TC2054 datasheet(13/22 Pages) MICROCHIP

13 / 22 page

DS21663D-page 13

TC2054/2055/2186

5.0

THERMAL CONSIDERATIONS

5.1

Thermal Shutdown

Integrated thermal protection circuitry shuts the

regulator off when the die temperature exceeds

approximately 160°C. The regulator remains off until

the die temperature cools to approximatley 150°C.

5.2

Power Dissipation

The amount of power the regulator dissipates is

primarily a function of input and output voltage, and

output current.

Equation 5-1 is used to calculate worst case power

dissipation:

EQUATION 5-1:

The

maximum

allowable

power

dissipation

(Equation 5-2) is a function of the maximum ambient

temperature (TA

MAX), the maximum allowable die

temperature (125 °C) and the thermal resistance from

junction-to-air (

JA). The 5-Pin SOT-23A package has

JA of approximately 220°C/Watt when mounted on a

typical two layer FR4 dielectric copper clad PC board.

EQUATION 5-2:

Equation 5-1

can

used

conjunction

with

Equation 5-2 to ensure regulator thermal operation is

within limits. For example:

Actual power dissipation:

Maximum allowable power dissipation:

In this example, the TC2054 dissipates a maximum of

only 26.7 mW; far below the allowable limit of 318 mW.

In a similar manner, Equation 5-1 and Equation 5-2 can

be used to calculate maximum current and/or input

voltage limits.

5.3

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

JA and, therefore,

increase the maximum allowable power dissipation

limit.

INMAX

OUTMIN

–

()I

LOADMAX

≈

Where:

= Worst-case actual power dissipation

VINMAX

= Maximum voltage on VIN

VOUTMIN

= Minimum regulator output voltage

ILOADMAX = Maximum output (load) current

Where all terms are previously defined.

DMAX

JMAX

AMAX

–

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

Given:

VINMAX

= 3.0V +10%

VOUTMIN

= 2.7V – 2.5%

ILOADMAX =40 mA

TAMAX

= +55°C

Find:

1. Actual power dissipation

2. Maximum allowable dissipation

INMAX

OUTMIN

–

()I

LOADMAX

3.0

1.1

()

2.7

0.975

()

–

[]40 10

–

26.7mW

DMAX

JMAX

AMAX

–

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

125

–

220

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

318mW

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