MIC4685WREV MICREL [Micrel Semiconductor], MIC4685WREV Datasheet - Page 11

• Manufacturer Part Number: MIC4685WREV
• Description: 3A SPAK SuperSwitcher Buck Regulator
• Manufacturer: MICREL [Micrel Semiconductor]

Application Information
Adjustable Regulators
Adjustable regulators require a 1.235V feedback signal.
Recommended
common output voltages are detailed in Table 1.
For other voltages, the resistor values can be
determined using the following formulas:
Thermal Considerations
The MIC4685 is capable of high current due to the
thermally optimized SPAK package.
One limitation of the maximum output current on any
MIC4685 design is the junction-to-ambient thermal
resistance (θ
plane).
Examining θ
where:
θ
θ
by the connection of pins 4, and Tab to the ground
plane. The purpose of the ground plane is to function as
a heat sink.
Checking the Maximum Junction Temperature:
For this example, with an output power (P
(5V output at 1.5A with V
ambient temperature, what is the junction temperature?
Referring to the “Typical Characteristics: 5V Output
Efficiency” graph, read the efficiency (η) for 1.5A output
current at V
Micrel, Inc.
JC
CA
January 2010
is a relatively constant 2.2°C/W for a 7-pin SPAK.
is dependent upon layout and is primarily governed
θ
θ
θ
η = 84%
R1
V
JA
JC
CA
V
OUT
REF
=
= (θ
IN
= junction-to-case thermal resistance
= case-to-ambient thermal resistance
JA
R2
JA
= 12V or perform you own measurement.
=
in more detail:
= 1.235V
) of the design (package and ground
JC
⎛
⎜ ⎜
⎝
V
V
voltage-divider
V
REF
+ θ
OUT
REF
CA
⎛
⎜
⎝
R2
R1
)
−
1
IN
+
⎞
⎟ ⎟
⎠
1
= 12V) and 60°C maximum
⎞
⎟
⎠
resistor
OUT
values
) of 7.5W,
for
11
The efficiency is used to determine how much of the
output power (P
(P
A worst-case rule of thumb is to assume that 80% of the
total output power dissipation is in the MIC4685 (P
and 20% is in the diode-inductor-capacitor circuit.
Calculate the worst-case junction temperature:
where:
The θ
2.2°C/W.
Calculating the maximum junction temperature given a
maximum ambient temperature of 60°C:
This value is within the allowable maximum operating
junction temperature of 125°C as listed in “Operating
Ratings.” Typical thermal shutdown is 160°C and is
listed in “Electrical Characteristics.” Also refer to the
“Typical Safe Operating Area (SOA)” graphs in this
document.
D
).
JC
P
P
P
P
T
T
P
θ
T
the Tab.
T
T
erature for the specific design.
T
T
P
P
for the MIC4685’s 7-pin SPAK is approximately
JC
J
J
C
A
A(max)
J
J
D
D(IC)
D(IC)
D(IC)
D(IC)
D
D
= P
= MIC4685 junction temperature
= 1.14 × 2.2°C + (46°C – 25°C) + 60°C
= 83.5°C
= ambient temperature
= 1.43W
= junction-to-case thermal resistance.
= “pin” temperature measurement taken at
=
=
= 0.8 P
= 0.8 × 1.43W
= 1.14W
= MIC4685 power dissipation
P
7.5W
D(IC)
0.84
OUT
= maximum ambient operating temp-
η
OUT
θ
) is dissipated in the regulator circuit
JC
−
−
D
P
7
+ (T
OUT
5 .
W
C
– T
A
) + T
A(max)
M9999-012610
MIC4685
D(IC)
)