Datasheet

Because R11 is less than 100kΩ, use 100kΩ for R11 and
recalculate C10 as (step 3):
1
C10 370pF
2µ 4.3kHz 100k
≈=
× ×Ω
Use the standard value of 470pF for C10 and recalculate
the crossover frequency as:
CROSSOVER
100µS 4180
f 70.8kHz
2µ 470pF 2000
×
≈=
××
Since the crossover frequency is less than 1/5th the
switching frequency, 470pF is an acceptable value for
C10.
Because the high-frequency pole of the current-mode
control is at 64kHz, the feed-forward capacitor is (step 4):
1
C23 140pF
2µ 64kHz 17.8k
≈=
× ×Ω
Use a standard value of 150pF for C23. The pole formed
by C23, R1 and R2 occur at 159kHz, above the 70.8kHz
crossover frequency.
Because a ceramic output capacitor is used in the circuit
of Figure1, the ESR zero occurs well above the crossover
frequency, so no additional compensation capacitor (C2)
is needed (step 5).
Output Voltage Selection
The MAX1530/MAX1531 step-down regulator’s output
voltage can be adjusted by connecting a resistive voltage-
divider from the output to AGND with the center tap con-
nected to FB (Figure 1). Select R2 in the 5kΩ to 50kΩ
range. Calculate R1 with the following equation:
OUT
FB
V
R1 R2 1
V

=×−


where V
FB
= 1.238V, and V
OUT
may vary from 1.238V to
approximately 0.6 × V
IN
(V
IN
is up to 28V).
Boost-Supply Diode
A signal diode, such as the 1N4148, works well in most
applications. If the input voltage goes below 6V, use
a small 100mA Schottky diode for slightly improved ef
ficiency and dropout characteristics. Do not use power di
odes, such as the 1N5817 or 1N4001, since high junction
capacitance can charge up VL to excessive voltages.
Charge Pumps
Selecting the Number of Charge-Pump Stages
For highest efficiency, always choose the lowest number
of charge-pump stages that meet the output requirement.
The number of positive charge-pump stages is given by:
POS DROPOUT IN
POS
IN D
VV V
N
V 2V
+−
=
−×
where N
POS
is the number of positive charge-pump
stages, V
POS
is the positive charge-pump output, V
IN
is the input voltage of the step-down regulator, V
D
is
the forward voltage drop of the charge-pump diode, and
V
DROPOUT
is the dropout margin for the linear regulator.
Use V
DROPOUT
= 0.3V. The number of negative charge-
pump stages is given by:
NEG DROPOUT
NEG
IN D
VV
N
V 2V
−+
=
−×
where N
NEG
is the number of negative charge-pump
stages, V
NEG
is the negative charge-pump output, VIN
is the input voltage of the step-down regulator, V
D
is
the forward voltage drop of the charge-pump diode, and
V
DROPOUT
is the dropout margin for the linear regulator.
Use V
DROPOUT
= 0.3V.
The above equations are derived based on the assump-
tion that the first stage of the positive charge pump is con-
nected to V
IN
and the first stage of the negative charge
pump is connected to ground. Sometimes fractional
stages are more desirable for better efficiency. This can
be done by connecting the first stage to V
OUT
or another
available supply. If the first stage of the positive charger
pump is powered from the output of the step-down regula-
tor V
OUT
, then the equation becomes:
POS DROPOUT OUT
POS
IN D
VV V
N
V 2V
−+ −
=
−×
If the first stage of the negative charge pump is powered
from the output of the step-down regulator V
OUT
, then the
equation becomes:
NEG DROPOUT OUT
NEG
IN D
VV V
N
V 2V
−+ +
=
−×
MAX1530/MAX1531 Multiple-Output Power-Supply
Controllers for LCD Monitors
www.maximintegrated.com
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