MIC39100_05 Datasheet, PDF (10/12 Pages) Micrel Semiconductor – 1A Low-Voltage Low-Dropout Regulator

MIC39100/39101/39102
Adjustable Regulator Design
V_IN
ENABLE
SHUTDOWN
MIC39102
IN
OUT
R1
EN
ADJ
GND
R2
V_OUT
C_OUT
V_OUT
= 1.240V
_{1 +}
R1
R2

Figure 2. Adjustable Regulator with Resistors
The MIC39102 allows programming the output voltage any-
where between 1.24V and the 16V 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 SOIC-8 Thermal Characteristics
One of the secrets of the MIC39101/2’s performance is its
power SO-8 package featuring half the thermal resistance of
a standard SO-8 package. Lower thermal resistance means
more output current or higher input voltage for a given pack-
age 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 (junc-
tion-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).
Micrel
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 capabil-
ity 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 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 operation
of the device. To prevent this maximum junction temperature
from being exceeded, the appropriate ground plane heat sink
must be used.
SOIC-8
JA
JC
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 maximum
allowable temperature rise must be calculated to determine
operation along which curve.
900
800
700 ^{∆T J A =}
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-SOIC
Power Dissipation
900
800 ^T _J ^{= 125°C}
700 T _A = 85°C
600
50°C 25°C
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-SOIC
Power Dissipation
M9999-082505
10
August 2005