MP2363 – 3A, 27V, 365KHz STEP-DOWN CONVERTER
Compensation Components
MP2363 employs current mode control for easy
compensation and fast transient response. The
system stability and transient response are
controlled through the COMP pin. COMP pin is
the output of the internal transconductance
error amplifier. A series capacitor-resistor
combination sets a pole-zero combination to
control the characteristics of the control system.
The DC gain of the voltage feedback loop is
given by:
A
VDC
=
R
LOAD
×
G
CS
×
A
VEA
×
V
FB
V
OUT
In this case, a third pole set by
compensation capacitor (C6) and
compensation resistor (R3) is used
compensate the effect of the ESR zero on
loop gain. This pole is located at:
f
P3
=
1
2
π ×
C6
×
R3
the
the
to
the
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 can cause system
instability. A good rule of thumb is to set the
crossover frequency to approximately one-tenth
of the switching frequency. Switching frequency
for the MP2363 is 365KHz, so the desired
crossover frequency is around 36.5KHz.
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
1.8V
2.5V
3.3V
5V
12V
Where A
VEA
is the error amplifier voltage gain,
400V/V;
G
CS
is
the
current
sense
transconductance, 7A/V, and R
LOAD
is the load
resistor value.
The system has two poles of importance. One
is due to the compensation capacitor (C3) and
the output resistor of error amplifier, and the
other is due to the output capacitor and the load
resistor. These poles are located at:
f
P1
=
f
P2
=
G
EA
2
π ×
C3
×
A
VEA
1
2
π ×
C2
×
R
LOAD
Where
G
EA
is
the
transconductance, 800µA/V.
error
amplifier
The system has one zero of importance, due to
the compensation capacitor (C3) and the
compensation resistor (R3). This zero is located
at:
f
Z1
1
=
2
π ×
C3
×
R3
L
4.7µH
4.7
–
10µH
6.8
–
10µH
10
–
15µH
15
–
20µH
C2
100µF
Ceramic
R3
5.6kΩ
C3
3.3nF
6.8nF
8.2nF
10nF
4.7nF
C6
None
None
None
None
None
47µF
3.32kΩ
Ceramic
22µFx2 4.02kΩ
Ceramic
22µFx2 6.49kΩ
Ceramic
22µFx2
Ceramic
15kΩ
The system may have another zero of
importance, if the output capacitor has a large
capacitance and/or a high ESR value. The zero,
due to the ESR and capacitance of the output
capacitor, is located at:
f
ESR
=
1
2
π ×
C2
×
R
ESR
MP2363 Rev. 1.0
6/15/2006
www.MonolithicPower.com
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8
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