TC1267 datasheet(5/14 Pages) MICROCHIP

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DS21378B-page 5

TC1267

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 TC1267

packages.

TABLE 4-1:

THERMAL RESISTANCE

GUIDELINES FOR TC1267 IN

5-PIN DDPAK

*Tab of device attached to topside copper.

Equation 4-1

can

used

conjunction

with

Equation 4-2 to ensure regulator thermal operation is

within limits. For example:

Given:

VINMAX

=5V± 5%

VOUTMIN = 3.217V

ILOADMAX = 400mA

TJMAX

= 95°C

TAMAX

= 70°C

θJA

= 27°C/W (DDPAK mounted on

1000 sq mm topside copper area)

Find: 1. Actual power dissipation

2. Maximum allowable dissipation

Actual power dissipation:

PD ≈ (VINMAX –VOUTMIN)ILOADMAX

= (5.25V - 3.217V) 400mA

=813mW

Maximum allowable power dissipation:

In this example, the TC1267 dissipates a maximum of

813mW; below the allowable limit of 926mW.

Copper

Area

(Topside)*

Copper

Area

(Backside)

Board

Area

Thermal

Resistance

(θJA)

2500 sq mm

25°C/W

1000 sq mm

2500 sq mm

27°C/W

125 sq mm

2500 sq mm

35°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

= (95 – 70)

= 423mW

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