MIC5236

Micrel, Inc.

MIC5236

10

July 2005

5V

IN

MIC5236

EN

200k

1N4148

200k

4.7µF

OUT

GND

SHUTDOWN

ENABLE

ERR

Figure 4. Remote Enable with Short-Circuit

Current Foldback

Thermal Characteristics

The MIC5236 is a high input voltage device, intended to

provide 150mAof continuous output current in two very small

profile packages. The power SOIC-8 and power MSOP-8 al-

low the device to dissipate about 50% more power than their

standard equivalents.

Power SOIC-8 Thermal Characteristics

One of the secrets of the MIC5236’s performance is its power

SO-8 package featuring half the thermal resistance of a

standard SO-8 package. Lower thermal resistance means

more output current or higher input voltage for a given pack-

age size.

Lower thermal resistance is achieved by joining the four

ground leads with the die attach paddle to create a single-

piece electrical and thermal conductor. This concept has

been used by MOSFET manufacturers for years, proving

very reliable and cost effective for the user.

resistance).

printed circuit board

ground plane

heat sink area

SOP-8

AMBIENT

Figure 5. Thermal Resistance

factor in calculating the maximum power dissipation capabil-

20°C/W, this is significantly lower than the standard SOIC-8

through 8 can now be soldered directly to a ground plane

whichsignificantlyreducesthecase-to-sinkthermalresistance

and sink to ambient thermal resistance.

Low-dropout linear regulators from Micrel are rated to a

maximum junction temperature of 125°C. It is important not to

exceed this maximum junction temperature during operation

of the device. To prevent this maximum junction temperature

from being exceeded, the appropriate ground plane heat sink

must be used.

0

100

200

300

400

500

600

700

800

900

0 0.25 0.50 0.75 1.00 1.25 1.50

POWER DISSIPATION (W)

Figure 6. Copper Area vs. Power-SOIC

Figure 6 shows copper area versus power dissipation with

each trace corresponding to a different temperature rise

above ambient.

Fromthesecurves,theminimumareaofcoppernecessaryfor

the part to operate safely can be determined. The maximum

allowable temperature rise must be calculated to determine

operation along which curve.

For example, the maximum ambient temperature is 50°C,

the ΔT is determined as follows:

ΔT = 125°C – 50°C

ΔT = 75°C

Using Figure 6, the minimum amount of required copper can

bedeterminedbasedontherequiredpowerdissipation.Power

dissipation in a linear regulator is calculated as follows:

If we use a 3V output device and a 28V input at moderate

output current of 25mA, then our power dissipation is as

follows:

From Figure 6, the minimum amount of copper required to

Quick Method

Determine the power dissipation requirements for the design

along with the maximum ambient temperature at which the

device will be operated. Refer to Figure 7, which shows safe

operating curves for three different ambient temperatures: