Micrel, Inc.
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-
ambient thermal resistance). See Figure 3. 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).
MIC37110/MIC37112
MIC37120/MIC37122
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.
Figure 4. Copper Area vs. Power SO-8 Power Dissipation
Figure 3. Thermal Resistance
Figure 4 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 4, 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
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 capability 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 sinks to
ambient thermal resistance.
December 2012
16
M9999-121312-A
MICREL [ MICREL SEMICONDUCTOR ]
