MIC5236 Micrel, Inc.
MIC5236 10 July 2005
C
OUT
V
OUT
V
IN
5V
V
ERR
IN
MIC5236
EN
200k
1N4148
200k
4.7µF
OUT
GND
SHUTDOWN
ENABLE
ERR
Figure 4. Remote Enable with Short-Circuit
Current Foldback
Thermal Characteristics
The MIC5236 is a high input voltage device, intended to
provide 150mA of continuous output current in two very small
profile packages. The power SOIC-8 and power MSOP-8 al-
low the device to dissipate about 50% more power than their
standard equivalents.
Power SOIC-8 Thermal Characteristics
One of the secrets of the MIC5236’s performance is its power
SO-8 package featuring half the thermal resistance of a
standard SO-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 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).
q
JA
q
JC
q
CA
printed circuit board
ground plane
heat sink area
SOP-8
AMBIENT
Figure 5. Thermal Resistance
Using the power SOIC-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 SOIC-8 has a θ
JC
of
20°C/W, this is significantly lower than the standard SOIC-8
which is typically 75°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.
0
100
200
300
400
500
600
700
800
900
0 0.25 0.50 0.75 1.00 1.25 1.50
m
m
(AERA
REP
P
OC
2
)
POWER DISSIPATION (W)
04 °C
05 °
C
55 °C
56 °C
57 °C
58 °C
001 °C
Figure 6. Copper Area vs. Power-SOIC
Power Dissipation (∆T
JA
)
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 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
T
A(max)
= maximum ambient operating temperature
For example, the maximum ambient temperature is 50°C,
the ΔT is determined as follows:
ΔT = 125°C – 50°C
ΔT = 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 follows:
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 ΔT of 75°C is 25mm
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:
