J

C

JC

T

T

R

Power

F

-

=

J

A

JA

T

T

R

Power

q

-

=

T

R

Power

q

D

=

LM2831

www.ti.com

SNVS422D – AUGUST 2006 – REVISED SEPTEMBER 2015

10.1.2 Thermal Definitions

TJ

Chip junction temperature

TA

Ambient temperature

RθJC

Thermal resistance from chip junction to device case

RθJA

Thermal resistance from chip junction to ambient air

Heat in the LM2831 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

→ package → 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:

(31)

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

(32)

The PCB size, weight of copper used to route traces and ground plane, and number of layers within the PCB can

greatly effect RθJA. 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. Four to six thermal vias should be placed under the exposed pad to the ground plane if the

WSON package is used.

Thermal impedance also depends on the thermal properties of the application operating conditions (Vin, Vo, Io,

and so forth), and the surrounding circuitry.

10.1.2.1 Silicon Junction Temperature Determination Method 1

To accurately measure the silicon temperature for a given application, two methods can be used. The first

method requires the user to know the thermal impedance of the silicon junction to top case temperature.

Some clarification must be made before we go any further.

RθJC is the thermal impedance from all six sides of an IC package to silicon junction.

RΦJC is the thermal impedance from top case to the silicon junction.

In this data sheet we will use RΦJC so that it allows the user to measure top case temperature with a small

thermocouple attached to the top case.

RΦJC is approximately 30°C/Watt for the 6-pin WSON package with the exposed pad. Knowing the internal

dissipation from the efficiency calculation given previously, and the case temperature, which can be empirically

measured on the bench we have:

(33)

Therefore:

Tj = (RΦJC × PLOSS) + TC

(34)

From the previous example:

Tj = (RΦJC × PINTERNAL) + TC

(35)

Tj = 30°C/W × 0.189 W + TC

(36)

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