LM27341_14 datasheet(21/42 Pages) TI1

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RTJA =

TJ - TA

Power

RT =

Power

LM27341, LM27342, LM27341Q, LM27342Q

www.ti.com

SNVS497D – NOVEMBER 2008 – REVISED FEBRUARY 2010

Figure 37. Cross-Sectional View of Integrated Circuit Mounted on a Printed Circuit Board.

Heat in the LM27341/LM27342 due to internal power dissipation is removed through conduction and/or

convection.

Conduction: Heat transfer occurs through cross sectional areas of material. Depending on the material, the

transfer of heat can be considered to have poor to good thermal conductivity properties (insulator vs conductor).

Heat Transfer goes as:

Silicon

→Lead Frame→PCB

Convection: Heat transfer is by means of airflow. This could be from a fan or natural convection. Natural

convection occurs when air currents rise from the hot device to cooler air.

Thermal impedance is defined as:

(48)

Thermal impedance from the silicon junction to the ambient air is defined as:

(49)

This impedance can vary depending on the thermal properties of the PCB. This includes PCB size, weight of

copper used to route traces , the ground plane, and the number of layers within the PCB. The type and number

of thermal vias can also make a large difference in the thermal impedance. Thermal vias are necessary in most

applications. They conduct heat from the surface of the PCB to the ground plane. Six to nine thermal vias should

be placed under the exposed pad to the ground plane. Placing more than nine thermal vias results in only a

small reduction to RθJA for the same copper area. These vias should have 8 mil holes to avoid wicking solder

away from the DAP. See SNOA401 and SNVA183 for more information on package thermal performance. If a

compromise for cost needs to be made, the thermal vias for the MSOP-PowerPad package can range from 8-14

mils, this will increase the possibility of solder wicking.

To predict the silicon junction temperature for a given application, three methods can be used. The first is useful

before prototyping and the other two can more accurately predict the junction temperature within the application.

Method 1:

The first method predicts the junction temperature by extrapolating a best guess RθJA from the table or graph.

The tables and graph are for natural convection. The internal dissipation can be calculated using the efficiency

calculations. This allows the user to make a rough prediction of the junction temperature in their application.

Methods two and three can later be used to determine the junction temperature more accurately.

The two tables below have values of RθJA for the SON and the MSOP-PowerPad packages.

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