LX8415-00 Просмотр технического описания (PDF) - Microsemi Corporation
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LX8415-00 Datasheet PDF : 7 Pages
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LX8415-xx
PRODUCT DATABOOK 1996/1997
0.5A LOW DROPOUT POSITIVE REGULATORS
PR E L I M I N A RY D ATA S H E E T
APPLICATION NOTES
THERMAL CONSIDERATIONS (continued)
Example
Given: V = 5.0V ±5%, V = 2.5V ±3%
IN
OUT
I = 0.5A, T = 55°C, T = 125°C
OUT
A
J
RθJT = 15°C/W, RθTS = 5°C/W
Find: The size of a square area of 1oz. copper circuit-
board trace-foil that will serve as a heatsink,
adequate to maintain the junction temperature of the
LX8415 in the ST (SOT-223) package within
specified limits.
Solution: The junction temperature is:
T
J
=
P
D
(RθJT
+
RθCS
+
R θSA)
+
T
A
where: PD ≡ Dissipated power.
RθJT ≡ Thermal resistance from the junction to the
mounting tab of the package.
RθTS ≡ Thermal resistance through the interface
between the IC and the surface on which
it is mounted.
RθSA ≡ Thermal resistance from the mounting surface
of the heatsink to ambient.
TS ≡ Heat sink temperature.
TJ
TC
TS
TA
Rq JT
Rq CS
Rq SA
First, find the maximum allowable thermal resistance of the
heat sink:
P
D
=
[[V
IN
*
(1
+ Tol )]
VIN
-
[V
OUT
*
(1
- Tol )]]
VOUT
*
I
OUT
PD = 1.4W
T -T
RθSA =
J
P
A - (RθJT + RθTS) ,
D
RθSA = 29.6°C/W
A test was conducted to determine the thermal characteristics of
1 oz. copper circuit-board trace material. The following equation
describes the observed relationship between the area of a square
copper pad, and the thermal resistance from the tab of a SOT-223
package soldered at the center of the pad to ambient.
3.1°C/W
AreaSINK = RθSA - 22.3°C/W in2
Substituting the value for RθSA calculated above, we find that a
square pad with area:
Area = 0.43 in2 (0.66" x 0.66"), 280mm2 (17 x 17 mm)
SINK
will be required to maintain the LX8415 junction temperature
within specified limits.
6
Copyright © 1999
Rev. 0.4 1/99
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