MIC4680_08 Datasheet, PDF (11/15 Pages) Micrel Semiconductor – 1A 200kHz SuperSwitcher Buck Regulator

Micrel, Inc.
Thermal Considerations
The MIC4680 SuperSwitcher features the power-SOIC-
8. This package has a standard 8-pin small-outline
package profile but with much higher power dissipation
than a standard SOIC-8. The MIC4680 SuperSwitcher is
the first dc-to-dc converter to take full advantage of this
package.
The reason that the power SOIC-8 has higher power
dissipation (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 improves 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
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 SOIC-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 SOIC-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 6cm². This
ground plane should be as close to the MIC4680 as
possible. The area maybe 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.
SOIC-8
MIC4680
JA
ground plane
JC
CA _{AMBIENTheat sink area}
printed circuit board
Figure 2. Power SOIC-8 Cross Section
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 output current.
Figure 3 assumes a constant die temperature of 75°C
above ambient.
1.5
12V
8V
1.0
24V
34V
0.5
T_A = 50°C
Minimum Current Limit = 1.3A
0
0
5
10
15
20 25
AREA (cm²)
Figure 3. Output Current vs. Ground Plane Area
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_OUT) of 5W, (5V
output at 1A maximum with V_IN = 12V) and 65°C
maximum ambient temperature, what is the maximum
junction temperature?
Referring to the “Typical Characteristics: 5V Output
Efficiency” 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).
March 2008
11
M9999-032808