Rainbow Electronics MAX1531 User Manual
Page 28
MAX1530/MAX1531
Multiple-Output Power-Supply Controllers for
LCD Monitors
28
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Because R11 is less than 100k
Ω, use 100kΩ for R11
and recalculate C10 as (step 3):
Use the standard value of 470pF for C10 and recalcu-
late the crossover frequency as:
Since the crossover frequency is less than 1/5th the switch-
ing 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):
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 cir-
cuit 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 volt-
age-divider from the output to AGND with the center
tap connected to FB (Figure 1). Select R2 in the 5k
Ω to
50k
Ω range. Calculate R1 with the following equation:
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 effi-
ciency and dropout characteristics. Do not use power
diodes, 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 num-
ber of charge-pump stages that meet the output
requirement. The number of positive charge-pump
stages is given by:
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 regula-
tor. Use V
DROPOUT
= 0.3V.
The number of negative charge-pump stages is given by:
where N
NEG
is the number of negative charge-pump
stages, V
NEG
is the negative 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 reg-
ulator. Use V
DROPOUT
= 0.3V.
The above equations are derived based on the
assumption that the first stage of the positive charge
pump is connected to V
IN
and the first stage of the
negative charge pump is connected to ground.
Sometimes fractional stages are more desirable for bet-
ter 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 regulator V
OUT
, then the equa-
tion becomes:
If the first stage of the negative charge pump is pow-
ered from the output of the step-down regulator V
OUT
,
then the equation becomes:
N
V
V
V
V
V
NEG
NEG
DROPOUT
OUT
IN
D
= −
+
+
− ×
2
N
V
V
V
V
V
POS
POS
DROPOUT
OUT
IN
D
= −
+
−
− ×
2
N
V
V
V
V
NEG
NEG
DROPOUT
IN
D
= −
+
− ×
2
N
V
V
V
V
V
POS
POS
DROPOUT
IN
IN
D
=
+
−
− ×
2
R
R
V
V
OUT
FB
1
2
1
=
×
−
C
kHz
k
pF
23
1
2
64
17 8
140
≈
Ч
Ч
=
.
π
Ω
f
S
pF
kHz
CROSSOVER
≈
Ч
Ч
Ч
=
.
100
4180
2
470
2000
70 8
µ
π
C
kHz
k
pF
10
1
2
4 3
100
370
≈
Ч
Ч
=
.
π
Ω