TJ2997GD datasheet(7/13 Pages) HTC

7 / 13 page

DDR Termination Regulator

TJ2997

Apr, 2011 - R1.0.1

7/13

HTC

THERMAL DISSIPATION

Since the TJ2997 is a linear regulator any current flow from VTT will result in internal power dissipation

generating heat.

To prevent damaging the part from exceeding the maximum allowable junction

temperature, care should be taken to derate the part dependent on the maximum expected ambient

temperature and power dissipation. The maximum allowable internal temperature rise, TRmax can be

calculated given the maximum ambient temperature, TAmax of the application and the maximum allowable

junction temperature, TJmax.

Rmax = TJmax − TAmax

From this equation, the maximum allowable power dissipation, P

Dmax of

the part can be

calculated:

Dmax

= T

Rmax

/ θ

The maximum allowable value for junction-to-ambient thermal resistance, θJA, can be calculated using

the formula:

θJA = TRmax / PD = (TJmax – TAmax) / PD

The θJA of the TJ2997 will be dependent on several variables: the package used; the thickness of

copper; the number of vias and the airflow. For instance, the θJA of the SOP8 is 165°C/W with the

package mounted to a standard 8x4 2-layer board with 1oz. copper, no airflow, and 0.5W dissipation at

room temperature. This value can be reduced to 152°C/W by changing to a 3x4 board with 2 oz. copper

that is the JEDEC standard.

Additional improvements can be made by the judicious use of vias to connect the part and

dissipate heat to an internal ground plane.

Using larger traces and more copper on the top side of

the board can also help.

With careful layout it is possible to reduce the θ

JA further than the nominal

values. Additional improvements in lowering the θJA can also be achieved with a constant airflow

across the package.

Optimizing the θ

JA and placing the TJ2997 in a section of a board exposed to lower ambient

temperature allows the part to operate with higher power dissipation. The internal power dissipation

can be calculated by summing the three main sources of loss: output current at V

TT, either sinking or

sourcing, and quiescent current at AVIN and VDDQ. During the active state (when enable is not held low)

the total internal power dissipation can be calculated from the following equations:

= P

AVIN

+ P

VDDQ

+ P

VTT

Where, PAVIN = IAVIN x VAVIN

VDDQ

= V

VDDQ

x I

VDDQ

= V

VDDQ2

x R

VDDQ

To calculate the maximum power dissipation at V

TT both conditions at VTT need to be examined,

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