mic4690 Micrel Semiconductor, mic4690 Datasheet - Page 9

• Manufacturer Part Number: mic4690
• Description: Mic4690 500khz 1a Superswitcher? Buck Regulator
• Manufacturer: Micrel Semiconductor

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
the design (package and ground plane).
Examining θ
where:
θ
θ
connection of pins 5 though 8 to the ground plane. The
purpose of the ground plane is to function as a heat sink.
θ
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 6cm
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.
April 2005
MIC4690
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.
θ
θ
θ
JC
CA
JA
Figure 2. Power SOP-8 Cross Section
= junction-to-case thermal resistance
= case-to-ambient thermal resistance
= (θ
JA
JC
in more detail:
+ θ
CA
printed circuit board
SOP-8
)
θ
JC
θ
JA
θ
CA
heat sink area
ground plane
2
. This ground
JA
) of
9
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
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 (η) 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 MIC4690 (P
is in the diode-inductor-capacitor circuit.
Calculate the worst-case junction temperature:
where:
The θ
20°C/W.
entry point of pins 6 or 7
for the specific design.
Calculating the maximum junction temperature given a
maximum ambient temperature of 50°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.
IN
= 12V or perform you own measurement.
η = 75%
P
P
P
P
T
T
P
θ
JC
T
T
T
T
T
P =
P =
JC
D
D
J
J
C
A
A(max)
J
J
D
D(IC)
D(IC)
D(IC)
D(IC)
for the MIC4690’s power-SOP-8 is approximately
= P
= MIC4690 junction temperature
= 1.336W × 20°C/W + (63°C – 25°C) + 50°C
= 114.72°C
= ambient temperature
OUT
= “pin” temperature measurement taken at the
= 1.67W
= junction-to-case thermal resistance.
P
0.75
5W
= 0.8 P
= 0.8 × 1.67W
= 1.336W
D(IC)
= MIC4690 power dissipation
OUT
) is dissipated in the regulator circuit (P
η
= maximum ambient operating temperature
−
θ
−
5W
JC
P
D
OUT
+ (T
C
– T
IN
A
) + T
= 12V) and 50°C maximum
A(max)
OUT
D(IC)
) of 5W, (5V
) and 20%
MIC4690
D
).
Micrel