TC1268 datasheet(6/12 Pages) MICROCHIP

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TC1268

DS21379B-page 6

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).

EQUATION 4-2:

Table 4-1 shows various values of

θJA for the TC1268

package.

TABLE 4-1:

THERMAL RESISTANCE

GUIDELINES FOR TC1268 IN

8-PIN SOIC PACKAGE

*Pin 2 is ground. Device is mounted on topside.

Equation 4-1

can

used

conjunction

with

Equation 4-2 to ensure regulator thermal operation is

within limits. For example:

Given:

VINMAX

= 3.3V ± 10%

VOUTMIN = 2.5V ± 0.5%

ILOADMAX = 275mA

TJMAX

= 125°C

TAMAX

=95°C

θJA

=60°C/W

Find: 1. Actual power dissipation

2. Maximum allowable dissipation

Actual power dissipation:

PD ≈ (VINMAX –VOUTMIN)ILOADMAX

= [(3.3 x 1.1) – (2.5 x .995)]275 x 10-3

= 314mW

Maximum allowable power dissipation:

In this example, the TC1268 dissipates a maximum of

314mW; below the allowable limit of 500mW. In a

similar manner, Equation 4-1 and Equation 4-2 can be

used to calculate maximum current and/or input

voltage limits. For example, the maximum allowable

VIN is found by substituting the maximum allowable

power dissipation of 500mW into Equation 4-1, from

which VINMAX =3.94V.

Copper

Area

(Topside)*

Copper

Area

(Backside)

Board

Area

Thermal

Resistance

(θJA)

2500 sq mm 2500 sq mm

2500 sq mm

60°C/W

1000 sq mm 2500 sq mm

2500 sq mm

60°C/W

225 sq mm

2500 sq mm

68°C/W

100 sq mm

2500 sq mm

74°C/W

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 – 95)

=500mW

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