MIC261203 Datasheet, PDF (21/31 Pages) Micrel Semiconductor – 28V, 12A Hyper Light Load™ Synchronous DC/DC 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 6a, 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 6b. 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.
MIC261203
Figure 6a. Enough Ripple at FB
MIC261203
MIC261203
MIC261203
Figure 6c. 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 6c. The injected ripple
is:
1
ΔV_FB(pp) = V_IN × K _div × D × (1- D) × _{fSW × 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 21 and 22, it is assumed that the time
constant associated with C_ff must be much greater than
the switching period:
1T
= << 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 in order to be considered as short
for a wide range of the frequencies.
Figure 6b. Inadequate Ripple at FB
July 2011
21
M9999-071311-A