MIC24051 Datasheet, PDF (22/34 Pages) Micrel Semiconductor – 12V, 6A High-Efficiency Buck Regulator

MIC24051
The power dissipated in the input capacitor is:
EQUATION 5-13:
P_{DISSCIN} = I_{CINRMS}  ESR_CIN
5.4 Ripple Injection
The V_FB ripple required for proper operation of the
MIC24051 g_m amplifier and error comparator is 20 mV
to 100 mV. However, the output voltage ripple is
generally designed as 1% to 2% of the output voltage.
For a low output voltage, such as a 1V, the output
voltage ripple is only 10 mV to 20 mV, and the feedback
voltage ripple is less than 20 mV. If the feedback
voltage ripple is so small that the g_m amplifier and error
comparator can’t sense it, then the MIC24051 will lose
control and the output voltage is not regulated. In order
to have some amount of V_FB ripple, a ripple injection
method is applied for low output voltage ripple
applications.
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.
SW
L
MIC24051 FB
R1
R2
C_OUT
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:
∆I_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.
SW
L
MIC24051 _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 feed-forward capacitor C_ff in this situation, as shown
in Figure 5-2. The typical C_ff value is between 1 nF and
100 nF. With the feed-forward 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.
SW
L
MIC24051 FB ^Rinj
C_{inj R1}
C_ff
R2
C_OUT
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 via a resistor R_INJ
and a capacitor C_INJ, as shown in Figure 5-3. The
injected ripple is:
EQUATION 5-16:
V _{F B  P P }
=
V_IN

K_DIV

D

1
–
D

--------1--------
f_SW  
Where:
V_IN Power stage input voltage
D Duty cycle
f_SW Switching frequency
τ (R1//R2//R_INJ) × C_ff
DS20005658A-page 22
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