MIC4685
Micrel
The efficiency is used to determine how much of the output
Applications Information
Adjustable Regulators
power (P
) is dissipated in the regulator circuit (P ).
OUT
D
P
Adjustable regulators require a 1.235V feedback signal.
Recommended voltage-divider resistor values for common
output voltages are included in Table 1.
OUT
P =
−P
OUT
D
η
7.5W
0.84
PD =
− 7.5W
For other voltages, the resistor values can be determined
using the following formulas:
P = 1.43W
D
A worst-case rule of thumb is to assume that 80% of the total
R1
VOUT = VREF
+1
output power dissipation is in the MIC4685 (P
is in the diode-inductor-capacitor circuit.
) and 20%
R2
D(IC)
VOUT
P
P
P
= 0.8 P
D
D(IC)
D(IC)
D(IC)
R1= R2
−1
VREF
= 0.8 × 1.43W
= 1.14W
VREF = 1.235V
Calculate the worst-case junction temperature:
T = P + (T – T ) + T
Thermal Considerations
θ
J
D(IC) JC
C
A
A(max)
The MIC4685 is capable of high current due to the thermally
optimized SPAK package.
where:
T = MIC4685 junction temperature
J
OnelimitationofthemaximumoutputcurrentonanyMIC4685
P
= MIC4685 power dissipation
D(IC)
design is the junction-to-ambient thermal resistance (θ ) of
JA
θ
= junction-to-case thermal resistance.
JC
the design (package and ground plane).
Examining θ in more detail:
JA
The θ for the MIC4685’s 7-lead SPAK is approximately
2.2°C/W.
JC
θ
= (θ + θ
)
CA
JA
JC
where:
T = “pin” temperature measurement taken at the
C
θ
θ
= junction-to-case thermal resistance
= case-to-ambient thermal resistance
Tab.
JC
CA
T = ambient temperature
A
θ
θ
is a relatively constant 2.2°C/W for a 7-lead SPAK.
T
= maximum ambient operating temperature
JC
A(max)
for the specific design.
is dependent on layout and is primarily governed by the
CA
connection of pins 4, and Tab to the ground plane. The
purpose of the ground plane is to function as a heat sink.
Calculating the maximum junction temperature given a
maximum ambient temperature of 60°C:
Checking the Maximum Junction Temperature:
T = 1.14 × 2.2°C + (46°C – 25°C) + 60°C
J
For this example, with an output power (P
) of 7.5W, (5V
T = 83.5°C
OUT
J
output at 1.5A with V = 12V) and 60°C maximum ambient
IN
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. Alsosee TypicalSafeOperatingArea(SOA)
graphs on page 7.
temperature, what is the junction temperature?
Referring to the “Typical Characteristics: 5V Output Effi-
ciency” graph, read the efficiency (η) for 1.5A output current
at V = 12V or perform you own measurement.
IN
η = 84%
MIC4685
10
September 2002
MICREL [ MICREL SEMICONDUCTOR ]
