FDD8896_12 FAIRCHILD [Fairchild Semiconductor], FDD8896_12 Datasheet - Page 7

• Manufacturer Part Number: FDD8896_12
• Manufacturer: FAIRCHILD [Fairchild Semiconductor]

FDD8896_F085 Rev. C2
Thermal Resistance vs. Mounting Pad Area
The maximum rated junction temperature, T
thermal resistance of the heat dissipating path determines
the maximum allowable device power dissipation, P
application.
temperature, T
must be reviewed to ensure that T
Equation 1 mathematically represents the relationship and
serves as the basis for establishing the rating of the part.
In using surface mount devices such as the TO-252
package, the environment in which it is applied will have a
significant influence on the part’s current and maximum
power dissipation ratings. Precise determination of P
complex and influenced by many factors:
1. Mounting pad area onto which the device is attached and
2. The number of copper layers and the thickness of the
3. The use of external heat sinks.
4. The use of thermal vias.
5. Air flow and board orientation.
6. For non steady state applications, the pulse width, the
Fairchild provides thermal information to assist the
designer’s preliminary application evaluation. Figure 21
defines the R
copper (component side) area. This is for a horizontally
positioned FR-4 board with 1oz copper after 1000 seconds
of steady state power with no air flow. This graph provides
the necessary information for calculation of the steady state
junction
applications can be evaluated using the Fairchild device
Spice thermal model or manually utilizing the normalized
maximum transient thermal impedance curve.
Thermal resistances corresponding to other copper areas
can be obtained from Figure 21 or by calculation using
Equation 2 or 3. Equation 2 is used for copper area defined
in inches square and equation 3 is for area in centimeters
square. The area, in square inches or square centimeters is
the top copper area including the gate and source pads.
R θJA
R θJA
P DM
whether there is copper on one side or both sides of the
board.
board.
duty cycle and the transient thermal response of the part,
the board and the environment they are in.
=
=
=
(
-----------------------------
temperature
T JM T A
33.32
33.32
R
θJA
θJA
A
–
(
Therefore
+
+
o
for the device as a function of the top
C), and thermal resistance R
)
------------------------------------ -
(
--------------------------------- -
(
0.268
1.73
23.84
154
or
+
+
Area
Area
the
power
)
)
Area in Centimeters Squared
application’s
JM
dissipation.
is never exceeded.
Area in Inches Squared
JM
θJA
, and the
(EQ. 1)
(EQ. 2)
(EQ. 3)
DM
ambient
(
o
, in an
Pulse
DM
C/W)
is
7
125
100
Figure 21. Thermal Resistance vs Mounting
75
50
25
(0.0645)
0.01
AREA, TOP COPPER AREA in
(0.645)
0.1
R
Pad Area
θJA
R
θJA
= 33.32+ 23.84/(0.268+Area) EQ.2
= 33.32+ 154/(1.73+Area) EQ.3
(6.45)
1
2
www.fairchildsemi.com
(cm
2
)
(64.5)
10