TC563302ECT datasheet(4/16 Pages) MICROCHIP

4 / 16 page

TC56

DS21436B-page 4

 2002 Microchip Technology Inc.

4.0

THERMAL CONSIDERATIONS

4.1

Power Dissipation

The amount of power the regulator dissipates is

primarily a function of input and output voltage, and

output current. The following equation is used to

calculate worst case actual power dissipation.

EQUATION 4-1:

The

maximum

allowable

power

dissipation

(Equation 4-2) is a function of the maximum ambient

temperature (TAMAX), the maximum allowable die

temperature (TJMAX) and the thermal resistance from

junction-to-air (

θJA). The 5-Pin SOT-23A package has

θJA of approximately 220°C/Watt.

EQUATION 4-2:

Equation 4-1

can

used

conjunction

with

Equation 4-2 to ensure regulator thermal operation is

within limits. For example:

Given:

VINMAX

=3.0V ±10%

VOUTMIN =2.7V – 2%

ILOADMAX = 98mA

TJMAX

= 125°C

TAMAX

=55°C

Find: 1. Actual power dissipation

2. Maximum allowable dissipation

Actual power dissipation:

PD ≈ (VINMAX – VOUTMIN)ILOADMAX

= [(3.0 x 1.1) – (2.7 x .98)]98 x 10-3

=64mW

Maximum allowable power dissipation:

In this example, the TC56 dissipates a maximum of

64mW; 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.

Where:

PD ≈ (VINMAX – VOUTMIN)ILOADMAX

VINMAX

VOUTMIN

ILOADMAX

= Worst case actual power dissipation

= Minimum regulator output voltage

= Maximum output (load) current

= Maximum voltage on VIN

PDMAX = (TJMAX – TAMAX)

θJA

Where all terms are previously defined.

PDMAX = (TJMAX – TAMAX)

θJA

= (125 – 55)

220

= 318mW

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