MIC3775 Datasheet, PDF (11/14 Pages) Micrel Semiconductor – 750mA UCap Low-Voltage Low-Dropout Regulator

MIC3775
Adjustable Regulator Design
V_IN
ENABLE
SHUTDOWN
MIC3775
IN OUT
R1
EN ADJ
GND
R2
V_OUT
C_OUT
V_OUT = 1.240V ^{}1+ _RR21^{}
Figure 2. Adjustable Regulator with Resistors
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 1MΩ, because of the very high input impedance
and low bias current of the sense comparator. The resistor
values are calculated by:
R1 =
R2


V_OUT
1.240
−
1^{}
Where V_O is the desired output voltage. Figure 2 shows
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, θ_JC
(junction-to-case thermal resistance) and θ_CA (case-to-ambi-
ent 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-
Micrel
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
heatsink must be used.
MSOP-8
θ_JA
θ_JC
θ_CA
ground plane
heat sink area
AMBIENT
printed circuit board
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.
900
800
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 4. Copper Area vs. Power-MSOP
Power Dissipation (∆T_JA)
900
800 ^{TJ = 125°C}
700
85°C 50°C 25°C
600
500
400
300
200
100
0
0 0.25 0.50 0.75 1.00 1.25 1.50
POWER DISSIPATION (W)
Figure 5. Copper Area vs. Power-MSOP
Power Dissipation (T_A)
March 2003
11
MIC3775