MIC24053 Datasheet, PDF (20/30 Pages) Micrel Semiconductor – 12V, 9A 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 the kΩ range.
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)
Eq. 21
Then the value of R_inj is calculated 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 MIC24053 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
MIC24053
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 MIC24053, as shown
in Figure 9. The inverting input voltage (V_INJ) is clamped
to 1.2V. As V_OUT increases, the swing of V_INJ is 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 MIC24053 should be
limited to 5.5V to avoid this problem.
Figure 9. Internal Ripple Injection
November 2012
20
M9999-110712-A