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TC1173

300mA CMOS LDO with Shutdown ERROR Output and Bypass

TelCom Semiconductor Inc => Microchip 

300mA CMOS LDO With Shutdown,

ERROR Output, And Bypass

TC1173

Bypass Input

A 470pF capacitor connected from the Bypass input to

ground reduces noise present on the internal reference,

which in turn significantly reduces output noise. If output

noise is not a concern, this input may be left unconnected.

Larger capacitor values may be used, but results in a longer

time period to rated output voltage when power is initially

applied.

Thermal Considerations

Thermal Shutdown

Integrated thermal protection circuitry shuts the regula-

tor off when die temperature exceeds 150°C. The regulator

remains off until the die temperature drops to approximately

140°C.

Power Dissipation

The amount of power the regulator dissipates is prima-

rily a function of input and output voltage, and output current.

The following equation is used to calculate worst case actual

power dissipation:

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 (␪JA). The 8-Pin

SOIC package has a ␪JA of approximately 160°C/Watt,

while the 8-Pin MSOP package has a ␪JA of approximately

200°C/Watt; both when mounted on a single layer FR4

dielectric copper clad PC board.

PDMAX = (TJMAX – TAMAX)

␪JA

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 ± 10%

VOUTMIN= 2.7V ± 0.5%

ILOADMAX = 250mA

TJMAX = 125°C

TAMAX = 55°C

␪JA = 200°C/W

8-Pin MSOP Package

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)]250 x 10-3

= 155mW

Maximum allowable power dissipation:

PD ≈ (TJMAX– TAMAX)

␪JA

= (125 – 55)

200

= 350mW

In this example, the TC1173 dissipates a maximum of

only 155mW; far below the allowable limit of 350mW. 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 substitut-

ing the maximum allowable power dissipation of 350mW

into Equation 1, from which VINMAX = 4.1V.

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 ␪JA and, therefore, increase the

maximum allowable power dissipation limit.

TC1173-2 2/2/00

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