MIC24053 Datasheet, PDF (22/32 Pages) Micrel Semiconductor – 12V, 9A High-Efficiency Buck Regulator

MIC24053
The applications are divided into three situations
according to the amount of the feedback voltage ripple:
1. Enough ripple at the feedback voltage due to the
large ESR of the output capacitors.
L
SW
MIC24053
FB
R1
C_OUT
R2
ESR
FIGURE 5-1:
Enough Ripple at FB.
As shown in Figure 5-1, the converter is stable without
any ripple injection. The feedback voltage ripple is:
EQUATION 5-14:
V _{FB pp} = -_R----1--^R--+--2---_R----2--  ESR_COUT  I _{L pp}
Where:
ΔI_L(pp) = Peak-to-peak value of the
inductor current ripple.
2. Inadequate ripple at the feedback voltage due to
the small ESR of the output capacitors.
L
SW
MIC24053
FB
R1
C_ff
R2
C_OUT
ESR
FIGURE 5-2:
Inadequate Ripple at FB.
The output voltage ripple is fed into the FB pin through
a feedforward capacitor (C_ff) in this situation, as shown
in Figure 5-2. The typical C_ff value is between 1nF and
100 nF. With the feedforward capacitor, the feedback
voltage ripple is very close to the output voltage ripple:
EQUATION 5-15:
V _{FB pp}  ESR  I _{L pp}
3. Virtually no ripple at the FB pin voltage due to
the very-low ESR of the output capacitors.
L
SW
MIC24053
FB
R_inj
C_inj
R1
C_ff
C_OUT
R2
ESR
FIGURE 5-3:
Invisible Ripple at FB.
In this situation, the output voltage ripple is less than
20 mV. Therefore, additional ripple is injected into the
FB pin from the switching node (SW) using a resistor
(R_inj) and a capacitor (C_inj), as shown in Figure 5-3.
The injected ripple is:
EQUATION 5-16:
L
V _{FB pp}
=
V
IN

K_div

D

1
–
D

---------1----------
f _SW  
EQUATION 5-17:
K _div = -_R----_i-_n--R-_j---_-1---R-^---R-1---2-_---R----2-
Where:
V_IN = Power stage input voltage
D = Duty Cycle
f_SW = Switching frequency
τ = (R1//R2//R_inj) × C_ff
In Equation 5-16 and Equation 5-17, it is assumed that
the time constant associated with C_ff must be much
greater than the switching period:
EQUATION 5-18:
---------1---------- = ^T--- » 1
f _SW   
If the voltage divider resistors (R1 and R2) are in the kΩ
range, a C_ff of 1 nF to 100 nF can easily satisfy the
large time constant requirement. Also, a 100 nF
injection capacitor (C_inj) is used in order to be
considered as short for a wide range of the
frequencies.
DS20005668A-page 22
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