TM
MP1580 – 2A, 380KHz 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
=
R
LOAD
×
G
CS
×
A
VEA
×
V
FB
V
OUT
In this case, the switching frequency is 380KHz,
so use a crossover frequency, f
C
, of 40KHz.
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 capacitor given.
Table 4—Compensation Values for Typical
Output Voltage/Capacitor Combinations
V
OUT
C2
R3
C3
C6
Where A
VEA
is the transconductance error
amplifier voltage gain, 400 V/V, G
CS
is the
current sense gain, (roughly the output current
divided by the voltage at COMP), 1.95 A/V and
R
LOAD
is the load resistance (V
OUT
/ I
OUT
where
I
OUT
is the output load current).
The system has 2 poles of importance, one is
due to the compensation capacitor (C3), and
the other is due to the output capacitor (C2).
These are:
f
P1
G
EA
=
2
π ×
C3
×
A
VEA
2.5V
3.3V
5V
12V
2.5V
3.3V
5V
12V
Where P1 is the first pole and G
EA
is the error
amplifier transconductance (770µA/V).
and
f
P2
1
=
2
π ×
C2
×
R
LOAD
22µF Ceramic
22µF Ceramic
22µF Ceramic
22µF Ceramic
560µF/6.3V
(30mΩ ESR)
560µF/6.3V
(30mΩ ESR)
470µF/10V
(30mΩ ESR)
220µF/25V
(30mΩ ESR)
7.5kΩ
10kΩ
15kΩ
33kΩ
200kΩ
200kΩ
250kΩ
250kΩ
2.2nF
2nF
1.2nF
1nF
1nF
1nF
1nF
1nF
None
None
None
None
100pF
82pF
56pF
27pF
To optimize the compensation components for
conditions not listed in Table 4, use the
following procedure:
Choose the compensation resistor to set the
desired crossover frequency. Determine the
value by the following equation:
R3
=
2
π ×
C2
×
f
C
V
OUT
×
G
EA
×
G
CS
V
FB
The system has one zero of importance, due to
the compensation capacitor (C3) and the
compensation resistor (R3). The zero is:
f
Z1
=
1
2
π ×
C3
×
R3
If a large value capacitor (C2) with relatively
high equivalent-series-resistance (ESR) is
used, the zero due to the capacitance and ESR
of the output capacitor can be compensated by
a third pole set by R3 and C6. The pole is:
f
P3
=
1
2
π ×
C6
×
R3
Putting in the known constants and setting the
crossover frequency to the desired 40KHz:
R3
≈
1.37
×
10
8
×
C2
×
V
OUT
Choose the compensation capacitor to set the
zero below ¼ of the crossover frequency.
Determine the value by the following equation:
C3
>
0.22
×
C2
×
V
OUT
R3
The system crossover frequency (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 1/10 of
the switching frequency.
MP1580 Rev. 3.0
12/5/2005
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7
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