MIC3975
Adjustable Regulator Design
V
IN
ENABLE
SHUTDOWN
Micrel
MIC3975
OUT
IN
EN
ADJ
GND
R1
R2
V
OUT
C
OUT
R1
V
OUT
�½
1.240V
1
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 1MΩ, because of the very high input impedance
and low bias current of the sense comparator: The resistor
values are calculated by:
V
R1
�½
R2
OUT
1
1.240
Where V
O
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,
θ
JC
(junc-
tion-to-case thermal resistance) and
θ
CA
(case-to-ambient
thermal resistance). See Figure 3.
θ
JC
is the resistance from
the die to the leads of the package.
θ
CA
is the resistance
from the leads to the ambient air and it includes
θ
CS
(case-
to-sink thermal resistance) and
θ
SA
(sink-to-ambient thermal
resistance).
Using the power MSOP-8 reduces the
θ
JC
dramatically and
allows the user to reduce
θ
CA
. The total thermal resistance,
θ
JA
(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
θ
JA
of
80°C/W, this is significantly lower than the standard MSOP-8
which is typically 160°C/W.
θ
CA
is reduced because pins 5
through 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.
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.
MSOP-8
JA
JC
CA
AMBIENT
ground plane
heat sink area
printed circuit board
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 maximum
allowable temperature rise must be calculated to determine
operation along which curve.
ΔT = T
J(max)
– T
A(max)
T
J(max)
= 125°C
Figure 4. Copper Area vs. Power-MSOP
Power Dissipation (∆T
JA
)
Figure 5. Copper Area vs. Power-MSOP
Power Dissipation (T
A
)
MIC3975
10
February 2005
MICREL [ MICREL SEMICONDUCTOR ]
