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TC1173 Datasheet(PDF) 4 Page - TelCom Semiconductor, Inc

Part No. TC1173
Description  300mA CMOS LDO with Shutdown, ERROR Output and Bypass
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Maker  TELCOM [TelCom Semiconductor, Inc]
Homepage  http://www.telcom-semi.com
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TC1173 Datasheet(HTML) 4 Page - TelCom Semiconductor, Inc

   
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4
TC1173
300mA CMOS LDO With Shutdown,
ERROR Output, And Bypass
TC1173-2
2/2/00
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.


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