MIC4680 Micrel Semiconductor, MIC4680 Datasheet - Page 11

• Manufacturer Part Number: MIC4680
• Description: 1A 200kHz SuperSwitcher Buck Regulator Final Information
• Manufacturer: Micrel Semiconductor

Thermal Considerations
The MIC4680 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. The MIC4680 SuperSwitcher is the first dc-to-dc
converter 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 though 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 of the limitation of the maximum output current on any
MIC4680 design is the junction-to-ambient thermal resis-
tance (
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.
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 MIC4680.
Quick Method
Make sure that MIC4680 pins 5 though 8 are connected to a
ground plane with a minimum area of 6cm
plane should be as close to the MIC4680 as possible. The
area maybe disributed 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.
June 2000
MIC4680
JC
CA
JA
is ideally 63 C/W but will vary depending on the size of the
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
JA
Figure 2. Power SOP-8 Cross Section
= junction-to-case thermal resistance
= case-to-ambient thermal resistance
) of the design (package and ground plane).
= (
JA
JC
in more detail:
+
printed circuit board
CA
SOP-8
)
JC
JA
CA
heat sink area
ground plane
2
. This ground
11
Minimum Copper/Maximum Current Method
minimum ground-plane heat-sink area required for the
application’s maximum output current. Figure 3 assumes a
constant die temperature of 75 C above ambient.
When designing with the MIC4680, 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 MIC4680 (P
is in the diode-inductor-capacitor circuit.
Calculate the worst-case junction temperature:
where:
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
T
P =
P =
JC
D
D
J
J
C
D
D(IC)
D(IC)
D(IC)
D(IC)
= 79%
= P
= MIC4680 junction temperature
The
approximately 20 C/W. (Also see Figure 1.)
entry point of pins 6 or 7 into the plastic package
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
= MIC4680 power dissipation
OUT
) is dissipated in the regulator circuit (P
JC
1.5
1.0
0.5
0
for the MIC4680’s power-SOP-8 is
0
5W
P
8V
D
1.33W
Minimum Current Limit = 1.3A
OUT
+ (T
24V
5
12V
C
34V
– T
AREA (cm
10
IN
A
) + T
= 12V) and 65 C maximum
15
2
T
)
A
A(max)
= 50 C
20
OUT
25
D(IC)
) of 5W, (5V
) and 20%
MIC4680
D
).
Micrel