WEITRON

http://www.weitron.com.tw

WT1117A

9/11

Rev-A 18-Oct-06

The thermal characteristics of an IC depend four factors:

The relationship of these four factors is expressed by equation (1):

Maximum ambient temperature and power dissipation are determined by the design while the maximum junction

temperature and thermal resistance depend on the manufacturer and the package type.

The maximum power dissipation for a regulator is expressed by equation (2):

where:

A heat sink effectively increases the surface area of the package to improve the flow of heat away from the IC into the air.

Each material in the heat flow path between the IC and the environment has a thermal resistance. Like series electrical

This is expressed by equation (3):

Where all of the following are in °C/W

3.5°C/W for a given package type based on an average die size. For a high current regulator such as the WT1117A the

majority of the heat is generated in the power transistor section.

P

= { V

- V

} I

+ V

I

D(max)

IN(max)

OUT(min)

OUT(max

IN(max) Q

)

........(2)