TC1073 datasheet(7/20 Pages) MICROCHIP

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DS21354B-page 7

TC1072/TC1073

4.0

THERMAL CONSIDERATIONS

4.1

Thermal Shutdown

Integrated

thermal

protection

circuitry

shuts

the

regulator off when die temperature exceeds 160°C.

The regulator remains off until the die temperature

drops to approximately 150°C.

4.2

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 6-Pin SOT-23A package has a θJA of

approximately 220°C/Watt.

EQUATION 4-2:

Equation 4-1 can be used in conjunction with Equation

4-2 to ensure regulator thermal operation is within

limits. For example:

Given:

VINMAX

=3.0V ±5%

VOUTMIN =2.7V – 2.5%

ILOADMAX =40mA

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 1.05) – (2.7 x .975)]40 x 10–3

= 20.7mW

Maximum allowable power dissipation:

In this example, the TC1072 dissipates a maximum of

20.7mW; 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.

4.3

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.

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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