MIC4685 datasheet(10/15 Pages) MICREL

10 / 15 page

MIC4685

Micrel

MIC4685

September 2002

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:

R1 R2

1.235V

OUT

REF

OUT

REF









=−









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 (

JA) of

the design (package and ground plane).

Examining

JA in more detail:

JA = (θJC + θCA)

where:

JC = junction-to-case thermal resistance

CA = case-to-ambient thermal resistance

JC is a relatively constant 2.2°C/W for a 7-lead SPAK.

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%

The efficiency is used to determine how much of the output

power (P

OUT) is dissipated in the regulator circuit (PD).

OUT

−

7.5W

0.84

− 7.

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.

D(IC) = 0.8 PD

D(IC) = 0.8 × 1.43W

D(IC) = 1.14W

Calculate the worst-case junction temperature:

J = PD(IC) θJC + (TC – TA) + TA(max)

where:

J = MIC4685 junction temperature

D(IC) = MIC4685 power dissipation

JC = junction-to-case thermal resistance.

The

JC for the MIC4685’s 7-lead SPAK is approximately

2.2

°C/W.

C = “pin” temperature measurement taken at the

Tab.

A = ambient temperature

A(max) = maximum ambient operating temperature

for the specific design.

Calculating the maximum junction temperature given a

maximum ambient temperature of 60

°C:

J = 1.14 × 2.2°C + (46°C – 25°C) + 60°C

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.

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