50mA ADJUSTABLE CMOS
LDO WITH SHUTDOWN
PRELIMINARY INFORMATION
TC1070
Equation 2 can be used in conjunction with Equation 3
to ensure regulator thermal operation is within limits. For
example:
Given:
V
INMAX
= 3.0V
±10%
V
OUTMIN
= 2.7V
±0.5V
I
LOAD
= 40mA
T
AMAX
= 55°C
1. Actual power dissipation
2. Maximum allowable dissipation
Find:
Actual power dissipation:
P
D
≈
(V
INMAX
– V
OUTMIN
)I
LOADMAX
= [(3.0 x 1.05) – (2.7 x .995)]40 x 10
–3
= 18.5mW
Maximum allowable power dissipation:
P
DMAX
= (T
JMAX
– T
AMAX
)
θ
JA
= (125 – 55)
220
= 318mW
In this example, the TC1070 dissipates a maximum of
only 60mW; far below the allowable limit of 318mW. In a
similar manner, Equation 2 and Equation 3 can be used to
calculate maximum current and/or input voltage limits.
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
θ
JA
and therefore increase the
maximum allowable power dissipation limit.
TC1070-01
6/13/97
5
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