MIC26903ZA_14 Datasheet, PDF (20/29 Pages) Micrel Semiconductor – 28V, 9A Hyper Speed Control Synchronous DC/DC Buck Regulator

Micrel, Inc.
Ripple Injection
The V_FB ripple required for proper operation of the
MIC26903-ZA g_m amplifier and error comparator is 20mV
to 100mV. 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 10mV to 20mV, and the feedback voltage ripple is
less than 20mV. If the feedback voltage ripple is so small
that the g_m amplifier and error comparator can’t sense it,
then the MIC26903-ZA 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 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:
ΔV_FB(pp)
=
R2
R1+ R2
×
ESR
C_OUT
× ΔI_{L (pp)}
Eq. 16
MIC26903-ZA
Figure 4. Enough Ripple at FB
Figure 5. Inadequate Ripple at FB
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:
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 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)
=
V_IN
× K _div
× D × (1- D) ×
1
f_SW ×τ
Eq. 18
K _div
=
R1//R2
R_INJ + R1//R2
where:
V_IN = Power stage input voltage
D = duty cycle
f_SW = switching frequency
τ = (R1//R2//R_INJ) × C_ff
Eq. 19
July 22, 2014
20
Revision 1.1