300mA CMOS LDO With Shutdown,
ERROR Output, And Bypass
TC1173
Bypass Input
Equation 1 can be used in conjunction with Equation 2
to ensure regulator thermal operation is within limits. For
example:
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.
GIVEN:
VINMAX= 3.0V ± 10%
VOUTMIN= 2.7V ± 0.5%
ILOADMAX = 250mA
TJ
TA
= 125°C
= 55°C
MAX
MAX
ꢀJA = 200°C/W
Thermal Considerations
8-Pin MSOP Package
Thermal Shutdown
FIND:
1. Actual power dissipation
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.
2. Maximum allowable dissipation
Actual power dissipation:
PD ≈ (VINMAX - VOUTMIN)ILOAD
MAX
= [(3.0 x 1.1) - (2.7 x .995)]250 x 10-3
Power Dissipation
The amount of power the regulator dissipates is prima-
rilyafunctionofinputandoutputvoltage,andoutputcurrent.
Thefollowingequationisusedtocalculateworstcaseactual
power dissipation:
= 155mW
Maximum allowable power dissipation:
J
A
MAX
PD ≈ (T MAX– T
)
ꢀJA
PD ≈ (VINMAX – VOUTMIN)ILOAD
MAX
Where:
PD = worst case actual power dissipation
VINMAX = maximum voltage on VIN
= (125 – 55)
200
VOUTMIN = minimum regulator output voltage
ILOADMAX = maximum output (load) current
= 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.
Equation 1.
The maximum allowablepower 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.
Layout Considerations
The primary path of heat conduction out of the package
isviathepackageleads. Therefore, layoutshavingaground
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.
PDMAX = (TJMAX – TA
)
MAX
ꢀJA
Where all terms are previously defined.
Equation 2.
TC1173-2 2/2/00
4
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