TC1267 View Datasheet(PDF) - Microchip Technology

Part Name

Description

Manufacturer

TC1267

400mA PCI LDO

Microchip Technology 

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:

PD ≈ (VINMAX – VOUTMIN)ILOADMAX

Where:

PD = Worst case actual power dissipation

VINMAX = Maximum voltage on VIN

VOUTMIN = Minimum regulator output voltage

ILOADMAX = Maximum output (load) current

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:

PDMAX = (TJMAX – TAMAX)

θJA

Where all terms are previously defined.

Table 4-1 shows various values of θJA for the TC1267

packages.

TABLE 4-1:

THERMAL RESISTANCE

GUIDELINES FOR TC1267 IN

5-PIN DDPAK

Copper

Area

(Topside)*

Copper

Area

(Backside)

Board

Area

Thermal

Resistance

(θJA)

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

*Tab of device attached to topside copper.

TC1267

Equation 4-1 can be used in 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:

PDMAX = (TJMAX – TAMAX)

θJA

= (95 – 70)

27

= 423mW

In this example, the TC1267 dissipates a maximum of

813mW; below the allowable limit of 926mW.

© 2002 Microchip Technology Inc.

DS21378B-page 5

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