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MP1591 – 2A, 32V, 330KHz STEP-DOWN CONVERTER
MP1591 Rev. 2.3
9/27/2006
www.MonolithicPower.com
7
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
2006 MPS. All Rights Reserved.
Compensation
The system stability is 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 loop gain is:
LOAD
CS
VEA
OUT
V
REF
V
VDC
R
G
A
A
×
×
×
=
Where V
REF
is the feedback threshold voltage,
1.230V, A
VEA
is the transconductance error
amplifier voltage gain, 400 V/V, and G
CS
is the
current sense gain (roughly the output current
divided by the voltage at COMP), 3.5 A/V.
The system has 2 poles of importance; one is
due to the compensation capacitor (C4) and the
other is due to the output capacitor (C5). These
are:
)
4
C
A
2
(
G
f
VEA
MEA
1
P
×
×
π
=
Where f
P1
is the first pole, and G
MEA
is the error
amplifier transconductance (770μS) and
)
5
C
R
2
(
1
f
LOAD
2
P
×
×
π
=
The system has one zero of importance due to
the compensation capacitor (C4) and the
compensation resistor (R3) which is
)
4
C
3
R
2
(
1
f
1
Z
×
×
π
=
If large value capacitors with relatively high
equivalent-series-resistance (ESR) are used,
the zero due to the capacitance and ESR of the
output capacitor can be compensated by a third
pole set by R3 and C3
)
3
C
3
R
2
(
1
f
3
P
×
×
π
=
The system crossover frequency f
C,
(the
frequency where the loop gain drops to 1, or
0dB) is important. A good rule of thumb is to set
the crossover frequency to approximately one
tenth of the switching frequency. In this case,
the switching frequency is 330KHz, so use a
crossover
frequency
crossover frequencies result in slower response
and worse transient load recovery. Higher
crossover frequencies can result in instability.
of
33KHz.
Lower
Choosing the Compensation Components
The values of the compensation components
given in Table 4 yield a stable control loop for
the output voltage and given capacitor.
Table 4—Compensation Values for Typical
Output Voltage/Capacitor Combinations
V
OUT
C5
R3
C3
C4
2.5V
22μF Ceramic
3.9k
None
4.7nF
3.3V
22μF Ceramic
5.1k
None
3.9nF
5V
22μF Ceramic
7.5k
None
2.7nF
12V
22μF Ceramic
18k
None
1.2nF
2.5V
47μF SP-Cap
8.2k
None
2.2nF
3.3V
47μF SP-Cap
10k
None
2.2nF
5V
47μF SP-Cap
16k
None
1.5nF
12V
47μF SP-Cap
36k
None
1nF
2.5V
560μF/6.3V, AL
30m
ESR
560μF/6.3V, AL
30m
ESR
470μF/10V, AL
30m
ESR
220μF/25V, AL
30m
ESR
100k
150pF
1nF
3.3V
120k
120pF
1nF
5V
150k
82pF
1nF
12V
180k
33pF
1nF
Note: “AL” = Electrolytic