MIC37100/37101/37102
Micrel
Minimum Load Current
been used by MOSFET manufacturers for years, proving
very reliable and cost effective for the user.
The MIC37100/01/02 regulator is specified between finite
loads. If the output current is too small, leakage currents
dominateandtheoutputvoltagerises.A10mAminimumload
current is necessary for proper regulation.
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
Adjustable Regulator Design
CA
from the leads to the ambient air and it includes θ (case-to-
CS
sink thermal resistance) and θ
resistance).
(sink-to-ambient thermal
SA
MIC37102
VIN
IN
OUT
VOUT
COUT
R1
R2
ENABLE
SHUTDOWN
EN
ADJ
GND
SOIC-8
R1
R2
V
= 1.240V 1+
OUT
Figure 2. Adjustable Regulator with Resistors
The MIC37102 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:
θJA
ground plane
heat sink area
θJC
θCA
AMBIENT
printed circuit board
VOUT
Figure 3. Thermal Resistance
R1= R2
−1
1.240
Using the power SOIC-8 reduces the θ dramatically and
JC
Where V is the desired output voltage. Figure 2 shows
allows the user to reduce θ . The total thermal resistance,
O
CA
component definition. Applications with widely varying load
currents may scale the resistors to draw the minimum load
current required for proper operation (see above).
θ
(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 of
JA
JC
20°C/W, this is significantly lower than the standard SOIC-8
Power SOIC-8 Thermal Characteristics
which is typically 75°C/W. θ
is reduced because pins 5
CA
One of the secrets of the MIC37101/02’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
package size.
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-
tionofthedevice.Topreventthismaximumjunctiontempera-
ture from being exceeded, the appropriate ground plane heat
sink must be used.
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
M9999-091604
12
September 2004
MICREL [ MICREL SEMICONDUCTOR ]
