TC1014 datasheet(4/10 Pages) TELCOM

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TC1014

50mA CMOS LDO WITH SHUTDOWN

AND REFERENCE BYPASS

PRELIMINARY INFORMATION

TC1014-01-6/5/97

PD ≈ (VINMAX – VOUTMIN)ILOADMAX

Where:

PD = Worst case actual power dissipation

VIN

MAX = Maximum voltage on VIN

VOUT

MIN = Minimum regulator output voltage

ILOAD

MAX = Maximum output (load) current

Equation 1.

The maximum

allowable power dissipation (Equation 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

SOT-23A-5 package has a

θJA of approximately 220°C/Watt

when mounted on a single layer FR4 dielectric copper clad

PC board.

MAX = (TJMAX – TJMAX)

θJA

Where all terms are previously defined.

Equation 2.

Equation 1 can be used in conjunction with Equation 2

to ensure regulator thermal operation is within limits. For

example:

Given:

VIN

MAX = 3.0V ±5%

VOUT

MIN = 2.7V ±0.5V

ILOAD = 40mA

TAMAX = 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 .995)]40 x 10–3

= 18.5mW

Maximum allowable power dissipation:

MAX = (TJMAX – TAMAX)

θJA

= (125 – 55)

220

= 318mW

In this example, the TC1014 dissipates a maximum of

only 60mW; far below the allowable limit of 318mW. In a

similar manner, Equation 1 and Equation 2 can be used to

calculate maximum current and/or input voltage limits. For

example, the maximum allowable VIN is found by sustituting

the maximum allowable power dissipation of 318mW into

Equation 1, from which VINMAX = 5.9V.

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.

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