MIC261203ZA_14 Datasheet, PDF (20/29 Pages) Micrel Semiconductor – Synchronous DC-to-DC Buck Regulator

Micrel, Inc.
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 caused by the
large ESR of the output capacitors.
As shown in Figure 4, the converter is stable without
any ripple injection. The feedback voltage ripple is:
MIC261203-ZA
Figure 5. Inadequate Ripple at FB
R2
ΔV_FB(pp) = _{R1 + R2} × ESR _COUT × 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 caused by
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 5. 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:
Figure 6. 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 6. The injected ripple
is:
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 4. Enough Ripple at FB
ΔV_FB(pp)
=
V_IN
× K _div
× D × (1- D) ×
1
f_SW ×τ
K _div
=
R1//R2
R_INJ + R1//R2
Eq. 18
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
July 22, 2014
20
Revision 1.1