MIC4685 Datasheet, PDF (10/15 Pages) Micrel Semiconductor – 3A SPAK SuperSwitcher™ Buck Regulator

MIC4685
Applications Information
Adjustable Regulators
Adjustable regulators require a 1.235V feedback signal.
Recommended 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:
V_OUT
=
V_REF

R1
R2
+ 1^{}
R1
=
R2


V_OUT
V_REF
− 1^{}
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-lead SPAK.
θ_CA is dependent on 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 Effi-
ciency” graph, read the efficiency (η) for 1.5A output current
at V_IN = 12V or perform you own measurement.
η = 84%
Micrel
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
=
7.5W
0.84
−
7.5W
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-lead 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 temperature
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 junc-
tion temperature of 125°C as listed in “Operating Ratings.”
Typical thermal shutdown is 160°C and is listed in Electrical
Characteristics. Also see Typical Safe Operating Area (SOA)
graphs on page 7.
MIC4685
10
September 2002