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TC1269 Datasheet(PDF) 5 Page - Microchip Technology

Part No. TC1269
Description  300mA CMOS LDO with Shutdown and VREF Bypass
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Maker  MICROCHIP [Microchip Technology]
Homepage  http://www.microchip.com
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TC1269 Datasheet(HTML) 5 Page - Microchip Technology

 
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© 2002 Microchip Technology Inc.
DS21380B-page 5
TC1269
4.0
THERMAL CONSIDERATIONS
4.1
Thermal Shutdown
Integrated
thermal
protection
circuitry
shuts
the
regulator off when die temperature exceeds 150°C.
The regulator remains off until the die temperature
drops to approximately 140°C.
4.2
Power Dissipation
The amount of power the regulator dissipates is
primarily a function of input and output voltage, and
output current. The following equation is used to
calculate worst case actual power dissipation:
EQUATION 4-1:
The
maximum
allowable
power
dissipation
(Equation 4-2) is a function of the maximum ambient
temperature (TAMAX), the maximum allowable die
temperature (TJMAX) and the thermal resistance from
junction-to-air (
θJA).
EQUATION 4-2:
Equation 4-1
can
be
used
in
conjunction
with
Equation 4-2 to ensure regulator thermal operation is
within limits. For example:
Given:
VINMAX
= 3.0V ± 10%
VOUTMIN =2.7V – 2.5%
ILOAD
= 250mA
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 .975)]250 x 10-3
= 167mW
Maximum allowable power dissipation:
In this example, the TC1269 dissipates a maximum of
167mW; below the allowable limit of 350mW. In a
similar manner, Equation 4-1 and Equation 4-2 can be
used to calculate maximum current and/or input
voltage limits.
4.3
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.
Where:
PD ≈ (VINMAX –VOUTMIN)ILOADMAX
PD
VINMAX
VOUTMIN
ILOADMAX
= Worst case actual power dissipation
= Minimum regulator output voltage
= Maximum output (load) current
= Maximum voltage on VIN
PDMAX =(TJMAX – TAMAX)
θJA
Where all terms are previously defined.
PDMAX =(TJMAX –TAMAX)
θJA
= (125 – 55)
200
= 350mW


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