Micrel, Inc.
MIC4685
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
−
P
OUT
η
Application Information
Adjustable Regulators
Adjustable regulators require a 1.235V feedback signal.
Recommended voltage-divider resistor values for
common output voltages are detailed in Table 1.
For other voltages, the resistor values can be
determined using the following formulas:
⎛
R1
⎞
V
OUT
=
V
REF
⎜
+
1
⎟
R2
⎝
⎠
⎛
V
⎞
R1
=
R2
⎜
OUT
−
1
⎟
⎜
V
⎟
⎝
REF
⎠
P
D
=
7.5W
−
7. 5 W
0.84
V
REF
= 1.235V
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
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 2.2°C/W for a 7-pin SPAK.
θ
CA
is dependent upon layout and is primarily governed
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
OUT
) of 7.5W,
(5V output at 1.5A with V
IN
= 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 1.5A output
current at V
IN
= 12V or perform you own measurement.
η
= 84%
P
D
= 1.43W
A worst-case rule of thumb is to assume that 80% of the
total output power dissipation is in the MIC4685 (P
D(IC)
)
and 20% is in the diode-inductor-capacitor circuit.
P
D(IC)
= 0.8 P
D
P
D(IC)
= 0.8 × 1.43W
P
D(IC)
= 1.14W
Calculate the worst-case junction temperature:
T
J
= P
D(IC)
θ
JC
+ (T
C
– T
A
) + T
A(max)
where:
T
J
= MIC4685 junction temperature
P
D(IC)
= MIC4685 power dissipation
θ
JC
= junction-to-case thermal resistance.
The
θ
JC
for the MIC4685’s 7-pin SPAK is approximately
2.2°C/W.
T
C
= “pin” temperature measurement taken at
the Tab.
T
A
= ambient temperature
T
A(max)
= maximum ambient operating temp-
erature for the specific design.
Calculating the maximum junction temperature given a
maximum ambient temperature of 60°C:
T
J
= 1.14 × 2.2°C + (46°C – 25°C) + 60°C
T
J
= 83.5°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.
January 2010
11
M9999-012610
MICREL [ MICREL SEMICONDUCTOR ]
