MIC261203ZA Datasheet, PDF (20/28 Pages) Micrel Semiconductor – 28V, 12A Hyper Speed Control Synchronous DC/DC Buck Regulator

Micrel, Inc.
Figure 5. Enough Ripple at FB
Figure 6. Inadequate Ripple at FB
Figure 7. Invisible Ripple at FB
In this situation, the output voltage ripple is less than
20mV. Therefore, additional ripple is injected into the FB
pin from the switching node SW via a resistor R_INJ and a
capacitor C_INJ, as shown in Figure 7. The injected ripple
is:
ΔV_FB(pp)
=
V_IN
× K _div
× D × (1- D) ×
1
f_SW ×τ
Eq. 18
K _div
=
R1//R2
R_INJ + R1//R2
where:
V_IN = Power stage input voltage
D = duty cycle
f_SW = switching frequency
τ = (R1//R2//R_INJ) × C_ff
Eq. 19
MIC261203-ZA
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:
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 MIC261203-ZA 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 = 0.6V.
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
June 11, 2013
20
Revision 1.0