MIC4682 Datasheet, PDF (11/15 Pages) Micrel Semiconductor – PRECISION CURRENT LIMIT SO-8 SUPERSWITCHER BUCK REGULATOR

MIC4682
Thermal Considerations
The MIC4682 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 MIC4682 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 2, 6, 7 and the die-
attach paddle are a single piece of metal. The die is attached
to the paddle with thermally conductive adhesive. This pro-
vides a low thermal resistance path from the junction of the
die to the ground pins. This design significantly improves
package power dissipation by allowing excellent heat trans-
fer through the ground leads to the printed circuit board.
One limitation of the maximum output current on any MIC4682
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
θ_CA = case-to-ambient thermal resistance
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 2, 6 and 7 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
There are two methods of determining the minimum ground
plane area required by the MIC4682.
Quick Method
Make sure that MIC4682 pins 2, 6 and 7 are connected to a
ground plane with a minimum area of 6cm². This ground
plane should be as close to the MIC4682 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
SOP-8
θ_JA
θ_JC
θ_CA
ground plane
AMBIENTheat sink area
printed circuit board
Figure 2. Power SOP-8 Cross Section
Micrel
pins 2, 6 and 7. This ground plane area is more than sufficient
for most designs.
Minimum Copper/Maximum Current Method
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.
1.5
12V
8V
1.0
24V
V_IN = 30V
0.5
T_A = 50°C
0
0
5
10 15 20 25
AREA (cm²)
Figure 3. Output Current vs. Ground Plane Area
When designing with the MIC4682, it is a good practice to
connect pins 2, 6 and 7 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_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: Efficiency vs. Out-
put Current” graph, read the efficiency (η) for 1A output
current at V_IN = 12V or perform you own measurement.
η = 81%
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_OUT
η
− P_OUT
P_D
=
5W _−
0.81
5W
P_D = 1.17W
A worst-case rule of thumb is to assume that 80% of the total
output power dissipation is in the MIC4682 (P_D(IC)) and 20%
is in the diode-inductor-capacitor circuit.
P_D(IC) = 0.8 P_D
P_D(IC) = 0.8 × 1.17W
P_D(IC) = 0.936W
Calculate the worst-case junction temperature:
T_J = P_D(IC) θ_JC + (T_C – T_A) + T_A(max)
where:
T_J = MIC4682 junction temperature
P_D(IC) = MIC4682 power dissipation
θ_JC = junction-to-case thermal resistance.
The θ_JC for the MIC4682’s power-SOP-8 is approximately
20°C/W.
October 2003
11
M0334-102203