MIC27600_11 Datasheet, PDF (20/28 Pages) Micrel Semiconductor – 36V, 7A Hyper Speed Control™ Synchronous DC-DC Buck Regulator

Micrel, Inc.
Setting Output Voltage
The MIC27600 requires two resistors to set the output
voltage as shown in Figure 6.
MIC27600
Figure 6. Voltage-Divider Configuration
The output voltage is determined by Equation 23:
V_OUT
=
V_FB
× (1+
R1)
R2
Eq. 23
where, V_FB = 0.8V. A typical value of R1 can be between
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:
R2 = ^{VFB × R1}
V_OUT − V_FB
Eq. 24
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 MIC27600, as shown
in Figure 7. The inverting input voltage V_INJ is clamped to
1.2V. As V_OUT increases, the swing of V_INJ will be
clamped. The clamped V_INJ reduces the line regulation
because it is reflected back as a DC error on the FB
terminal. To avoid this line regulation problem, the
maximum output voltage of the MIC27600 should be
limited to 5.5V for up to 28V V_IN and 3.6V for V_IN higher
than 28V.
Figure 7. Internal Ripple Injection
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 then (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 possible, an infrared thermometer is
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.
July 2011
20
M9999-070811