MIC26950 MICREL, MIC26950 Datasheet - Page 19

• Manufacturer Part Number: MIC26950
• Description: 12A Hyper Speed Control Synchronous DC-DC Buck Regulator
• Manufacturer: MICREL

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Micrel, Inc.
Setting Output Voltage
The MIC26950 requires two resistors to set the output
voltage as shown in Figure 6.
The output voltage is determined by the equation:
where, V
3kΩ and 10kΩ. If R1 is too large, it may allow noise to be
introduced into the voltage feedback loop. If R1 is too
small, it will decrease the efficiency of the power supply,
especially at light loads. Once R1 is selected, R2 can be
calculated using:
In addition to the external ripple injection added at the
FB pin, internal ripple injection is added at the inverting
input of the comparator inside the MIC26950, as shown
in Figure 7. The inverting input voltage V
1.2V. As V
clamped. The clamped V
because it is reflected back as a DC error on the FB
terminal. Therefore, the maximum output voltage of the
MIC26950 should be limited to 5.5V to avoid this line
regulation problem.
September 2010
FB
Figure 6. Voltage-Divider Configuration
= 0.8V. A typical value of R1 can be between
OUT
R2
V
OUT
=
is increased, the swing of V
V
=
OUT
V
V
FB
FB
×
−
×
R1
V
(1
FB
INJ
+
R2
R1
reduces the line regulation
)
INJ
is clamped to
INJ
will be
(24)
(25)
19
Thermal Measurements
Measuring the IC’s case temperature is recommended to
ensure it is within its operating limits. Although this might
seem like a very elementary task, it is easy to get
erroneous results. The most common mistake is to use
the standard thermal couple that comes with a thermal
meter. This thermal couple wire gauge is large, typically
22 gauge, and behaves like a heatsink, resulting in a
lower case measurement.
Two methods of temperature measurement are using a
smaller thermal couple wire or an infrared thermometer.
If a thermal couple wire is used, it must be constructed
of 36 gauge wire or higher (smaller wire size) to
minimize the wire heat-sinking 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 with the case of
the IC. Omega brand thermal couple (5SC-TT-K-36-36)
is adequate for most applications.
Wherever
recommended. The measurement spot size of most
infrared thermometers is too large for an accurate
reading on a small form factor ICs. However, a IR
thermometer from Optris has a 1mm spot size, which
makes it a good choice for measuring the hottest point
on the case. An optional stand makes it easy to hold the
beam on the IC for long periods of time.
Figure 7. Internal Ripple Injection
possible,
an
infrared
thermometer
M9999-091710-C
MIC26950
is