MIC5236
1N4148
200k
MIC5236
V
IN
5V
IN
200k
EN
SHUTDOWN
ENABLE
Micrel
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擄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.
900
COPPER AREA (mm
2
)
V
ERR
V
OUT
C
OUT
OUT
ERR
GND
4.7碌F
700
600
500
400
300
200
100
0
0
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 SOP-8 and power MSOP-8
allow the device to dissipate about 50% more power than
their standard equivalents.
Power SOP-8 Thermal Characteristics
0.25 0.50 0.75 1.00 1.25 1.50
POWER DISSIPATION (W)
One of the secrets of the MIC5236鈥檚 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
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,
胃
JC
(junction-to-case thermal resistance) and
胃
CA
(case-to-ambi-
ent thermal resistance). See Figure 5.
胃
JC
is the resistance
from the die to the leads of the package.
胃
CA
is the resistance
from the leads to the ambient air and it includes
胃
CS
(case-to-
sink thermal resistance) and
胃
SA
(sink-to-ambient thermal
resistance).
Figure 6. Copper Area vs. Power-SOP
Power Dissipation
(鈭員
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 maxi-
mum allowable temperature rise must be calculated to deter-
mine operation along which curve.
鈭員
= T
J(max)
鈥?T
A(max)
T
J(max)
= 125擄C
T
A(max)
= maximum ambient operating temperature
For example, the maximum ambient temperature is 50擄C, the
鈭員
is determined as follows:
鈭員
= 125擄C 鈥?50擄C
鈭員
= 75擄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
鈥?V
OUT
) I
OUT
+ V
IN
路 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 鈥?3V)
脳
25mA + 28V
脳
250碌A(chǔ)
P
D
= 625mW + 7mW
P
D
= 632mW
From Figure 6, the minimum amount of copper required to
operate this application at a
鈭員
of 75擄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:
25擄C, 50擄C and 85擄C. From these curves, the minimum
10
November 2000
SOP-8
胃
JA
胃
JC
胃
CA
AMBIENT
ground plane
heat sink area
printed circuit board
Figure 5. Thermal Resistance
Using the power SOP-8 reduces the
胃
JC
dramatically and
allows the user to reduce
胃
CA
. The total thermal resistance,
胃
JA
(junction-to-ambient thermal resistance) is the limiting
factor in calculating the maximum power dissipation capabil-
ity of the device. Typically, the power SOP-8 has a
胃
JC
of
20擄C/W, this is significantly lower than the standard SOP-8
which is typically 75擄C/W.
胃
CA
is reduced because pins 5
MIC5236
40擄C
50擄C
55擄C
65擄C
75擄C
85擄C
800
100擄C
Figure 4. Remote Enable with Short-Circuit
Current Foldback
MICREL
1
2
3
4
5
6
7
8
9
10
11
12
