TC1269 查看數據表（PDF） - Microchip Technology

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TC1269

300mA CMOS LDO with Shutdown and VREF Bypass

Microchip Technology 

4.0 THERMAL CONSIDERATIONS

4.1 Thermal Shutdown

Integrated thermal protection circuitry shuts the

regulator off when die temperature exceeds 150°C.

The regulator remains off until the die temperature

drops to approximately 140°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.

TC1269

Equation 4-1 can be used in conjunction with

Equation 4-2 to ensure regulator thermal operation is

within limits. For example:

Given:

VINMAX

VOUTMIN

ILOAD

TAMAX

= 3.0V ± 10%

= 2.7V – 2.5%

= 250mA

= 55°C

Find: 1. Actual power dissipation

2. Maximum allowable dissipation

Actual power dissipation:

PD ≈ (VINMAX – VOUTMIN) ILOADMAX

= [(3.0 x 1.1) – (2.7 x .975)]250 x 10-3

= 167mW

Maximum allowable power dissipation:

PDMAX = (TJMAX – TAMAX)

θJA

= (125 – 55)

200

= 350mW

In this example, the TC1269 dissipates a maximum of

167mW; below the allowable limit of 350mW. In a

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

used to calculate maximum current and/or input

voltage limits.

4.3 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.

© 2002 Microchip Technology Inc.

DS21380B-page 5

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