MIC24054 Datasheet, PDF (21/30 Pages) Micrel Semiconductor – 12V, 9A High-Efficiency Buck Regulator

Micrel, Inc.
MIC24054
The process of sizing the ripple injection resistor and
capacitors is:
Step 1. Select C_ff to feed all output ripples into the
feedback pin and make sure the large time constant
assumption is satisfied. Typical choice of C_ff is 1nF to
100nF if R1 and R2 are in kΩ range.
Step 2. Select R_inj according to the expected feedback
voltage ripple using Equation 19:
K _div
=
ΔV_FB(pp)
V_IN
×
f_SW × τ
D × (1 - D)
Eq. 21
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 MIC24054, 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 MIC24054
should be limited to 5.5V to avoid this problem.
Then the value of R_inj is obtained as:
R _inj
= (R1//R2) × ( ¹
K _div
− 1)
Eq. 22
Step 3. Select C_inj as 100nF, which could be considered
as short for a wide range of the frequencies.
Setting Output Voltage
The MIC24054 requires two resistors to set the output
voltage as shown in Figure 9.
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 Equation 24:
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 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, then 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.
October 2012
21
M9999-102512-A