 2002 Microchip Technology Inc.

DS21385C-page 9

TC1300

4.0

THERMAL CONSIDERATIONS

4.1

Thermal Shutdown

Integrated thermal protection circuitry shuts the regula-

tor off when the die temperature exceeds 150°C. The

regulator remains off until the die temperature drops to

approximately 140°C.

4.2

Power Dissipation

The amount of power the regulator dissipates is prima-

rily a function of input and output voltage, and output

current. The following equation is used to calculate

worst case actual power dissipation:

EQUATION

the maximum recommended die temperature (125°C)

and the thermal resistance from junction-to-air (

The MSOP-8 package has a

200°C/Watt when mounted on a FR4 dielectric copper

clad PC board.

EQUATION

The worst case actual power dissipation equation can

be used in conjunction with the LDO maximum allow-

able power dissipation equation to ensure regulator

thermal operation is within limits. For example:

Find:

EQUATION:

ACTUAL POWER

DISSIPATION

EQUATION:

MAXIMUM ALLOWABLE

POWER DISSIPATION

In this example, the TC1300 dissipates a maximum of

only 220 mW; below the allowable limit of 350 mW. In a

similar manner, the maximum actual power dissipation

equation and the maximum allowable power dissipa-

tion equation can be used to calculate maximum cur-

rent and/or input voltage limits. For example, the

maximum allowable power dissipation of 350 mW into

the actual power dissipation equation, from which

4.3

Layout Considerations

The primary path of heat conduction out of the package

is via the package leads. Therefore, layouts having a

ground plane, wide traces at the pads and wide power

supply bus lines combine to lower

increase the maximum allowable power dissipation

limit.

–

≈

Where:

–

()

--------------------------------------------

=

Given:

=4.1V

= 3.0V -2.5%

= 125°C

=55°C

= 200°C/W

–

≈

4.1

()

3.0 .975

×

()

–

[]

=

200 10

3

–

×

220 mW

=

–

()

--------------------------------------------

=

125 55

–

()

200

-------------------------

=

350 mW

=