TJ2995DP datasheet(7/9 Pages) HTC

7 / 9 page

DDR Termination Regulator

TJ2995

Oct. 2009 - Rev. 1.1

7/9

HTC

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 TJ2995 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 TJ2995 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 shutdown 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,

sinking and sourcing current. Although only one equation will add into the total, V

TT cannot source and

sink current simultaneously.

VTT = VVTT x ILOAD (Sinking)

or P

VTT = ( VPVIN - VVTT) x ILOAD (Sourcing)

The power dissipation of the TJ2995 can also be calculated during the shutdown state. During this

condition the output V

TT will tri-state, therefore that term in the power equation will disappear as it

cannot sink or source any current (leakage is negligible). The only losses during shutdown will be

the reduced quiescent current at AVIN and the constant impedance that is seen at the VDDQ pin.

D = PAVIN + PVDDQ ,

Where, PAVIN = IAVIN x VAVIN

VDDQ

= V

VDDQ

x I

VDDQ

= V

VDDQ2

x R

VDDQ

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