MP1591 – 2A, 32V, 330KHz STEP-DOWN CONVERTER
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:
A
VDC
=
V
REF
×
A
VEA
×
G
CS
×
R
LOAD
V
OUT
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
2.5V
3.3V
5V
12V
2.5V
3.3V
5V
12V
2.5V
3.3V
5V
12V
C5
22µF Ceramic
22µF Ceramic
22µF Ceramic
22µF Ceramic
47µF SP-Cap
47µF SP-Cap
47µF SP-Cap
47µF SP-Cap
560µF/6.3V, AL
30mΩ ESR
560µF/6.3V, AL
30mΩ ESR
470µF/10V, AL
30mΩ ESR
220µF/25V, AL
30mΩ ESR
R3
3.9kΩ
5.1kΩ
7.5kΩ
18kΩ
8.2kΩ
10kΩ
16kΩ
36kΩ
100kΩ
120kΩ
150kΩ
180kΩ
C3
None
None
None
None
None
None
None
None
150pF
120pF
82pF
33pF
C4
4.7nF
3.9nF
2.7nF
1.2nF
2.2nF
2.2nF
1.5nF
1nF
1nF
1nF
1nF
1nF
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:
f
P1
=
G
MEA
(2
π ×
A
VEA
×
C4)
Where f
P1
is the first pole, and G
MEA
is the error
amplifier transconductance (770µS) and
f
P2
=
1
(2
π ×
R
LOAD
×
C5)
The system has one zero of importance due to
the compensation capacitor (C4) and the
compensation resistor (R3) which is
f
Z1
=
1
(2
π ×
R3
×
C4)
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
f
P3
=
1
(2
π ×
R3
×
C3)
Note: “AL” = Electrolytic
MP1591 Rev. 2.3
9/27/2006
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