8

Thermal Resistance vs. Mounting Pad Area

mal resistance of the heat dissipating path determines the

application. Therefore the application’s ambient tempera-

mathematically represents the relationship and serves as

the basis for establishing the rating of the part.

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

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

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.

Thermal resistances corresponding to other copper areas can

be obtained from Figure 23 or by calculation using Equation 2.

added to a constant. The area, in square inches is the top

copper area including the gate and source pads.

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.

(EQ. 1)

P

DM

T

JM

T

A

–

()

ZθJA

-------------------------------

=

(EQ. 2)

RθJA

83.2

23.6

Area

()

ln

×

–

=

120

160

200

240

0.1

1.0

80

0.01

FIGURE 23. THERMAL RESISTANCE vs MOUNTING PAD AREA

30

60

90

120

150

0

FIGURE 24. THERMAL IMPEDANCE vs MOUNTING PAD AREA

t, RECTANGULAR PULSE DURATION (s)

COPPER BOARD AREA - DESCENDING ORDER

HUF76121SK8