MIC26603ZA_14 Datasheet, PDF (20/29 Pages) Micrel Semiconductor – 28V, 6A Hyper Speed Control Synchronous DC/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 5, the converter is stable without
any ripple injection. The feedback voltage ripple is:
MIC26603-ZA
Figure 6. Inadequate Ripple at FB
R2
∆V_FB(PP) = _{R1 + R2} × ESR _COUT × ∆I_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 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:
∆V_FB(PP) ≈ ESR × ∆I_L(PP)
Eq. 17
3. Virtually no ripple at the FB pin voltage due to the
very-low ESR of the output capacitors.
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 via a resistor R_INJ and a
capacitor C_INJ, as shown in Figure 7. The injected ripple
is:
∆V_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
Figure 5. Enough Ripple at FB
where:
V_IN = power stage input voltage
D = duty cycle
f_SW = switching frequency
τ = (R1//R2//R_INJ) × C_ff
July 16, 2014
20
Revision 1.1