MIC79050 datasheet(13/20 Pages) MICREL

13 / 20 page

June 2000

MIC79050

Micrel

been used by MOSFET manufacturers for years, proving

very reliable and cost effective for the user.

Thermal resistance consists of two main elements,

JC, or

thermal resistance junction to case and

CA, thermal resis-

tance case to ambient (Figure 8).

JC is the resistance from

the die to the leads of the package.

CA is the resistance from

the leads to the ambient air and it includes

CS, thermal

resistance case to sink, and

SA, thermal resistance sink to

ambient. Using the power SOP-8 reduces the

JC dramati-

cally and allows the user to reduce

CA. The total thermal

resistance,

JA, junction to ambient thermal resistance, is the

limiting factor in calculating the maximum power dissipation

capability of the device. Typically, the power SOP-8 has a

of 20

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

8 which is typically 75

°C/W. θ

CA is reduced because pins 5-

8 can now be soldered directly to a ground plane, which

significantly reduces the case to sink thermal resistance and

sink to ambient thermal resistance.

printed circuit board

ground plane

heat sink area

SOP-8

AMBIENT

Figure 8. Thermal Resistance

The MIC79050 is 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.

Figure 9 shows curves of copper area versus power dissipa-

tion, each trace corresponding to different temperature rises

above ambient. From these curves, the minimum area of

copper necessary for the part to operate safely can be

determined. The maximum allowable temperature rise must

be calculated to determine operation along which curve.

100

200

300

400

500

600

700

800

900

0.25 0.50 0.75 1.00 1.25 1.50

POWER DISSIPATION (W)

∆T

JA =

Figure 9. Copper Area vs. Power-SOP

Power Dissipation

(∆T

JA)

Where

∆T = T

j(max) – Ta(max)

j(max) = 125°C

a(max) = maximum ambient operating

temperature

For example, the maximum ambient temperature is 40

°C, the

∆T is determined as follows:

∆T = +125°C – 40°C

∆T = +85°C

Using Figure 9, 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:

D = (Vin-Vout)*Iout + Vin*Ignd

For example, using the charging circuit in Figure 7, assume

the input is a fixed 5V and the output is pulled down to 4.2V

at a charge current of 500mA. The power dissipation in the

MIC79050 is calculated as follows:

D = (5V – 4.2V)*0.5A + 5V*0.012A

D = 0.460W

From Figure 9, the minimum amount of copper required to

operate this application at a

∆T of 85C is less than 50mm2.

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 10 , which shows safe

operating curves for 3 different ambient temperatures: +25

°C,

+50

°C and +85°C. From these curves, the minimum amount

of copper can be determined by knowing the maximum power

dissipation required. If the maximum ambient temperature is

+40

°C and the power dissipation is as above, 0.46W, the

curve in Figure 10 shows that the required area of copper is

50mm2.

The

JA of this package is ideally 63°C/W, but it will vary

depending upon the availability of copper ground plane to

which it is attached.

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