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MP2363 – 3A, 27V, 365KHz STEP-DOWN CONVERTER
MP2363 Rev. 1.0
6/15/2006
www.MonolithicPower.com
8
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
2006 MPS. All Rights Reserved.
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:
OUT
V
FB
VEA
CS
LOAD
VDC
V
A
G
R
A
×
×
×
=
Where A
VEA
is the error amplifier voltage gain,
400V/V;
G
CS
is
transconductance, 7A/V, and R
LOAD
is the load
resistor value.
the
current
sense
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:
VEA
EA
3
1
P
A
C
2
G
f
×
×
π
=
LOAD
2
P
R
2
C
2
1
×
f
×
π
=
Where
transconductance, 800μA/V.
G
EA
is
the
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:
3
R
3
C
2
1
f
1
Z
×
×
π
=
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:
ESR
ESR
R
2
C
2
1
f
×
×
π
=
In this case, a third pole set by the
compensation
capacitor
compensation
resistor
compensate the effect of the ESR zero on the
loop gain. This pole is located at:
(C6)
and
used
the
to
(R3)
is
3
R
6
C
2
1
f
3
P
×
×
π
=
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
responses
and
good
conditions.
for
stability
fast
transient
at
given
Table 3—Compensation Values for Typical
Output Voltage/Capacitor Combinations
V
OUT
1.8V
L
C2
100
μ
F
Ceramic
47
μ
F
Ceramic
22
μ
Fx2
Ceramic
22
μ
Fx2
Ceramic
22
μ
Fx2
Ceramic
R3
5.6k
C3
3.3nF
C6
None
4.7
μ
H
2.5V
4.7
–
10
μ
H
3.32k
6.8nF
None
3.3V
6.8
–
10
μ
H
4.02k
8.2nF
None
5V
10
–
15
μ
H
6.49k
10nF
None
12V
15
–
20
μ
H
15k
4.7nF
None