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MIC2104 数据表(PDF) 31 Page - Microchip Technology

部件名 MIC2104
功能描述  75V Synchronous Buck Controllers Featuring Adaptive ON-Time Control
PDF  42 Pages
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制造商  MICROCHIP [Microchip Technology]
网页  http://www.microchip.com
标志 MICROCHIP - Microchip Technology

MIC2104 数据表(HTML) 31 Page - Microchip Technology

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 2017 Microchip Technology Inc.
DS20005899A-page 31
MIC2103/4
5.7
Ripple Injection
The VFB ripple required for proper operation of the
MIC2103/4 gm amplifier and error comparator is 20 mV
to 100 mV. 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 10 mV to 20 mV, and the feedback
voltage ripple is less than 20 mV. If the feedback
voltage ripple is so small that the gm amplifier and error
comparator cannot sense it, then the MIC2103/4 will
lose control and the output voltage is not regulated. In
order to have some amount of VFB 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.
FIGURE 5-4:
Enough Ripple at FB.
As shown in Figure 5-4, the converter is stable without
any ripple injection. The feedback voltage ripple is:
EQUATION 5-24:
2.
Inadequate ripple at the feedback voltage due to
the small ESR of the output capacitors.
FIGURE 5-5:
Inadequate Ripple at FB.
The output voltage ripple is fed into the FB pin through
a feed-forward capacitor Cff in this situation, as shown
in Figure 5-5. The typical Cff value is between 1 nF and
100 nF. With the feed-forward capacitor, the feedback
voltage ripple is very close to the output voltage ripple:
EQUATION 5-25:
3.
Virtually no ripple at the FB pin voltage due to
the very-low ESR of the output capacitors:
FIGURE 5-6:
Invisible Ripple at FB.
In this situation, the output voltage ripple is less than
20 mV. Therefore, additional ripple is injected into the
FB pin from the switching node SW via a resistor Rinj
and a capacitor Cinj, as shown in Figure 5-6. The
injected ripple is:
EQUATION 5-26:
EQUATION 5-27:
In Equation 5-26 and Equation 5-27, it is assumed that
the time constant associated with Cff must be much
greater than the switching period:
L
SW
FB
R1
R2
ESR
C
OUT
MIC2103/04
V
FB PP

R2
R1
R2
+
-------------------- ESR
COUT
I
LPP

=
Where:
∆IL(PP) = Peak-to-peak inductor current ripple.
L
SW
FB
R1
R2
ESR
C
OUT
MIC2103/04
C
ff
V
FB PP

ESR
I
LPP

L
SW
FB
R1
R2
ESR
C
OUT
MIC2103/04
C
ff
C
inj
R
inj
V
FB PP

V
IN
K
DIV
D
1 D
–

1
f
SW
-----------------
=
Where:
VIN = Power stage input voltage.
D = Duty cycle.
fSW = Switching frequency.
= (R1//R2//Rinj) x Cff.
τ
K
DIV
R1//R2
R
inj
R1//R2
+
---------------------------------
=



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