PRELIMINARY INFORMATION
100mA CMOS LDO WITH SHUTDOWN
AND REFERENCE BYPASS
TC1015
In this example, the TC1015 dissipates a maximum of
only 60mW; far below the allowable limit of 318mW. In a
similar manner, Equation 1 and Equation 2 can be used to
calculate maximum current and/or input voltage limits. For
example,themaximumallowableVIN isfoundbysustituting
the maximum allowable power dissipation of 318mW into
Equation 1, from which VINMAX = 5.9V.
PD ≈ (VIN
VOUT
ILOAD
)
MIN MAX
–
MAX
Where:
PD = Worst case actual power dissipation
= Maximum voltage on VIN
= Minimum regulator output voltage
ILOADMAX = Maximum output (load) current
VIN
MAX
VOUT
MIN
Layout Considerations
Equation 1.
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.
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 (θJA). The
SOT-23A-5packagehasaθJA ofapproximately220°C/Watt
when mounted on a single layer FR4 dielectric copper clad
PC board.
PD MAX (TJMAX – TJ
)
=
MAX
θJA
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:
VIN
VOUT
= 3.0V ±10%
= 2.7V ±0.5V
MAX
MIN
ILOAD = 98mA
TAMAX = 55°C
Find: 1. Actual power dissipation
2. Maximum allowable dissipation
Actual power dissipation:
PD ≈ (VIN
VOUT
ILOAD
)
MIN MAX
–
MAX
= [(3.0 x 1.1) – (2.7 x .995)]98 x 10–3
= 60mW
Maximum allowable power dissipation:
PDMAX = (TJMAX – TAMAX
)
θJA
= (125 – 55)
220
= 318mW
4
TC1015-01-6/5/97
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