MIC4690 datasheet(9/12 Pages) MICREL

9 / 12 page

June 2001

MIC4690

Micrel

When designing with the MIC4690, it is a good practice to

connect pins 5 through 8 to the largest ground plane that is

practical for the specific design.

Checking the Maximum Junction Temperature:

For this example, with an output power (P

OUT) of 5W, (5V

output at 1A maximum with V

IN = 12V) and 50°C maximum

ambient temperature, what is the maximum junction tem-

perature?

Referring to the “Typical Characteristics: 5V Output Effi-

ciency” graph, read the efficiency (

η) for 1A output current at

IN = 12V or perform you own measurement.

η = 75%

The efficiency is used to determine how much of the output

power (P

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

OUT

−

0.75

−

D = 1.67W

A worst-case rule of thumb is to assume that 80% of the total

output power dissipation is in the MIC4690 (P

D(IC)) and 20%

is in the diode-inductor-capacitor circuit.

D(IC) = 0.8 PD

D(IC) = 0.8 × 1.67W

D(IC) = 1.336W

Calculate the worst-case junction temperature:

J = PD(IC) θJC + (TC – TA) + TA(max)

where:

J = MIC4690 junction temperature

D(IC) = MIC4690 power dissipation

JC = junction-to-case thermal resistance.

The

JC for the MIC4690’s power-SOP-8 is approximately

°C/W.

C = “pin” temperature measurement taken at the

entry point of pins 6 or 7

A = ambient temperature

A(max) = maximum ambient operating temperature

for the specific design.

Calculating the maximum junction temperature given a

maximum ambient temperature of 50

°C:

J = 1.336W × 20°C/W + (63°C – 25°C) + 50°C

J = 114.72°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.”

Layout Considerations

Layout is very important when designing any switching regu-

lator. Rapidly changing currents through the printed circuit

board traces and stray inductance can generate voltage

transients which can cause problems.

AMBIENT

printed circuit board

ground plane

heat sink area

SOP-8

Figure 2. Power SOP-8 Cross Section

Thermal Considerations

The MIC4690 SuperSwitcher™ features the power-SOP-8.

This package has a standard 8-lead small-outline package

profile, but with much higher power dissipation than a stan-

dard SOP-8. Micrel's MIC4690 SuperSwitcher™ family are

the first dc-to-dc converters to take full advantage of this

package.

The reason that the power SOP-8 has higher power dissipa-

tion (lower thermal resistance) is that pins 5 through 8 and the

die-attach paddle are a single piece of metal. The die is

attached to the paddle with thermally conductive adhesive.

This provides a low thermal resistance path from the junction

of the die to the ground pins. This design significantly im-

proves package power dissipation by allowing excellent heat

transfer through the ground leads to the printed circuit board.

One limitation of the maximum output current on any MIC4690

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 20°C/W for a power SOP-8.

CA is dependent on layout and is primarily governed by the

connection of pins 5 though 8 to the ground plane. The

purpose of the ground plane is to function as a heat sink.

JA is ideally 63°C/W, but will vary depending on the size of

the ground plane to which the power SOP-8 is attached.

Determining Ground-Plane Heat-Sink Area

There are two methods of determining the minimum ground

plane area required by the MIC4690.

Quick Method

Make sure that MIC4690 pins 5 though 8 are connected to a

ground plane with a minimum area of 6cm2. This ground

plane should be as close to the MIC4690 as possible. The

area may be distributed in any shape around the package or

on any pcb layer

as long as there is good thermal contact to

pins 5 though 8. This ground plane area is more than

sufficient for most designs.

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