January 2002

9

MIC5239

MIC5239

Micrel

Thermal Characteristics

The MIC5239 is a high input voltage device, intended to

provide 500mA of continuous output current in two very small

profile packages. The power MSOP-8 allow the device to

dissipate about 50% more power than their standard equiva-

lents.

Power MSOP-8 Thermal Characteristics

One of the secrets of the MIC5239’s performance is its power

MSOP-8 package featuring half the thermal resistance of a

standard MSOP-8 package. Lower thermal resistance means

more output current or higher input voltage for a given

package 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.

Thermal resistance consists of two main elements,

θ

JC

(junction-to-case thermal resistance) and

θ

ent thermal resistance). See Figure 5.

θ

from the die to the leads of the package.

θ

from the leads to the ambient air and it includes

θ

sink thermal resistance) and

θ

resistance).

θ

JA

θ

JC

θ

CA

printed circuit board

ground plane

heat sink area

MSOP-8

AMBIENT

Figure 5. Thermal Resistance

Using the power MSOP-8 reduces the

θ

allows the user to reduce

θ

θ

factor in calculating the maximum power dissipation capabil-

ity of the device. Typically, the power MSOP-8 has a

θ

80

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

which is typically 200

°C/W. θ

through 8 can now be soldered directly to a ground plane

which significantly reduces the case-to-sink thermal resis-

tance 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 opera-

tion of the device. To prevent this maximum junction tempera-

ture 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-MSOP

Power Dissipation

(∆

(∆

(∆

(∆

(∆T

Figure 6 shows copper area versus power dissipation with

each trace corresponding to a different temperature rise

above ambient.

From these curves, the minimum area of copper necessary

for the part to operate safely can be determined. The maxi-

mum allowable temperature rise must be calculated to deter-

mine operation along which curve.

∆T = T

T

T

For example, the maximum ambient temperature is 50

°C, the

∆T is determined as follows:

∆T = 125°C – 50°C

∆T = 75°C

Using Figure 6, the minimum amount of required copper can

be determined based on the required power dissipation.

Power dissipation in a linear regulator is calculated as fol-

lows:

P

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

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

follows:

P

P

P

From Figure 6, the minimum amount of copper required to

operate this application at a

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:

25

°C, 50°C and 85°C. From these curves, the minimum

amount of copper can be determined by knowing the maxi-

mum power dissipation required. If the maximum ambient

temperature is 50

°C and the power dissipation is as above,

639mW, the curve in Figure 7 shows that the required area of

The

θ

depending upon the availability of copper ground plane to

which it is attached.