MIC3975

Micrel

MIC3975

10

February 2003

Adjustable Regulator Design

IN

R1

R2

EN

OUT

ADJ

GND

MIC3975

ENABLE

SHUTDOWN

V

1.240V 1

R1

R2









Figure 2. Adjustable Regulator with Resistors

The MIC3975 allows programming the output voltage any-

where between 1.24V and the 16V maximum operating rating

of the family. Two resistors are used. Resistors can be quite

large, up to 1M

Ω, because of the very high input impedance

and low bias current of the sense comparator: The resistor

values are calculated by:

R1 R2

V

1.240

1

OUT

=−









Where V

component definition. Applications with widely varying load

currents may scale the resistors to draw the minimum load

current required for proper operation (see above).

Power MSOP-8 Thermal Characteristics

One of the secrets of the MIC3975’s performance is its power

MSO-8 package featuring half the thermal resistance of a

standard MSO-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 3.

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).

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 160

°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.

θ

JA

θ

JC

θ

CA

printed circuit board

ground plane

heat sink area

MSOP-8

AMBIENT

Figure 3. Thermal Resistance

Figure 4 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.

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 4. Copper Area vs. Power-MSOP

Power Dissipation (

∆∆∆∆∆T

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)

85

°C

50

°C 25°C

T

J

= 125

°C

Figure 5. Copper Area vs. Power-MSOP

Power Dissipation (T