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TC1306 Datasheet(PDF) 6 Page - Microchip Technology

Part No. TC1306
Description  Dual 150mA CMOS LDO With Select Mode™ Operation, Shutdown and RESET Output
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Maker  MICROCHIP [Microchip Technology]
Homepage  http://www.microchip.com
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TC1306 Datasheet(HTML) 6 Page - Microchip Technology

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TC1306
DS21527C-page 6
© 2007 Microchip Technology Inc.
5.0
THERMAL CONSIDERATIONS
5.1
Thermal Shutdown
Integrated thermal protection circuitry shuts the
regulator off when die exceeds approximately 160°C.
The regulator remains off until the die temperature
drops to approximately 145°C.
Thermal shutdown is intended to protect the device
under transient accidental (fault) overload conditions.
Thermal Shutdown may not protect the LDO while
operating above junction temperatures of 125°C
continuously. Sufficient thermal evaluation of the
design needs to be conducted to ensure that the
junction temperature does not exceed 125°C.
5.2
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 5-1:
The
maximum
allowable
power
dissipation
(Equation 5-2) 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). The MSOP-8 package has a θJA of
approximately 200°C/W when mounted on a four layer
FR4 dielectric copper clad PC board.
EQUATION 5-2:
Equation 5-1 can be used in conjunction with
Equation 5-2 to ensure regulator thermal operation is
within limits. For example:
Given:
VINMAX
= 3.8V ± 5%
VOUT1MIN = 1.8V ± 2.5%
VOUT2MIN = 3.0V ± 2.5%
ILOAD1MAX= 60mA
ILOAD2MAX= 120mA
TJMAX
= 125°C
TAMAX
= 55°C
θJA
= 200°C/W
Find: 1. Actual power dissipation
2. Maximum allowable dissipation
Actual power dissipation:
PD ≈ [(VINMAX – VOUT1MIN)] x ILOAD1MAX
+ [(VINMAX – VOUT2MIN)] x ILOAD2MAX
[(3.8 x 1.05) – (1.8 x .975)] x 60 x 10-3
+ [(3.8 x 1.05) – (3.0 x .975)] x 120 x 10-3
= 256mW
Maximum allowable power dissipation:
In this example, the TC1306 dissipates a maximum of
262mW; below the allowable limit of 350mW. In a
similar manner, Equation 5-1 and Equation 5-2 can be
used to calculate maximum current and/or input
voltage limits. For example, the maximum allowable
VIN is found by substituting the maximum allowable
power dissipation of 350mW into Equation 5-1, from
which VINMAX = 4.5V.
5.3
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 – VOUT1MIN)ILOAD1MAX +
PD
VINMAX
VOUT1MIN
ILOAD1MAX
= Worst case actual power dissipation
= Minimum regulator output voltage1
= Maximum output (load) current1
= Maximum voltage on VIN
(VINMAX – VOUT2MIN)ILOAD2MAX
VOUT2MIN
ILOAD2MAX
= Minimum regulator output voltage2
= Maximum output (load) current2
PDMAX = (TJMAX – TAMAX)
θJA
Where all terms are previously defined.
PD = (TJMAX – TAMAX)
θJA
= (125 – 55)
200
= 350mW


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