MIC3775 Micrel Semiconductor, MIC3775 Datasheet - Page 11

• Manufacturer Part Number: MIC3775
• Description: 750mA UCap Low-Voltage Low-Dropout Regulator
• Manufacturer: Micrel Semiconductor

Adjustable Regulator Design
The MIC3775 allows programming the output voltage any-
where between 1.24V and the 6V maximum operating rating
of the family. Two resistors are used. Resistors can be quite
large, up to 1M , because of the very high input impedance
and low bias current of the sense comparator. The resistor
values are calculated by:
Where V
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
One of the secrets of the MIC3775’s performance is its power
MSOP-8 package featuring half the thermal resistance of a
standard MSOP-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,
(junction-to-case thermal resistance) and
ent thermal resistance). See Figure 3.
from the die to the leads of the package.
from the leads to the ambient air and it includes
March 2003
MIC3775
Figure 2. Adjustable Regulator with Resistors
R1 R2
Figure 4. Copper Area vs. Power-MSOP
O
SHUTDOWN
is the desired output voltage. Figure 2 shows
ENABLE
1.240
V
Power Dissipation ( T
OUT
900
800
700
600
500
400
300
200
100
V
IN
0
0
V
OUT
0.25 0.50 0.75 1.00 1.25 1.50
POWER DISSIPATION (W)
1
IN
EN
MIC3775
1.240V 1
GND
OUT
ADJ
R2
R1
R1
R2
JC
JA
CA
CA
)
is the resistance
C
is the resistance
OUT
(case-to-ambi-
V
OUT
CS
(case-to-
JC
11
sink thermal resistance) and
resistance).
Using the power MSOP-8 reduces the
allows the user to reduce
factor in calculating the maximum power dissipation capabil-
ity of the device. Typically, the power MSOP-8 has a
80 C/W, this is significantly lower than the standard MSOP-8
which is typically 160 C/W.
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
heatsink must be used.
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.
JA
(junction-to-ambient thermal resistance) is the limiting
Figure 5. Copper Area vs. Power-MSOP
Figure 3. Thermal Resistance
900
800
700
600
500
400
300
200
100
Power Dissipation (T
printed circuit board
MSOP-8
0
0
T
0.25 0.50 0.75 1.00 1.25 1.50
J
POWER DISSIPATION (W)
= 125 C
85 C
JC
CA
JA
CA
. The total thermal resistance,
50 C 25 C
CA
SA
is reduced because pins 5
(sink-to-ambient thermal
AMBIENT
A
JC
heat sink area
)
ground plane
dramatically and
MIC3775
Micrel
JA
of