MIC3975 datasheet(10/12 Pages) MICREL

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MIC3975

Micrel

MIC3975

February 2005

Adjustable Regulator Design

VOUT

VIN

COUT

OUT

ADJ

GND

MIC3975

ENABLE

SHUTDOWN

1.240V 1 R1

OUT 











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 1MΩ, because of the very high input impedance

and low bias current of the sense comparator: The resistor

values are calculated by:

R1 R2

1.240

OUT













Where VO is the desired output voltage. Figure 2 shows

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 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.

Thermal resistance consists of two main elements, θJC (junc-

tion-to-case thermal resistance) and θCA (case-to-ambient

thermal resistance). See Figure 3. θJC is the resistance from

the die to the leads of the package. θCA is the resistance

from the leads to the ambient air and it includes θCS (case-

to-sink thermal resistance) and θSA(sink-to-ambient thermal

resistance).

Using the power MSOP-8 reduces the θJC dramatically and

allows the user to reduce θCA. The total thermal resistance,

θJA (junction-to-ambient thermal resistance) is the limiting

factor in calculating the maximum power dissipation capabil-

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

80°C/W, this is signiﬁcantly lower than the standard MSOP-8

which is typically 160°C/W. θCA is reduced because pins 5

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.

JA

JC

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.

Fromthesecurves,theminimumareaofcoppernecessaryfor

the part to operate safely can be determined. The maximum

allowable temperature rise must be calculated to determine

operation along which curve.

ΔT = TJ(max) – TA(max)

TJ(max) = 125°C

Figure 4. Copper Area vs. Power-MSOP

Power Dissipation (∆TJA)

Figure 5. Copper Area vs. Power-MSOP

Power Dissipation (TA)

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