MP1583 – 3A, 23V, 385KHz STEP-DOWN CONVERTER
The MP1583 can be optimized for a wide range
of capacitance and ESR values.
Compensation Components
The MP1583 employs current mode control for
easy compensation and fast transient response.
The system stability and transient response are
controlled through the COMP pin. COMP 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:
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:
the
the
to
the
f
P
3
=
1
2
π
×
C
6
×
R
3
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 unity gain) is
important.
Lower crossover frequencies result in slower
line and load transient responses, while higher
crossover frequencies could cause system
instability. A good standard is to set the
crossover frequency to approximately one-tenth
of the switching frequency. The switching
frequency for the MP1583 is 385KHz, so the
desired crossover frequency is around 38KHz.
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
(Please reference Fig. 3 and Fig. 4)
A
VDC
=
R
LOAD
×
G
CS
×
A
VEA
×
V
FB
V
OUT
Where A
VEA
is the error amplifier voltage gain,
G
CS
is the current sense transconductance 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 while the
other is due to the output capacitor and the load
resistor. These poles are located at:
f
P
1
=
f
P
2
=
G
EA
2
π
×
C
3
×
A
VEA
1
2
π
×
C
2
×
R
LOAD
the
error
amplifier
is
Where
G
EA
transconductance.
V
OUT
2.5V
3.3V
5V
12V
2.5V
3.3V
5V
12V
C2
22μF
Ceramic
22μF
Ceramic
22μF
Ceramic
22μF
Ceramic
560μF Al.
30mΩ ESR
560μF Al
30mΩ ESR
470μF Al.
30mΩ ESR
220μF Al.
30mΩ ESR
R3
3.9kΩ
4.7kΩ
7.5kΩ
16.9kΩ
91kΩ
120kΩ
100kΩ
169kΩ
C3
5.6nF
4.7nF
4.7nF
1.5nF
1nF
1nF
1nF
1nF
C6
None
None
None
None
150pF
120pF
120pF
39pF
The system has one zero of importance, due to
the compensation capacitor (C3) and the
compensation resistor (R3). This zero is located
at:
f
Z
1
=
1
2
π
×
C
3
×
R
3
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
×
C
2
×
R
ESR
MP1583 Rev. 3.1
6/20/2011
www.MonolithicPower.com
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2011 MPS. All Rights Reserved.
8
MPS [ MONOLITHIC POWER SYSTEMS ]
