Micrel, Inc. MIC49150
November 2006
10
M9999-111306
capacitor of at least 1µF is needed directly between the
input and regulator ground. Refer to “
Application Note 9
”
for further details and examples on thermal design and
heat sink specification.
Minimum Load Current
The MIC49150, unlike most other high current
regulators, does not require a minimum load to maintain
output voltage regulation.
Power MSOP-8 Thermal Characteristics
One of the secrets of the MIC49150’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,
JC
(junction-to-case thermal resistance) and
CA
(case-to-
ambient thermal resistance). See Figure 1.
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
(case-to-sink thermal resistance) and
SA
(sink-to-ambient thermal resistance).
Using the power MSOP-8 reduces the
JC
dramatically
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
MSOP-8 has a
JA
of 80°C/W, this is significantly lower
than the standard MSOP-8 which is typically 160°C/W.
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.
q
JA
q
JC
q
CA
printed circuit board
ground plane
heat sink area
MSOP-8
AMBIENT
Figure 1. Thermal Resistance
Figure 2 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.
0
100
200
300
400
500
600
700
800
900
0 0.25 0.50 0.75 1.00 1.25 1.50
COPPER AREA (mm
2
)
POWER DISSIPATION (W)
Figure 2. Copper Area vs. Power-MSOP
Power Dissipation (∆T
JA
)
0
100
200
300
400
500
600
700
800
900
0 0.25 0.50 0.75 1.00 1.25 1.50
COPPER AREA (mm
2
)
POWER DISSIPATION (W)
85°C 50°C 25°C
T
J
=125°C
Figure 3. Copper Area vs. Power-MSOP
Power Dissipation (T
A
)
