MIC26901 MICREL [Micrel Semiconductor], MIC26901 Datasheet - Page 20

• Manufacturer Part Number: MIC26901
• Manufacturer: MICREL [Micrel Semiconductor]

Micrel, Inc.
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
As shown in Figure 5a, the converter is stable without
any ripple injection. The feedback voltage ripple is:
where ΔI
current ripple.
2. Inadequate ripple at the feedback voltage due to the
The output voltage ripple is fed into the FB pin through a
feedforward capacitor C
Figure 5b. The typical C
100nF. With the feedforward capacitor, the feedback
voltage ripple is very close to the output voltage ripple:
3. Virtually no ripple at the FB pin voltage due to the
July 2011
ΔV
large ESR of the output capacitors.
small ESR of the output capacitors.
very-low ESR of the output capacitors.
FB(pp)
MIC26901
MIC26901
L(pp)
ΔV
=
Figure 5b. Inadequate Ripple at FB
R1
FB(pp)
Figure 5a. Enough Ripple at FB
is the peak-to-peak value of the inductor
R2
+
R2
≈
ESR
×
ESR
ff
×
ff
in this situation, as shown in
C
ΔI
value is between 1nF and
OUT
L
(pp)
×
ΔI
L
(pp)
Eq. 16
Eq. 17
20
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
capacitor C
is:
where:
V
D = duty cycle
f
τ = (R1//R2//R
In Equations 21 and 22, it is assumed that the time
constant associated with C
the switching period:
If the voltage divider resistors R1 and R2 are in the kΩ
range, a C
time constant requirements. Also, a 100nF injection
capacitor C
for a wide range of the frequencies.
SW
IN
= Power stage input voltage
= switching frequency
MIC26901
K
ΔV
ff
inj
div
inj
of 1nF to 100nF can easily satisfy the large
Figure 5c. Invisible Ripple at FB
FB(pp)
, as shown in Figure 5c. The injected ripple
is used in order to be considered as short
inj
=
) × C
R
f
SW
inj
=
R1//R2
1
V
+
ff
×
IN
R1//R2
τ
×
=
K
T
τ
div
ff
<<
×
must be much greater than
D
1
×
(1
-
D)
×
M9999-071311-A
f
SW
1
×
MIC26901
τ
inj
Eq. 18
Eq. 19
Eq. 20
and a