MIC4684 Datasheet, PDF (10/16 Pages) Micrel Semiconductor – 2A High-Efficiency SuperSwitcher Buck Regulator

MIC4684
Applications Information
Adjustable Regulators
Adjustable regulators require a 1.23V feedback signal. Rec-
ommended voltage-divider resistor values for common out-
put voltages are included in Table 1.
For other voltages, the resistor values can be determined
using the following formulas:
V_OUT
=
V_REF

R1
R2
+ 1^{}
R1
=
R2


V_OUT
V_REF
− 1^{}
V_REF = 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 V_OUT.
Max. V_IN for a Given V_OUT 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 stan-
dard 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 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 MIC4684
design is the junction-to-ambient thermal resistance (θ_JA) of
the design (package and ground plane).
Micrel
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 25°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 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 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
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 (P_OUT) of 5W, (5V
output at 1A with V_IN = 12V) and 60°C maximum ambient
temperature, what is the junction temperature?
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 = 84%
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
0.84
−
5W
P_D = 0.95W
MIC4684
10
July 2001