Micrel, Inc.
MIC37110/MIC37112
MIC37120/MIC37122
Minimum Load Current
The MIC37112/22 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.
Adjustable Regulator Design
V
OUT
1.0V
1.1V
1.2V
1.5V
1.8V
2.2V
2.5V
3.0V
3.3V
3.6V
R1
0
10.0Ω
20.0Ω
49.9Ω
80.6Ω
121Ω
150Ω
200Ω
232Ω
261Ω
R2
−
100Ω
100Ω
100Ω
100Ω
100Ω
100Ω
100Ω
100Ω
100Ω
Table 1. Resistor Selection for Specific V
OUT
Figure 2. Adjustable Regulator with Resistors
The MIC37112 and MIC37122 allow programming the
output voltage anywhere between 1.0V and 5.0V by
placing a resistor divider network from OUT to GND and
is determined by the following equation:
R1
V
OUT
=
V
ADJ
×
+
1
R2
where:
V
OUT
is the desired output voltage and V
ADJ
= 1.0V.
Two resistors are used. Resistors can be quite large, but
the resistor (R1) value between the OUT pin and the
ADJ pin should not exceed 10kΩ. Larger values can
cause instability.
The resistor values are calculated from the previous
equation, resulting in the following:
R1
=
R2
×
(
V
OUT
−
1
)
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 Table 1 for a list of resistor combinations to set the
output voltage. A 1% tolerance is recommended for both
R1 and R2.
Thermal Measurements
It is always wise the measure the IC’s case temperature
to make sure that it is within its operating limits. Although
this might seem like a very elementary task, it is very
easy to get to get erroneous results. The most common
mistake is to use the standard thermal couple that
comes with the thermal voltage meter. This thermal
couple wire gauge is large, typically 22 gauge, and
behaves like a heatsink, resulting in a lower case
measurement.
There are two suggested methods for measuring the IC
case temperature: a thermal couple or an infrared
thermometer. If a thermal couple is used, it must be
constructed of 36 gauge wire or higher to minimize the
wire heatsinking effect. In addition, the thermal couple tip
must be covered in either thermal grease or thermal glue
to make sure that the thermal couple junction is making
good contact to the case of the IC. This thermal couple
from Omega (5SC-TT-K-36-36) is adequate for most
applications.
To avoid this messy thermal couple grease or glue, an
infrared thermometer is recommended. Most infrared
thermometers’ spot size is too large for an accurate
reading on small form factor ICs. However, an IR
thermometer from Optris has a 1mm spot size, which
makes it ideal for the MIC371xx 2mm x 2mm Thin MLF
package. Also, get the optional stand. The stand makes
it easy to hold the beam on the IC for long periods of
time.
Power SOIC-8 Thermal Characteristics
One of the secrets of the MIC37110/37120’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.
15
M9999-121312-A
December 2012
MICREL [ MICREL SEMICONDUCTOR ]
