MIC5239
Thermal Characteristics
COPPER AREA (mm
2
)
Micrel
900
700
600
500
400
300
200
100
0
0
0.25 0.50 0.75 1.00 1.25 1.50
POWER DISSIPATION (W)
The MIC5239 is a high input voltage device, intended to
provide 500mA of continuous output current in two very small
profile packages. The power MSOP-8 allow the device to
dissipate about 50% more power than their standard equiva-
lents.
Power MSOP-8 Thermal Characteristics
One of the secrets of the MIC5239’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,
θ
JC
(junction-to-case thermal resistance) and
θ
CA
(case-to-ambi-
ent thermal resistance). See Figure 5.
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).
Figure 6. Copper Area vs. Power-MSOP
Power Dissipation
(∆
JA
)
(∆T
Figure 6 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.
∆T
= T
J(max)
– T
A(max)
T
J(max)
= 125°C
T
A(max)
= maximum ambient operating temperature
For example, the maximum ambient temperature is 50°C, the
∆T
is determined as follows:
∆T
= 125°C – 50°C
∆T
= 75°C
Using Figure 6, 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:
P
D
= (V
IN
– V
OUT
) I
OUT
+ V
IN
· I
GND
If we use a 3V output device and a 28V input at moderate
output current of 25mA, then our power dissipation is as
follows:
P
D
= (28V – 3V)
×
25mA + 28V
×
250µA
P
D
= 625mW + 7mW
P
D
= 632mW
From Figure 6, the minimum amount of copper required to
operate this application at a
∆T
of 75°C is 110mm
2
.
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 7, which shows safe
operating curves for three different ambient temperatures:
25°C, 50°C and 85°C. From these curves, the minimum
amount of copper can be determined by knowing the maxi-
mum power dissipation required. If the maximum ambient
temperature is 50°C and the power dissipation is as above,
639mW, the curve in Figure 7 shows that the required area of
copper is 110mm
2
.
The
θ
JA
of this package is ideally 80°C/W, but it will vary
depending upon the availability of copper ground plane to
which it is attached.
MSOP-8
θ
JA
θ
JC
θ
CA
AMBIENT
ground plane
heat sink area
printed circuit board
Figure 5. 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
JC
of
80°C/W, this is significantly lower than the standard MSOP-8
which is typically 200°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 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.
January 2002
9
40°C
50°C
55°C
65°C
75°C
85°C
800
100°C
MIC5239
MICREL [ MICREL SEMICONDUCTOR ]
