MP1580 – 2A, 380KHz STEP-DOWN CONVERTER
MP1580 Rev. 3.0 www.MonolithicPower.com 7
10/10/2007 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
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TM
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:
OUT
FB
VEACSLOADVDC
V
V
AGRA ×××=
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:
VEA
EA
1P
A3C2
G
f
××π
=
Where P1 is the first pole and G
EA
is the error
amplifier transconductance (770µA/V).
and
LOAD
2P
R2C2
1
f
××π
=
The system has one zero of importance, due to
the compensation capacitor (C3) and the
compensation resistor (R3). The zero is:
3R3C2
1
f
1Z
××π
=
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:
3R6C2
1
f
3P
××π
=
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.
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
2.5V 22F Ceramic 7.5k 2.2nF None
3.3V 22F Ceramic 10k 2nF None
5V 22F Ceramic 15k 1.2nF None
12V 22F Ceramic 33k 1nF None
2.5V
560F/6.3V
(30m ESR)
200k 1nF 100pF
3.3V
560F/6.3V
(30m ESR)
200k 1nF 82pF
5V
470F/10V
(30m ESR)
250k 1nF 56pF
12V
220F/25V
(30m ESR)
250k 1nF 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:
FB
OUT
CSEA
C
V
V
GG
f2C2
3R ×
×
×
=
Putting in the known constants and setting the
crossover frequency to the desired 40KHz:
OUT
8
V2C1037.13R ×××≈
Choose the compensation capacitor to set the
zero below ¼ of the crossover frequency.
Determine the value by the following equation:
3R
V2C22.0
3C
OUT
××
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