76112SK8 View Datasheet(PDF) - Intersil
Part Name
Description
Manufacturer
76112SK8 Datasheet PDF : 12 Pages
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HUF76112SK8
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--Z-M--θ----J-–---A-T----A-----)
(EQ. 1)
In using surface mount devices such as the SO8 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.
Intersil 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
Intersil 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
150
COPPER BOARD AREA - DESCENDING ORDER
0.04 in2
120 0.28 in2
0.52 in2
0.76 in2
90 1.00 in2
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 coefficient
added to a constant. The area, in square inches is the top
copper area including the gate and source pads.
RθJA = 83.2 – 23.6 × ln (Area)
(EQ. 2)
The transient thermal impedance (ZθJA) is also effected by
varied top copper board area. Figure 24 shows the effect of
copper pad area on single pulse transient thermal
impedance. Each trace represents a copper pad area in
square inches corresponding to the descending list in the
graph. Spice and SABER thermal models are provided for
each of the listed pad areas.
Copper pad area has no perceivable effect on transient
thermal impedance for pulse widths less than 100ms. For
pulse widths less than 100ms the transient thermal
impedance is determined by the die and package. Therefore,
CTHERM1 through CTHERM5 and RTHERM1 through
RTHERM5 remain constant for each of the thermal models. A
listing of the model component values is available in Table 1.
240
RθJA = 83.2 - 23.6*ln(AREA)
200
189oC/W - 0.0115in2
160
152oC/W - 0.054in2
120
80
0.01
0.1
1.0
AREA, TOP COPPER AREA (in2)
FIGURE 23. THERMAL RESISTANCE vs MOUNTING PAD AREA
60
30
0
10-1
100
101
102
103
t, RECTANGULAR PULSE DURATION (s)
FIGURE 24. THERMAL IMPEDANCE vs MOUNTING PAD AREA
7
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