MIC4681_05 MICREL [Micrel Semiconductor], MIC4681_05 Datasheet - Page 12

• Manufacturer Part Number: MIC4681_05
• Description: 2A-Peak SuperSwitcher SOP-8 Buck Regulator
• Manufacturer: MICREL [Micrel Semiconductor]

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 (
the design (package and ground plane).
Examining
where:
connection of pins 5 though 8 to the ground plane. The
purpose of the ground plane is to function as a heat sink.
the ground plane to which the power SOP-8 is attached.
Determining Ground-Plane Heat-Sink Area
There are two methods of determining the minimum ground
plane area required by the MIC4681.
Quick Method
Make sure that MIC4681 pins 5 though 8 are connected to a
ground plane with a minimum area of 6cm
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.
MIC4681
JC
CA
JA
is ideally 63 C/W, but will vary depending on the size of
is dependent on layout and is primarily governed by the
is a relatively constant 20 C/W for a power SOP-8.
JC
CA
JA
Figure 2. Power SOP-8 Cross Section
= junction-to-case thermal resistance
= case-to-ambient thermal resistance
= (
JA
JC
in more detail:
+
printed circuit board
CA
SOP-8
)
JC
JA
CA
heat sink area
ground plane
2
. This ground
JA
) of
12
Minimum Copper/Maximum Current Method
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.
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:
For this example, with an output power (P
output at 1A maximum with V
ambient temperature, what is the maximum junction tem-
perature?
Referring to the “Typical Characteristics: 5V Output Effi-
ciency” graph, read the efficiency ( ) for 1A output current at
V
The efficiency is used to determine how much of the output
power (P
A worst-case rule of thumb is to assume that 80% of the total
output power dissipation is in the MIC4681 (P
is in the diode-inductor-capacitor circuit.
Calculate the worst-case junction temperature:
where:
The
20 C/W.
entry point of pins 6 or 7
Using Figure 3, for a given input voltage range, determine the
IN
Figure 3. Output Current vs. Ground Plane Area
= 12V or perform you own measurement.
P
P
P
P
T
T
P
JC
T
P =
P =
JC
D
D
J
J
C
D
D(IC)
D(IC)
D(IC)
D(IC)
= 79%
for the MIC4681’s power-SOP-8 is approximately
= P
= MIC4681 junction temperature
OUT
= “pin” temperature measurement taken at the
= 1.33W
= junction-to-case thermal resistance.
P
0.79
5W
= 0.8 P
= 0.8
= 1.064W
D(IC) JC
= MIC4681 power dissipation
OUT
) is dissipated in the regulator circuit (P
1.5
1.0
0.5
0
0
5W
8V
P
D
1.33W
OUT
+ (T
24V
5
12V
C
V
IN
– T
AREA (cm
= 30V
10
IN
A
) + T
= 12V) and 65 C maximum
15
2
)
A(max)
T
A
20
= 50 C
OUT
D(IC)
25
) of 5W, (5V
) and 20%
April 2005
D
Micrel
).