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 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 dependent
on the ambient air conditions and the physical connection made
between the IC package and the PCB.
Adequate dissipation of power 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 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, can be a ground plane. Sockets
for either package style of the device are not recommended.
Package style
Selected mode of operation
Internal system clock speed
Supply voltage
Ambient temperature
Rev. D | Page 39 of 52
Note that the AD9854ASQ exposed heat sink surface finish is
nickel. It is suggested that soldering materials be selected with
adequate fluxing strength to establish a solid conduit between
this pad and the PCB surface.
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 not exceed 150°C
and that the ambient temperature can be 85°C, the maximum
power consumption is 1.7 W for the AD9854AST and 4.1 W for
the AD9854ASQ (thermally enhanced package). Factors affecting
the power dissipation are described in the Supply Voltage section.
Supply Voltage
The supply voltage affects power dissipation and junction
temperature because P
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× the
highest desired output frequency.
Mode of Operation
The selected mode of operation of the AD9854 has a great
influence on the total power consumption. The AD9854 offers
many features and modes, each of which imposes an additional
power requirement. 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 any 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 the individual features of the AD9854. These
graphs should be used as a guide in determining which feature
to use for optimum performance and reliable operation in a
given application of the AD9854.
(Thermal Impedance × Power Consumption) +
Ambient Temperature
DISS
= V × I. Users should design for 3.3 V
DISS
) of the AD9854 in a given
DISS
AD9854
, such as