TC1072 datasheet(13/22 Pages) MICROCHIP

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DS21354D-page 13

TC1072/TC1073

5.0

THERMAL CONSIDERATIONS

5.1

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.

5.2

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 5-1:

The

maximum

allowable

power

dissipation

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

temperature (TAMAX), the maximum allowable die tem-

perature (TJMAX) and the thermal resistance from junc-

tion-to-air (

θJA). The 6-Pin SOT-23 package has a θJA

of approximately 220°C/Watt.

EQUATION 5-2:

Equation 5-1 can be used in conjunction with

Equation 5-2 to ensure regulator thermal operation is

within limits. For example:

Given:

VINMAX

= 3.0V ±5%

VOUTMIN

= 2.7V – 2.5%

ILOADMAX

= 40 mA

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 0.975)] x 40 x 10–3

= 20.7 mW

Maximum allowable power dissipation:

In this example, the TC1072 dissipates a maximum of

20.7 mW; 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

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

= 318 mW

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