AD9857SLASHPCB datasheet(28/31 Pages) AD

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AD9857

–28–

REV. 0

Power Management Considerations

The thermal impedance for the AD9857 80-lead LQFP package

θJA = 35°C/W. The maximum allowable power dissipation

using this value is calculated using

∆T = P × θ

JA.

∆

150

185

–

The AD9857 power dissipation is at or below this value when

the SYSCLK frequency is at 200 MHz or lower with all optional

features enabled. The maximum power dissipation occurs while

operating the AD9857 as a quadrature modulator at the maxi-

mum system clock frequency with TxENABLE in a logic high

state 100% of the time the device is powered. Under these con-

ditions the device operates with all possible circuits enabled at

maximum speed.

Signiﬁcant power saving may be seen by using a TxENABLE

signal that toggles low during times when the device does not

modulate.

The thermal impedance of the AD9857 package was measured

in a controlled temperature environment at temperatures ranging

from 28

°C to 85°C with no air flow. The device under test was

soldered to an AD9857 evaluation board and operated under

conditions that generate maximum power dissipation. The ther-

mal resistance 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 pack-

age 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

AD9857 relies upon all power and ground pins of the device

being soldered directly to copper planes on a PCB.

Many variables contribute to the operating junction temperature

within a device. They include:

1. Package Style

2. Selection Mode of Operation

3. Internal System Clock Speed

4. Supply Voltage

5. Ambient Temperature

The power dissipation of the AD9857 in a given application is

determined by several operating conditions. Some of these con-

ditions, such as supply voltage and clock speed, have a direct

relationship with power dissipation. The most important factors

affecting power dissipation are the following:

Supply Voltage

This affects power dissipation and junction temperature since

power dissipation equals supply voltage multiplied by supply

current. It is recommended that the user design for a 3.3 V

nominal supply voltage in order to manage the affect of supply

voltage on the junction temperature of the AD9857.

Clock Speed

This directly and linearly influences the total power dissipation

of the device and, therefore, junction temperature. 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 output bandwidth from a DDS

is limited to 40% of the system clock rate to keep reasonable

requirements on the output low-pass filter. This means that for

the typical DDS application, the system clock frequency should

be 2.5 times the highest output frequency.

Mode of Operation

The AD9857 has three modes of operation that consume sig-

nificantly different amounts of power. When operating in the

Quadrature Modulation Mode, the AD9857 will dissipate about

twice the power as when operating as a single-tone DDS. When

operating as a quadrature modulator, the AD9857 has features

that facilitate power management tactics. For example, the

TxENABLE pin may be used in conjunction with the auto

power-down bit to frame bursts of data and automatically switch

the device into a low-power state when there is no data to

be modulated.

Equivalent I/O Circuits

VDD

DAC OUTPUTS

IOUT IOUB

VDD

DIGITAL

VDD

DIGITAL

OUT

Figure 33. Equivalent I/O Circuits

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