MIC2103_13 Datasheet, PDF (27/38 Pages) Micrel Semiconductor – 75V, Synchronous Buck Controllers featuring Adaptive On-Time Control

Micrel, Inc.
Ripple Injection
The V_FB ripple required for proper operation of the
MIC2103/04 gm 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 cannot sense it, then the MIC2103/04 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 due to the
large ESR of the output capacitors.
As shown in Figure 8a, 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. 28
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 feed-forward capacitor C_ff in this situation,
as shown in Figure 8b. The typical C_ff value is
between 1nF and 100nF. With the feed-forward
capacitor, the feedback voltage ripple is very close
to the output voltage ripple:
ΔV_FB(pp) ≈ ESR × ΔI_{L (pp)}
Eq. 29
3. Virtually no ripple at the FB pin voltage due to the
very-low ESR of the output capacitors:
MIC2103/04
Figure 8b. Inadequate Ripple at FB
Figure 8c. 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 8c. The injected ripple
is:
ΔV_FB(pp)
=
V_IN
× K _div
× D × (1- D) ×
1
f_SW ×τ
Eq. 30
K _div
=
R1//R2
R_inj + R1//R2
Eq. 31
where:
V_IN = Power stage input voltage
D = Duty cycle
f_SW = Switching frequency
τ = (R1//R2//R_inj) × C_ff
In Equations 30 and 32, 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. 32
Figure 8a. Enough Ripple at FB
November 26, 2013
27
Revision 2.0