76121SK8 View Datasheet(PDF) - Intersil
Part Name
Description
Manufacturer
76121SK8 Datasheet PDF : 11 Pages
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HUF76121SK8
Thermal Resistance vs. Mounting Pad Area
The maximum rated junction temperature, TJM, and the ther-
mal resistance of the heat dissipating path determines the
maximum allowable device power dissipation, PDM, in an
application. Therefore the application’s ambient tempera-
ture, 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 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.
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 (compo-
nent 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 tem-
perature 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.
150
COPPER BOARD AREA - DESCENDING ORDER
0.04 in2
120 0.28 in2
0.52 in2
0.76 in2
90 1.00 in2
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 = 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 ther-
mal 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
ln RθJA = 83.2 - 23.6* (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
8
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