LP2960 Datasheet, PDF (13/17 Pages) National Semiconductor (TI) – Adjustable Micropower 0.5A Low-Dropout Regulators

Application Hints (Continued)
The next parameter which must be calculated is the maxi-
mum allowable temperature rise, T_R (max). This is calcu-
lated by using the formula:
where:
T_R (max) = T_J (max) − T_A (max)
T_J (max) is the maximum allowable junction temperature,
which is 125˚C for commercial grade parts.
T_A (max) is the maximum ambient temperature which will be
encountered in the application.
Using the calculated values for T_R (max) and P_D, the maxi-
mum allowable value for the junction-to-ambient thermal re-
sistance, θ_(J−A), can now be found:
θ_(J−A) = T_R (max)/P_D
The heatsink for the LP2960 is made using the PC board
copper, with the heat generated on the die being conducted
through the lead frame and out to the pins which are sol-
dered to the PC board.
The GND pins are the only ones capable of conducting any
significant amount of heat, as they are internally attached to
the lead frame on which the die is mounted.
The figure below shows recommended copper foil patterns
to be used for heatsinking the DIP and Surface Mount pack-
ages:
Heat Sink Foil Patterns
DS011962-31
The table below shows measured values of θ_(J−A) for a PC
board with 1 ounce copper weight:
Package
DIP
L (in.)
1
2
H (in.)
0.5
0.2
θ_J−A(˚C/W)
50
52
Surface
1
0.5
72
Mount
2
0.2
74
As the heat must transfer from the copper to the surrounding
air, best results (lowest θ_J−A) will be obtained by using a sur-
face copper layer with the solder resist opened up over the
heatsink area.
If an internal copper layer of a multi-layer board is used for
heatsinking, the board material acts as an insulator, inhibit-
ing heat transfer and increasing θ_J−A.
As with any heatsink, increasing the airflow across the board
will significantly improve the heat transfer.
13
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