TC2054 datasheet(5/16 Pages) MICROCHIP

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DS21663B-page 5

TC2054/2055/2186

4.0

THERMAL CONSIDERATIONS

4.1

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

power dissipation:

EQUATION 4-1:

The maximum allowable power dissipation (Equation

4-2) 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

(

θJA). The 5-Pin SOT-23A package has a θJA of

approximately 220°C/Watt when mounted on a typical

two layer FR4 dielectric copper clad PC board.

EQUATION 4-2:

Equation 4-1 can be used in conjunction with Equation

4-2 to ensure regulator thermal operation is within lim-

its. For example:

Maximum allowable power dissipation:

= 318mW

In this example, the TC2054 dissipates a maximum of

only 20.7mW; far below the allowable limit of 318mW.

In a similar manner, Equation 4-1 and Equation 4-2 can

be used to calculate maximum current and/or input

voltage limits.

4.2

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.

PD ≈ (VIN –VOUT

MIN

)ILOAD

MAX

Where:

Worst case actual power dissipation

VIN

MAX

Maximum voltage on VIN

VOUT

MIN

Minimum regulator output voltage

ILOAD

MAX

Maximum output (load) current

Where all terms are previously defined

MA X

MAX

MA X

–

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

Given:

VINMAX

=3.0V ±5%

VOUTMIN = 2.7V – 2.5%

ILOADMAX = 40mA

TAMAX

= 55°C

Find

1. Actual power dissapation

2. Maximum allowable dissapation

Actual power dissipation:

≈ (VIN

MAX

–VOUT

MIN

)ILOADMAX

= [(3.0 x 1.05) – (2.7 x .975)]40 x 10–3

= 20.7mW

MA X

MAX

MA X

–

()

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

125

–

()

220

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

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