76407DK8 Ver la hoja de datos (PDF) - Fairchild Semiconductor
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76407DK8
Fairchild Semiconductor
76407DK8 Datasheet PDF : 12 Pages
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HUFA76407DK8
Thermal Resistance vs. Mounting Pad Area
The maximum rated junction temperature, TJM, and the
thermal resistance of the heat dissipating path determines
the maximum allowable device power dissipation, PDM, in an
application. Therefore the application’s ambient
temperature, TA (oC), and thermal resistance RθJA (oC/W)
must be reviewed to ensure that TJM is never exceeded.
Equation 1 mathematically represents the relationship and
serves as the basis for establishing the rating of the part.
PDM
=
-(--T----J---M-------–----T----A-----)
RθJA
(EQ. 1
In using surface mount devices such as the SOP-8 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 PDM is complex
and influenced by many factors:
1. Mounting pad area onto which the device is attached and
whether there is copper on one side or both sides of the
board.
2. The number of copper layers and the thickness of the
board.
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
duty cycle and the transient thermal response of the part,
the board and the environment they are in.
Fairchild provides thermal information to assist the
designer’s preliminary application evaluation. Figure 23
defines the RθJA for the device as a function of the top
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 temperature or power dissipation. Pulse applications
can be evaluated using the Fairchild device Spice thermal
model or manually utilizing the normalized maximum
transient thermal impedance curve.
Displayed on the curve are RθJA values listed in the
Electrical Specifications table. The points were chosen to
depict the compromise between the copper board area, the
thermal resistance and ultimately the power dissipation,
PDM.
Thermal resistances corresponding to other copper areas
can be obtained from Figure 23 or by calculation using
Equation 2. RθJA is defined as the natural log of the area
times a cofficient added to a constant. The area, in square
inches is the top copper area including the gate and source
pads.
RθJA = 103.2 – 24.3 × ln (Area)
(EQ. 2)
300
RθJA = 103.2 - 24.3 * ln(AREA)
250
228 oC/W - 0.006in2
200
191 oC/W - 0.027in2
150
100
50
Rθβ = 46.4 - 21.7 * ln(AREA)
0
0.001
0.01
0.1
1
AREA, TOP COPPER AREA (in2) PER DIE
FIGURE 23. THERMAL RESISTANCE vs MOUNTING PAD AREA
While Equation 2 describes the thermal resistance of a
single die, several of the new UltraFETs are offered with two
die in the SOP-8 package. The dual die SOP-8 package
introduces an additional thermal component, thermal
coupling resistance, Rθβ. Equation 3 describes Rθβ as a
function of the top copper mounting pad area.
Rθβ = 46.4 – 21.7 × ln (Area)
(EQ. 3)
The thermal coupling resistance vs. copper area is also
graphically depicted in Figure 23. It is important to note the
thermal resistance (RθJA) and thermal coupling resistance
(Rθβ) are equivalent for both die. For example at 0.1 square
inches of copper:
RθJA1 = RθJA2 = 159oC/W
Rθβ1 = Rθβ2 = 97oC/W
TJ1 and TJ2 define the junction temerature of the respective
die. Similarly, P1 and P2 define the power dissipated in each
die. The steady state junction temperature can be calculated
using Equation 4 for die 1and Equation 5 for die 2.
Example: To calculate the junction temperature of each die
when die 2 is dissipating 0.5 Watts and die 1 is dissipating 0
Watts. The ambient temperature is 70oC and the package is
mounted to a top copper area of 0.1 square inches per die.
Use Equation 4 to calulate TJ1 and and Equation 5 to
calulate TJ2.
.
TJ1 = P1RθJA + P2Rθβ + TA
(EQ. 4)
TJ1 = (0 Watts)(159oC/W) + (0.5 Watts)(97oC/W) + 70oC
TJ1 = 119oC
©2001 Fairchild Semiconductor Corporation
HUFA76407DK8 Rev. B
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