MIC26903ZA_14 Datasheet, PDF (21/29 Pages) Micrel Semiconductor – 28V, 9A Hyper Speed Control Synchronous DC/DC Buck Regulator

Micrel, Inc.
In Equations 18 and 19, it is assumed that the time
constant associated with C_ff must be much greater than
the switching period:
1 ₌ T _{<< 1}
f_SW ×τ τ
Eq. 20
If the voltage divider resistors R1 and R2 are in the kΩ
range, a C_ff of 1nF to 100nF can easily satisfy the large
time constant requirements. Also, a 100nF injection
capacitor C_INJ is used, which could be considered as
short for a wide range of the frequencies.
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:
MIC26903-ZA
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
K _div
=
ΔV_FB(pp)
V_IN
× ^{fSW ×τ}
D × (1− D)
Then the value of R_INJ is obtained as:
Eq. 21
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 MIC26903-ZA requires two resistors to set the output
voltage, as shown in Figure 7.
The output voltage is determined by Equation 23:
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 MIC26903-ZA, as shown in
Figure 8. 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 MIC26903-ZA should be
limited to 5.5V to avoid this problem.
V_OUT
=
V_FB
× (1+
R1)
R2
where V_FB = 0.6V.
July 22, 2014
Eq. 23
21
Figure 8. Internal Ripple Injection
Revision 1.1