MP1584 – 3A, 1.5MHz, 28V STEP-DOWN CONVERTER
MP1584 Rev. 1.0
www.MonolithicPower.com
13
8/8/2011
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In this case (as shown in Figure 2), a third pole
set by the compensation capacitor (C6) and the
compensation resistor (R3) is used to
compensate the effect of the ESR zero on the
loop gain. This pole is located at:
3
R
6
C
2
1
f
3
P
The goal of compensation design is to shape
the converter transfer function to get a desired
loop gain. The system crossover frequency
where the feedback loop has the unity gain is
important. Lower crossover frequencies result
in slower line and load transient responses,
while higher crossover frequencies could cause
system unstable. A good rule of thumb is to set
the crossover frequency to approximately one-
tenth of the switching frequency. The Table 3
lists the typical values of compensation
components for some standard output voltages
with various output capacitors and inductors.
The values of the compensation components
have been optimized for fast transient
responses and good stability at given conditions.
Table 3—Compensation Values for Typical
Output Voltage/Capacitor Combinations
V
OUT
(V)
L (µH)
C2
(µF)
R3
(k
Ω
)
C3
(pF)
C6
1.8 4.7 47
105
100
None
2.5
4.7 - 6.8
22
54.9
220
None
3.3 6.8
-10 22 68.1 220 None
5
15 - 22
22
100
150
None
12
22 - 33
22
147
150
None
To optimize the compensation components for
conditions not listed in Table 3, the following
procedure can be used.
1. Choose the compensation resistor (R3) to set
the desired crossover frequency. Determine the
R3 value by the following equation:
FB
OUT
CS
EA
C
V
V
G
G
f
2
C
2
3
R
Where f
C
is the desired crossover frequency.
2. Choose the compensation capacitor (C3) to
achieve the desired phase margin. For
applications with typical inductor values, setting
the compensation zero, f
Z1
, below one forth of
the crossover frequency provides sufficient
phase margin. Determine the C3 value by the
following equation:
C
f
3
R
2
4
3
C
3. Determine if the second compensation
capacitor (C6) is required. It is required if the
ESR zero of the output capacitor is located at
less than half of the switching frequency, or the
following relationship is valid:
2
f
R
2
C
2
1
S
ESR
If this is the case, then add the second
compensation capacitor (C6) to set the pole f
P3
at the location of the ESR zero. Determine the
C6 value by the equation:
3
R
R
2
C
6
C
ESR