MPS MP1591, Manual, Page: 7 / 11

MP1591 – 2A, 32V, 330KHz STEP-DOWN CONVERTER
MP1591 Rev. 2.3 www.MonolithicPower.com
7
9/21/2007 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2007 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
REF
VDC
R G A
V
V
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 (770 μ S) 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 of 33KHz. Lower
crossover frequencies result in slower response
and worse transient load recovery. Higher
crossover frequencies can result in instability.
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 22 μ F Ceramic 3.9k Ω None 4.7nF
3.3V 22 μ F Ceramic 5.1k Ω None 3.9nF
5V 22 μ F Ceramic 7.5k Ω None 2.7nF
12V 22 μ F Ceramic 18k Ω None 1.2nF
2.5V 47 μ F SP-Cap 8.2k Ω None 2.2nF
3.3V 47 μ F SP-Cap 10k Ω None 2.2nF
5V 47 μ F SP-Cap 16k Ω None 1.5nF
12V 47 μ F SP-Cap 36k Ω None 1nF
2.5V
560 μ F/6.3V, AL
30m Ω ESR
100k Ω 150pF 1nF
3.3V
560 μ F/6.3V, AL
30m Ω ESR
120k Ω 120pF 1nF
5V
470 μ F/10V, AL
30m Ω ESR
150k Ω 82pF 1nF
12V
220 μ F/25V, AL
30m Ω ESR
180k Ω 33pF 1nF
Note: “AL” = Electrolytic