MIC3975
Micrel
Adjustable Regulator Design
sink thermal resistance) and θ
(sink-to-ambient thermal
SA
resistance).
Using the power MSOP-8 reduces the θ dramatically and
JC
MIC3975
allows the user to reduce θ . The total thermal resistance,
VIN
IN
OUT
VOUT
COUT
CA
R1
R2
θ
(junction-to-ambient thermal resistance) is the limiting
JA
ENABLE
SHUTDOWN
EN
ADJ
factor in calculating the maximum power dissipation capabil-
GND
ity of the device. Typically, the power MSOP-8 has a θ of
JA
80°C/W, this is significantly lower than the standard MSOP-8
R1
R2
which is typically 160°C/W. θ is reduced because pins 5
V
= 1.240V 1+
CA
OUT
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.
Figure 2. Adjustable Regulator with Resistors
The MIC3975 allows programming the output voltage any-
wherebetween1.24Vandthe16Vmaximumoperatingrating
of the family. Two resistors are used. Resistors can be quite
large, up to 1MΩ, because of the very high input impedance
and low bias current of the sense comparator: The resistor
values are calculated by:
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-
tionofthedevice.Topreventthismaximumjunctiontempera-
ture from being exceeded, the appropriate ground plane heat
sink must be used.
VOUT
R1= R2
−1
1.240
Where V is the desired output voltage. Figure 2 shows
MSOP-8
O
component definition. Applications with widely varying load
currents may scale the resistors to draw the minimum load
current required for proper operation (see above).
Power MSOP-8 Thermal Characteristics
θJA
One of the secrets of the MIC3975’s performance is its power
MSO-8 package featuring half the thermal resistance of a
standard MSO-8 package. Lower thermal resistance means
more output current or higher input voltage for a given
package size.
ground plane
heat sink area
θJC
θCA
AMBIENT
printed circuit board
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.
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 maxi-
mum allowable temperature rise must be calculated to deter-
mine operation along which curve.
Thermal resistance consists of two main elements, θ
JC
(junction-to-casethermalresistance)andθ (case-to-ambi-
CA
ent thermal resistance). See Figure 3. θ is the resistance
JC
from the die to the leads of the package. θ is the resistance
CA
from the leads to the ambient air and it includes θ (case-to-
CS
900
800
700
600
500
400
300
200
100
0
900
T
= 125°C
85°C
800
700
600
500
400
300
200
100
0
J
50°C 25°C
0
0.25 0.50 0.75 1.00 1.25 1.50
POWER DISSIPATION (W)
0
0.25 0.50 0.75 1.00 1.25 1.50
POWER DISSIPATION (W)
Figure 4. Copper Area vs. Power-MSOP
Power Dissipation (∆T
Figure 5. Copper Area vs. Power-MSOP
)
Power Dissipation (T )
JA
A
MIC3975
10
February 2003
MICREL [ MICREL SEMICONDUCTOR ]
