MIC26603 MICREL [Micrel Semiconductor], MIC26603 Datasheet - Page 21

• Manufacturer Part Number: MIC26603
• Description: 28V, 6A 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)
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.
The process of sizing the ripple injection resistor and
capacitors is:
Step 1. Select C
feedback pin and make sure the large time constant
assumption is satisfied. Typical choice of C
100nF if R1 and R2 are in kΩ range.
SW
IN
= Power stage input voltage
= switching frequency
ΔV
ff
FB(pp)
inj
inj
of 1nF to 100nF can easily satisfy the large
Figure 6c. Invisible Ripple at FB
, as shown in Figure 6c. The injected ripple
is used in order to be considered as short
inj
) × C
f
=
SW
K
ff
V
1
div
IN
ff
×
to feed all output ripples into the
×
τ
=
K
=
R
div
inj
T
τ
R1//R2
ff
×
<<
+
must be much greater than
D
R1//R2
1
×
(1
-
D)
×
M9999-071311-A
f
SW
1
×
MIC26603
ff
τ
is 1nF to
inj
Eq. 18
Eq. 19
Eq. 20
and a