MIC24052 Datasheet, PDF (21/30 Pages) Micrel Semiconductor – 12V, 6A High-Efficiency Buck Regulator

Micrel, Inc.
MIC24052
Step 2. Select R_inj according to the expected feedback
voltage ripple using Equation 19:
K _div
=
ΔV_FB(pp)
V_IN
× ^{fSW × τ}
D × (1- D)
Then the value of R_inj is obtained as:
R _inj
= (R1//R2) × ( ¹
K _div
− 1)
Eq. 21
Eq. 22
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 MIC24052, as shown
in Figure 10. The inverting input voltage V_INJ is clamped
to 1.2V. As V_OUT is increased, the swing of V_INJ will be
clamped. The clamped V_INJ reduces the line regulation
because it is reflected as a DC error on the FB terminal.
Therefore, the maximum output voltage of the MIC24052
should be limited to 5.5V to avoid this problem.
Step 3. Select C_inj as 100nF, which could be considered
as short for a wide range of the frequencies.
Setting Output Voltage
The MIC24052 requires two resistors to set the output
voltage as shown in Figure 9.
The output voltage is determined by the equation:
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
Figure 9. Voltage-Divider Configuration
Figure 10. Internal Ripple Injection
Thermal Measurements
Measuring the IC’s case temperature is recommended to
insure it is within 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.
December 2012
21
M9999-120312-A