MIC4684YM データシートの表示（PDF） - Micrel
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MIC4684YM Datasheet PDF : 16 Pages
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Micrel, Inc.
Applications Information
Adjustable Regulators
Adjustable regulators require a 1.23V feedback signal. Rec-
ommended voltage-divider resistor values for common output
voltages are included in Table 1.
For other voltages, the resistor values can be determined
using the following formulas:
VOUT
= VREF
R1
R2
+ 1
R1
=
R2
VOUT
VREF
− 1
VREF = 1.235V
Minimum Pulse Width
The minimum duty cycle of the MIC4684 is approximately
10%. See Minimum Duty Cycle Graph. If this input-to-output
voltage characteristic is exceeded, the MIC4684 will skip
cycles to maintain a regulated VOUT.
Max. VIN for a Given VOUT for
Constant-Frequency Switching
40
35
30
25
20
15
10
5
00 1 2 3 4 5 6
OUTPUT VOLTAGE (V)
Figure 1. Minimum Pulse Width Characteristic
Thermal Considerations
The MIC4684 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 standard
SOP‑8. Micrel’s MIC4684 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, and 7 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 limitation of the maximum output current on any MIC4684
design is the junction-to-ambient thermal resistance (θJA) of
the design (package and ground plane).
MIC4684
Examining θJA in more detail:
θJA = (θJC + θCA)
where:
θJC = junction-to-case thermal resistance
θCA = case-to-ambient thermal resistance
θJC is a relatively constant 25°C/W for a power SOP-8.
θCA is dependent on layout and is primarily governed by the
connection of pins 2, 6, and 7 to the ground plane. The pur-
pose of the ground plane is to function as a heat sink.
θJA is ideally 75°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
Make sure that MIC4684 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 MIC4684 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 2, 6, and 7. This ground plane area is more than suf-
ficient for most designs.
SOP-8
θJA
θJC
θCA
ground plane
AMBIENTheat sink area
printed circuit board
Figure 2. Power SOP-8 Cross Section
When designing with the MIC4684, 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 (POUT) of 5W, (5V
output at 1A with VIN = 12V) and 60°C maximum ambient
temperature, what is the junction temperature?
Referring to the “Typical Characteristics: 5V Output Efficiency”
graph, read the efficiency (η) for 1A output current at VIN =
12V or perform you own measurement.
η = 84%
The efficiency is used to determine how much of the output
power (POUT) is dissipated in the regulator circuit (PD).
PD
=
POUT
η
− POUT
PD
= 5W
0.84
− 5W
PD = 0.95W
January 2010
10
M9999-012610
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