Micrel, Inc.
MIC37100/37101/37102
without operating in dropout, NPN-based regulators
require an input voltage of 3.7V at the very least. The
MIC37100 regulator will provide excellent performance
with an input as low as 3.0V or 2.5V respectively. This
gives the PNP based regulators a distinct advantage
over older, NPN based linear regulators.
Thermal resistance consists of two main elements, θJC
(junction-to-case thermal resistance) and θCA (case-to-
ambient thermal resistance). See Figure 3. θJC is the
resistance from the die to the leads of the package. θCA
is the resistance from the leads to the ambient air and it
includes θCS (case-to-sink thermal resistance) and θSA
(sink-to-ambient thermal resistance).
Minimum Load Current
The MIC37100/01/02 regulator is specified between
finite loads. If the output current is too small, leakage
currents dominate and the output voltage rises. A 10mA
minimum load current is necessary for proper regulation.
SOIC-8
Adjustable Regulator Design
MIC37102
VIN
VOUT
COUT
IN
OUT
JA
ground plane
heat sink area
R1
R2
JC
CA
ENABLE
EN
ADJ
SHUTDOWN
AMBIENT
GND
printed circuit board
Figure 3. Thermal Resistance
R1
⎛
⎞
⎟
VOUT = 1.240V 1+
⎜
R2
⎝
⎠
Using the power SOIC-8 reduces the θJC dramatically
and allows the user to reduce θCA. The total thermal
resistance, θJA (junction-to-ambient thermal resistance)
is the limiting factor in calculating the maximum power
dissipation capability of the device. Typically, the power
SOIC-8 has a θJC of 20°C/W, this is significantly lower
than the standard SOIC-8 which is typically 75°C/W. θCA
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.
Figure 2. Adjustable Regulator with Resistors
The MIC37102 allows programming the output voltage
anywhere between 1.24V and the 6V maximum
operating rating of the family. Two resistors are used.
Resistors can be quite large, up to 1Mꢀ, because of the
very high input impedance and low bias current of the
sense comparator. The resistor values are calculated by:
V
⎛
⎞
OUT
R1 = R2⎜
− 1⎟
⎜
⎟
1.240
⎝
⎠
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.
Where VO 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
TJA
=
700
600
500
400
300
200
100
0
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.
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.
0
0.25 0.50 0.75 1.00 1.25 1.50
POWER DISSIPATION (W)
Figure 4. Copper Area vs. Power SO-8
Power Dissipation
M9999-090607
September 2007
12
MICREL [ MICREL SEMICONDUCTOR ]
