MIC4690 Datasheet, PDF (9/12 Pages) Micrel Semiconductor – 500kHz 1A SuperSwitcher Buck Regulator

MIC4690
Thermal Considerations
The MIC4690 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 MIC4690 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 MIC4690
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 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
There are two methods of determining the minimum ground
plane area required by the MIC4690.
Quick Method
Make sure that MIC4690 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 MIC4690 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
ground plane
AMBIENTheat sink area
printed circuit board
Figure 2. Power SOP-8 Cross Section
Micrel
When designing with the MIC4690, 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 50°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 = 75%
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
0.75
−
5W
P_D = 1.67W
A worst-case rule of thumb is to assume that 80% of the total
output power dissipation is in the MIC4690 (P_D(IC)) and 20%
is in the diode-inductor-capacitor circuit.
P_D(IC) = 0.8 P_D
P_D(IC) = 0.8 × 1.67W
P_D(IC) = 1.336W
Calculate the worst-case junction temperature:
T_J = P_D(IC) θ_JC + (T_C – T_A) + T_A(max)
where:
T_J = MIC4690 junction temperature
P_D(IC) = MIC4690 power dissipation
θ_JC = junction-to-case thermal resistance.
The θ_JC for the MIC4690’s power-SOP-8 is approximately
20°C/W.
T_C = “pin” temperature measurement taken at the
entry point of pins 6 or 7
T_A = ambient temperature
T_A(max) = maximum ambient operating temperature
for the specific design.
Calculating the maximum junction temperature given a
maximum ambient temperature of 50°C:
T_J = 1.336W × 20°C/W + (63°C – 25°C) + 50°C
T_J = 114.72°C
This value is within the allowable maximum operating junc-
tion temperature of 125°C as listed in “Operating Ratings.”
Typical thermal shutdown is 160°C and is listed in “Electrical
Characteristics.”
Layout Considerations
Layout is very important when designing any switching regu-
lator. Rapidly changing currents through the printed circuit
board traces and stray inductance can generate voltage
transients which can cause problems.
June 2001
9
MIC4690