MIC4681
Thermal Considerations
The MIC4681 SuperSwitcher™ features the power-SOP-8.
This package has a standard 8-lead small-outline package
profile, but with much higher power dissipation than a stan-
dard SOP-8. Micrel's MIC4681 SuperSwitcher™ family are
the first dc-to-dc converters to take full advantage of this
package.
The reason that the power SOP-8 has higher power dissipa-
tion (lower thermal resistance) is that pins 5 through 8 and the
die-attach paddle are a single piece of metal. The die is
attached to the paddle with thermally conductive adhesive.
This provides a low thermal resistance path from the junction
of the die to the ground pins. This design significantly im-
proves package power dissipation by allowing excellent heat
transfer through the ground leads to the printed circuit board.
One limitation of the maximum output current on any MIC4681
design is the junction-to-ambient thermal resistance (θ
JA
) of
the design (package and ground plane).
Examining
θ
JA
in more detail:
θ
JA
= (θ
JC
+
θ
CA
)
where:
θ
JC
= junction-to-case thermal resistance
Minimum Copper/Maximum Current Method
Micrel
Using Figure 3, for a given input voltage range, determine the
minimum ground-plane heat-sink area required for the
application’s maximum continuous output current. Figure 3
assumes a constant die temperature of 75°C above ambient.
CONTINUOUS OUTPUT CURRENT (A)
1.5
8V
1.0
12V
24V
V
IN
= 30V
0.5
T
A
= 50°C
0
0
5
10
AREA
15
(cm
2
)
20
25
Figure 3. Output Current vs. Ground Plane Area
When designing with the MIC4681, it is a good practice to
connect pins 5 through 8 to the largest ground plane that is
practical for the specific design.
Checking the Maximum Junction Temperature:
θ
JC
is a relatively constant 20°C/W for a power SOP-8.
θ
CA
is dependent on layout and is primarily governed by the
connection of pins 5 though 8 to the ground plane. The
purpose of the ground plane is to function as a heat sink.
θ
JA
is ideally 63°C/W, but will vary depending on the size of
the ground plane to which the power SOP-8 is attached.
Determining Ground-Plane Heat-Sink Area
θ
CA
= case-to-ambient thermal resistance
There are two methods of determining the minimum ground
plane area required by the MIC4681.
Quick Method
For this example, with an output power (P
OUT
) of 5W, (5V
output at 1A maximum with V
IN
= 12V) and 65°C maximum
ambient temperature, what is the maximum junction tem-
perature?
Referring to the “Typical Characteristics: 5V Output Effi-
ciency” graph, read the efficiency (η) for 1A output current at
V
IN
= 12V or perform you own measurement.
η
= 79%
The efficiency is used to determine how much of the output
power (P
OUT
) is dissipated in the regulator circuit (P
D
).
P
D
=
P
D
=
P
OUT
η
−
P
OUT
Make sure that MIC4681 pins 5 though 8 are connected to a
ground plane with a minimum area of 6cm
2
. This ground
plane should be as close to the MIC4681 as possible. The
area may be distributed in any shape around the package or
on any pcb layer
as long as there is good thermal contact to
pins 5 though 8.
This ground plane area is more than
sufficient for most designs.
SOP-8
θ
JA
θ
JC
θ
CA
AM
BIE
ground plane
heat sink area
NT
printed circuit board
5W
−
5W
0.79
P
D
= 1.33W
A worst-case rule of thumb is to assume that 80% of the total
output power dissipation is in the MIC4681 (P
D(IC)
) and 20%
is in the diode-inductor-capacitor circuit.
P
D(IC)
= 0.8 P
D
P
D(IC)
= 0.8
×
1.33W
P
D(IC)
= 1.064W
Calculate the worst-case junction temperature:
T
J
= P
D(IC)
θ
JC
+ (T
C
– T
A
) + T
A(max)
where:
T
J
= MIC4681 junction temperature
P
D(IC)
= MIC4681 power dissipation
θ
JC
= junction-to-case thermal resistance.
The
θ
JC
for the MIC4681’s power-SOP-8 is approximately
20°C/W.
T
C
= “pin” temperature measurement taken at the
entry point of pins 6 or 7
12
April 2005
Figure 2. Power SOP-8 Cross Section
MIC4681
MICREL [ MICREL SEMICONDUCTOR ]
