TC1300 datasheet(9/16 Pages) MICROCHIP

9 / 16 page

 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 allowable power dissipation, PDMAX, is a

function of the maximum ambient temperature (TAMAX),

the maximum recommended die temperature (125°C)

and the thermal resistance from junction-to-air (

θJA).

The MSOP-8 package has a

θJA of approximately

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 VIN is found by substituting the

maximum allowable power dissipation of 350 mW into

the actual power dissipation equation, from which

VINMAX = 4.97V.

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

θJA and, therefore,

increase the maximum allowable power dissipation

limit.

VINMAX VOUTMIN

–

()ILOADMAX

≈

Where:

PD = worst case actual power dissipation

VINMAX = maximum voltage on VIN

VOUTMIN = minimum regulator output voltage

ILOADMAX = maximum output (load) current

PDMAX

TJMAX TAMAX

–

()

θJA

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

Given:

VINMAX

=4.1V

VOUTMIN

= 3.0V -2.5%

ILOADMAX =200 mA

TJMAX

= 125°C

TAMAX

=55°C

θJA

= 200°C/W

VINMAX VOUTMIN

–

()ILOADMAX

≈

4.1

()

3.0 .975

()

–

[]

200 10

–

220 mW

PDMAX

TJMAX TAMAX

–

()

θJA

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

125 55

–

()

200

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

350 mW

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