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TC1070 Datasheet(PDF) 6 Page - Microchip Technology

Part No. TC1070
Description  50mA, 100mA and 150mA Adjustable CMOS LDOs with Shutdown
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Maker  MICROCHIP [Microchip Technology]
Homepage  http://www.microchip.com
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TC1070 Datasheet(HTML) 6 Page - Microchip Technology

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TC1070/TC1071/TC1187
DS21353B-page 6
© 2002 Microchip Technology Inc.
4.0
THERMAL CONSIDERATIONS
4.1
Thermal Shutdown
Integrated
thermal
protection
circuitry
shuts
the
regulator off when die temperature exceeds 160°C.
The regulator remains off until the die temperature
drops to approximately 150°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). The 5-Pin SOT-23A package has a θJA of
approximately 220°C/Watt.
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%
ILOADMAX =40mA
TJMAX
= 125°C
TAMAX
=55°C
Find: 1. Actual power dissipation
2. Maximum allowable dissipation
Actual power dissipation:
PD ≈ (VINMAX –VOUTMIN)ILOADMAX
= [(3.0 x 1.10) – (2.7 x .0.98)]40 x 10–3
= 26.2mW
Maximum allowable power dissipation:
In this example, the TC1070 dissipates a maximum of
26.2mW; below the allowable limit of 318mW. 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)
220
=318mW


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