TC1264

DS21375B-page 4

© 2002 Microchip Technology Inc.

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

(

EQUATION 4-2:

Table 4-1 and Table 4-2 show various values of

the TC1264 packages.

TABLE 4-1:

THERMAL RESISTANCE

GUIDELINES FOR TC1264 IN

SOT-223 PACKAGE

*Tab of device attached to topside copper

TABLE 4-2:

THERMAL RESISTANCE

GUIDELINES FOR TC1264 IN

3-PIN DDPAK/TO-220

PACKAGE

*Tab of device attached to topside copper

Equation 4-1 can be used in conjunction with Equation

4-2 to ensure regulator thermal operation is within

limits. For example:

Given:

= 3.3V ± 10%

= 125°C

=95°C

= 59°C/W (SOT-223)

Find: 1. Actual power dissipation

2. Maximum allowable dissipation

Actual power dissipation:

= 260mW

Maximum allowable power dissipation:

In this example, the TC1264 dissipates a maximum of

260mW; below the allowable limit of 508mW. 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

power dissipation of 508mW into Equation 4-1, from

Copper

Area

(Topside)*

Copper

Area

(Backside)

Board

Area

Thermal

Resistance

2500 sq mm 2500 sq mm

2500 sq mm

45°C/W

1000 sq mm 2500 sq mm

2500 sq mm

45°C/W

225 sq mm

2500 sq mm

2500 sq mm

53°C/W

100 sq mm

2500 sq mm

2500 sq mm

59°C/W

1000 sq mm 1000 sq mm

1000 sq mm

52°C/W

1000 sq mm

0 sq mm

1000 sq mm

55°C/W

Where:

= Worst case actual power dissipation

= Minimum regulator output voltage

= Maximum output (load) current

Where all terms are previously defined.

Copper

Area

(Topside)*

Copper

Area

(Backside)

Board

Area

Thermal

Resistance

2500 sq mm 2500 sq mm

2500 sq mm

25°C/W

1000 sq mm 2500 sq mm

2500 sq mm

27°C/W

125 sq mm

2500 sq mm

2500 sq mm

35°C/W

= (125 – 95)

59

= 508mW