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MIC4685_10 Datasheet, PDF (11/17 Pages) Micrel Semiconductor – 3A SPAK SuperSwitcher Buck Regulator | |||
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Micrel, Inc.
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:
VOUT
=
VREF
ââ
â
R1
R2
+
1ââ
â
R1
=
R2ââââ
VOUT
VREF
â 1âââ â
VREF = 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 (θJA) of the design (package and ground
plane).
Examining θJA in more detail:
θJA = (θJC + θCA)
where:
θJC = junction-to-case thermal resistance
θCA = case-to-ambient thermal resistance
θJC is a relatively constant 2.2°C/W for a 7-pin SPAK.
θCA 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 (POUT) of 7.5W,
(5V output at 1.5A with VIN = 12V) and 60°C maximum
ambient temperature, what is the junction temperature?
Referring to the âTypical Characteristics: 5V Output
Efï¬ciencyâ graph, read the efï¬ciency (η) for 1.5A output
current at VIN = 12V or perform you own measurement.
η = 84%
MIC4685
The efï¬ciency is used to determine how much of the
output power (POUT) is dissipated in the regulator circuit
(PD).
PD
=
POUT
η
â POUT
PD
=
7.5W
0.84
â 7.5W
PD = 1.43W
A worst-case rule of thumb is to assume that 80% of the
total output power dissipation is in the MIC4685 (PD(IC))
and 20% is in the diode-inductor-capacitor circuit.
PD(IC) = 0.8 PD
PD(IC) = 0.8 Ã 1.43W
PD(IC) = 1.14W
Calculate the worst-case junction temperature:
TJ = PD(IC) θJC + (TC â TA) + TA(max)
where:
TJ = MIC4685 junction temperature
PD(IC) = MIC4685 power dissipation
θJC = junction-to-case thermal resistance.
The θJC for the MIC4685âs 7-pin SPAK is approximately
2.2°C/W.
TC = âpinâ temperature measurement taken at
the Tab.
TA = ambient temperature
TA(max) = maximum ambient operating temp-
erature for the speciï¬c design.
Calculating the maximum junction temperature given a
maximum ambient temperature of 60°C:
TJ = 1.14 à 2.2°C + (46°C â 25°C) + 60°C
TJ = 83.5°C
This value is within the allowable maximum operating
junction temperature of 125°C as listed in âOperating
Ratings.â Typical thermal shutdown is 160°C and is
listed in âElectrical Characteristics.â Also refer to the
âTypical Safe Operating Area (SOA)â graphs in this
document.
January 2010
11
M9999-012610
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