MIC24051 Datasheet, PDF (20/30 Pages) Micrel Semiconductor – 12V, 6A High-Efficiency Buck Regulator

Micrel, Inc.
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
=
∆V_FB(PP)
V_IN
× ^{fSW × τ}
D × (1 − D)
Then the value of R_inj is obtained as:
Eq. 21
Rinj
=
(R1
//
R2)
×

K
1
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 MIC24051 requires two resistors to set the output
voltage as shown in Figure 8.
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
MIC24051
Figure 8. Voltage-Divider Configuration
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 MIC24051, as shown
in Figure 9. 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 MIC24051
should be limited to 5.5V to avoid this problem.
Figure 9. 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.
November 2012
20
M9999-112612-A