MIC26903 MICREL [Micrel Semiconductor], MIC26903 Datasheet - Page 21

• Manufacturer Part Number: MIC26903
• Description: 28V, 9A Hyper Light Load Synchronous DC/DC Buck Regulator
• 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 6a, 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 6b. 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)
MIC26903
MIC26903
L(pp)
ΔV
=
Figure 6b. Inadequate Ripple at FB
R1
FB(pp)
Figure 6a. 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
21
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
MIC26903
K
ΔV
ff
inj
div
inj
of 1nF to 100nF can easily satisfy the large
Figure 6c. Invisible Ripple at FB
, as shown in Figure 6c. The injected ripple
FB(pp)
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
×
1
D
×
(1
-
D)
M9999-071311-A
×
f
SW
MIC26903
1
×
τ
inj
Eq. 18
Eq. 19
Eq. 20
and a