TC1054 データシートの表示（PDF） - TelCom Semiconductor Inc => Microchip

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TC1054

50mA CMOS LDO WITH SHUTDOWN AND ERROR OUTPUT

TelCom Semiconductor Inc => Microchip 

50mA CMOS LDO WITH SHUTDOWN

AND ERROR OUTPUT

PRELIMINARY INFORMATION

TC1054

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

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). SOT-23A-5

package has a θJA of approximately 220°C/Watt when

mounted on a single layer FR4 dielectric copper clad PC

board.

PD 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:

VINMAX = 3.0V ±5%

VOUTMIN = 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 1.05) – (2.7 x .995)]40 x 10–3

= 18.5mW

Maximum allowable power dissipation:

PDMAX = (TJMAX – TAMAX)

θJA

= (125 – 55)

220

= 318mW

In this example, the TC1054 dissipates a maximum of

only 18.5mW; 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.

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 in-

crease the maximum allowable power dissipation limit.

TC1054-01 6/12/97

5

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