MIC49150
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 1.
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 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.
900
Micrel
40°C
50°C
55°C
65°C
75°C
85°C
T = 125°C
J
85°C
50°C 25°C
800
COPPER AREA (mm
2
)
700
600
500
400
300
200
100
0
0
0.25 0.50 0.75 1.00 1.25 1.50
POWER DISSIPATION (W)
Figure 2. Copper Area vs. Power-MSOP
Power Dissipation (∆T
JA
)
900
800
COPPER AREA (mm
2
)
700
600
500
400
300
200
100
0
0
0.25 0.50 0.75 1.00 1.25 1.50
POWER DISSIPATION (W)
Figure 3. Copper Area vs. Power-MSOP
Power Dissipation (T
A
)
∆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 2, 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
×
I
IN
+ V
BIAS
×
I
BIAS
– V
OUT
×
I
OUT
Using a typical application of 750mA output current, 1.2V
output voltage, 1.8V input voltage and 3.3V bias voltage, the
power dissipation is as follows:
P
D
= (1.8V)
×
(730mA) + 3.3V(30mA) – 1.2V(750mA)
At full current, a small percentage of the output current is
supplied from the bias supply, therefore the input current is
less than the output current.
P
D
= 513mW
From Figure 2, the minimum current of copper required to
operate this application at a
∆T
of 75°C is less than 100mm
2
.
MSOP-8
θ
JA
θ
JC
θ
CA
AMBIENT
ground plane
heat sink area
printed circuit board
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 maxi-
mum allowable temperature rise must be calculated to deter-
mine operation along which curve.
January 2002
9
100°C
MIC49150
MICREL [ MICREL SEMICONDUCTOR ]
