TC1306_07 datasheet(6/18 Pages) MICROCHIP

Teilenummer	TC1306

Bauteilbeschribung	Dual 150mA CMOS LDO With Select Mode??Operation, Shutdown and RESET Output

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TC1306

DS21527C-page 6

5.0

THERMAL CONSIDERATIONS

5.1

Thermal Shutdown

Integrated thermal protection circuitry shuts the

regulator off when die exceeds approximately 160°C.

The regulator remains off until the die temperature

drops to approximately 145°C.

Thermal shutdown is intended to protect the device

under transient accidental (fault) overload conditions.

Thermal Shutdown may not protect the LDO while

operating above junction temperatures of 125°C

continuously. Sufficient thermal evaluation of the

design needs to be conducted to ensure that the

junction temperature does not exceed 125°C.

5.2

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 5-1:

The

maximum

allowable

power

dissipation

(Equation 5-2) is a function of the maximum ambient

temperature (TAMAX), the maximum allowable die

temperature (125°C), and the thermal resistance from

junction-to-air (

θJA). The MSOP-8 package has a θJA of

approximately 200°C/W when mounted on a four layer

FR4 dielectric copper clad PC board.

EQUATION 5-2:

Equation 5-1 can be used in conjunction with

Equation 5-2 to ensure regulator thermal operation is

within limits. For example:

Given:

VINMAX

= 3.8V ± 5%

VOUT1MIN = 1.8V ± 2.5%

VOUT2MIN = 3.0V ± 2.5%

ILOAD1MAX= 60mA

ILOAD2MAX= 120mA

TJMAX

= 125°C

TAMAX

= 55°C

θJA

= 200°C/W

Find: 1. Actual power dissipation

2. Maximum allowable dissipation

Actual power dissipation:

PD ≈ [(VINMAX – VOUT1MIN)] x ILOAD1MAX

+ [(VINMAX – VOUT2MIN)] x ILOAD2MAX

[(3.8 x 1.05) – (1.8 x .975)] x 60 x 10-3

+ [(3.8 x 1.05) – (3.0 x .975)] x 120 x 10-3

= 256mW

Maximum allowable power dissipation:

In this example, the TC1306 dissipates a maximum of

262mW; below the allowable limit of 350mW. In a

similar manner, Equation 5-1 and Equation 5-2 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 350mW into Equation 5-1, from

which VINMAX = 4.5V.

5.3

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.

Where:

PD ≈ (VINMAX – VOUT1MIN)ILOAD1MAX +

PD

VINMAX

VOUT1MIN

ILOAD1MAX

= Worst case actual power dissipation

= Minimum regulator output voltage1

= Maximum output (load) current1

= Maximum voltage on VIN

(VINMAX – VOUT2MIN)ILOAD2MAX

VOUT2MIN

ILOAD2MAX

= Minimum regulator output voltage2

= Maximum output (load) current2

PDMAX = (TJMAX – TAMAX)

θJA

Where all terms are previously defined.

PD = (TJMAX – TAMAX)

θJA

= (125 – 55)

200

= 350mW

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