Datasheet

Current-mode control has the effect of splitting the com-
plex pole pair of the output LC filter into a single low-
frequency pole and a single high-frequency pole. The
low-frequency current-mode pole depends on output
capacitor C
OUT
and the equivalent load resistance R
LE
,
given by the following:
POLE(LOW)
LE OUT
1
f
2µ R C
=
××
The high-frequency current-mode pole is given by:
SW
POLE(HIGH)
f
f
2µ n D'
=
××
The COMP pin, which is the output of the IC’s internal
transconductance error amplifier, is used to stabilize the
control loop. A series resistor (R11) and capacitor (C10)
are connected between COMP and AGND to form a
pole-zero pair. Another pole-zero pair can be added by
connecting a feed-forward capacitor (C23) in parallel with
feedback resistor R1. The compensation resistor and
capacitors are selected to optimize the loop stability.
The compensation capacitor (C10) creates a dominant
pole at very low frequency (a few hertz). The zero formed
by R11 and C10 cancels the low-frequency current-
mode pole. The zero formed by R1 and C23 cancels
the high-frequency current-mode pole and introduces a
preferable higher frequency pole. In applications where
ceramic capacitors are used, the ESR zero is usually
not a concern because the ESR zero occurs at very high
frequency. If the ESR zero does not occur at a frequency
at least one decade above the crossover, connect a sec-
ond parallel capacitor (C2) between COMP and AGND to
cancel the ESR zero. The component values shown in the
standard application circuits (Figure 1 and Figure 2) yield
stable operation and fast transient response over a broad
range of input-to-output voltages.
To design a compensation network for other components
or applications, use the following procedure to achieve
stable operation:
1) Select the crossover frequency f
CROSSOVER
(band-
width) to be 1/5th the switching frequency f
SW
or less:
SW
CROSSOVER
f
f
5
≤
Unnecessarily high bandwidth can increase noise sen-
sitivity while providing little benefit. Good transient
response with low amounts of output capacitance is
achieved with a crossover frequency between 20kHz
and 100kHz. The series compensation capacitor (C10)
generates a dominant pole that sets the desired cross-
over frequency. Determine C10 using the following
expression:
m DC
CROSSOVER VEA
g A
C10
2µ f A
×
≈
××
where g
m
is the error amplifier’s transconductance
(100μS typ).
2) The compensation resistor R11, together with capaci-
tor C10, provides a zero that is used to cancel the low-
frequency current-mode pole. Determine R11 using
the following expression:
POLE(LOW)
1
R11
2µ f C10
≈
××
3) Because the error amplifier has limited output current
(16μA typ), small values of R11 can prevent the error
amplifier from providing an immediate COMP voltage
change required for good transient response with mini-
mal output capacitance. If the calculated R11 value
is less than 100kΩ, use 100kΩ and recalculate C10
using the following formula:
POLE(LOW)
1
C10
2µ f 100k
≈
× ×Ω
Changing C10 also changes the crossover frequency;
the new crossover frequency is:
m DC
CROSSOVER
VEA
g A
f
2µ C10 A
×
=
××
The calculated crossover frequency should be less
than 1/5th the switching frequency. There are two
ways to lower the crossover frequency if the calculated
value is greater than 1/5th the switching frequency:
increase the high-side MOSFET R
DS(ON)
, or increase
the output capacitance. Increasing R
DS(ON)
reduces
the DC loop gain, which results in lower crossover
frequency. Increasing output capacitance reduces the
frequency of the lower low-frequency current-mode
MAX1530/MAX1531 Multiple-Output Power-Supply
Controllers for LCD Monitors
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