MIC26600_1107 Datasheet, PDF (19/28 Pages) Micrel Semiconductor – 7A Hyper Speed ControlTM Synchronous DC-DC Buck Regulator

Micrel, Inc.
The output voltage ripple is fed into the FB pin through a
feedforward capacitor C_ff in this situation, as shown in
Figure 5b. 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)}
(18)
3) Virtually no ripple at the FB pin voltage due to the very
low ESR of the output capacitors.
Figure 5a. Enough Ripple at FB
MIC26600
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 5c. The injected ripple
is:
ΔV_FB(pp)
=
V_IN
× K _div
× D × (1- D) ×
1
f_SW ×τ
(19)
K _div
=
R1//R2
R_inj + R1//R2
(20)
where
V_IN = Power stage input voltage
D = duty cycle
f_SW = switching frequency
τ = (R1//R2//R_inj) × C_ff
In equations (19) and (20), it is assumed that the time
constant associated with C_ff must be much greater than
the switching period:
Figure 5b. Inadequate Ripple at FB
Figure 5c. Invisible Ripple at FB
1 ₌ T _{<< 1}
(21)
f_SW ×τ τ
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.
The process of sizing the ripple injection resistor and
capacitors is:
Step 1. Select C_ff to feed all output ripples into the
feedback pin and make sure the large time constant
assumption is satisfied. Typical choice of C_ff is 1nF to
100nF if R1 and R2 are in kΩ range.
Step 2. Select R_inj according to the expected feedback
voltage ripple using equation (22):
K _div
=
ΔV_FB(pp)
V_IN
× ^{fSW ×τ}
D × (1− D)
(22)
July 2011
19
M9999-070111-C