MIC24053 Datasheet, PDF (19/30 Pages) Micrel Semiconductor – 12V, 9A High-Efficiency Buck Regulator

Micrel, Inc.
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.
As shown in Figure 5, the converter is stable without any
ripple injection. The feedback voltage ripple is:
ΔV_FB(pp)
=
R2
R1 + R2
×
ESR
C_OUT
× ΔI_{L (pp)}
Eq. 16
where ΔI_L(pp) is the 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.
The output voltage ripple is fed into the FB pin through a
feedforward capacitor (C_ff) in this situation, as shown in
Figure 6. The typical C_ff value is between 1nF and
100nF. With the feedforward capacitor, the feedback
voltage ripple is very close to the output voltage ripple:
ΔV_FB(pp) ≈ ESR × ΔI_{L (pp)}
Eq. 17
3. Virtually no ripple at the FB pin voltage due to
the very-low ESR of the output capacitors.
Figure 5. Enough Ripple at FB
MIC24053
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) using 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
Eq. 19
where:
V_IN = power stage input voltage
D = duty cycle
f_SW = switching frequency
τ = (R1//R2//R_inj) × C_ff
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 requirement. Also, a 100nF injection
capacitor (C_inj) is used in order to be considered as short
for a wide range of the frequencies.
Figure 6. Inadequate Ripple at FB
November 2012
19
M9999-110712-A