MIC261203 Datasheet, PDF (22/31 Pages) Micrel Semiconductor – 28V, 12A Hyper Light Load™ Synchronous DC/DC 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 22:
K _div
=
ΔV_FB(pp)
V_IN
× ^{fSW × τ}
D × (1- D)
Eq. 21
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 MIC261203 requires two resistors to set the output
voltage as shown in Figure 7.
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
MIC261203
Figure 7. 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 MIC261203, as shown
in Figure 8. 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
MIC261203 should be limited to 5.5V to avoid this
problem.
Figure 8. Internal Ripple Injection
July 2011
22
M9999-071311-A