Rainbow Electronics MAX8712 User Manual
Page 20
MAX8710/MAX8711/MAX8712
Low-Cost Linear-Regulator
LCD Panel Power Supplies
20
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Charge-Pump Regulators
Number of Charge-Pump Stages
For highest efficiency, always choose the lowest num-
ber of charge-pump stages that meets 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
P
is the positive charge-pump regulator out-
put, V
INPUT
is the supply voltage for the charge-pump
regulators (V
SUPCP
, MAX8710 or V
OUTL
, MAX8711/
MAX8712), V
DIODE
is the forward-voltage drop of the
charge-pump diode, and V
SWITCH
is the voltage drop
of the internal switches. Use V
SWITCH
= 0.3V.
The number of negative charge-pump stages is given by:
where n
NEG
is the number of negative charge-pump
stages and V
GOFF
is the negative charge-pump regula-
tor output.
The above equations are derived based on the
assumption that the first stage of the positive charge
pump is connected to V
MAIN
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 another available supply, such as a 5V supply.
If the first charge-pump stage is powered from 5V, then
the above equations become:
Output Voltage Selection
Adjust the positive charge-pump-regulator output volt-
age by connecting a resistive voltage-divider from the
regulator output V
P
to GND with the center tap connect-
ed to FBP (Figure 1). Select the lower resistor of divider
R4 in the range of 10kΩ to 50kΩ. Calculate upper resistor
R3 with the following equation:
where V
FBP
= 2.5V (typ) is the regulation point of the
positive charge-pump regulator.
Adjust the negative charge-pump-regulator output volt-
age by connecting a resistive voltage-divider from the
negative charge-pump output V
GOFF
to REF with the
center tap connected to FBN (Figure 1). Select R6 in
the 20kΩ to 100kΩ range. Calculate R5 with the follow-
ing equation:
where V
REF
= 5V and V
FBN
= 250mV is the regulation
point of the negative charge-pump regulator.
Flying Capacitor
Increasing the flying-capacitor (C
X
) value lowers the
effective source impedance and increases the output-
current capability of the charge pump. Increasing the
capacitance indefinitely has a negligible effect on out-
put-current capability because the internal switch resis-
tance and the diode impedance place a lower limit on
the source impedance. A 0.1µF ceramic capacitor
works well in most low-current applications. The flying
capacitor’s voltage rating must exceed the following:
V
CX
> n x V
INPUT
where n is the stage number in which the flying capaci-
tor is used, and V
INPUT
is the supply voltage for the
charge-pump regulators (V
SUPCP
, MAX8710 or V
OUTL
,
MAX8711/MAX8712).
Charge-Pump Input Capacitor
Use an input capacitor with a value equal to or greater
than the flying capacitor. Place the capacitor as close
to the IC as possible. Connect the capacitor directly
to PGND.
R
R
V
V
V
V
FBN
GOFF
REF
FBN
5
6
=
×
−
−
R
R
V
V
P
FBP
3
4
1
=
×
−
n
V
V
V
V
V
n
V
V
V
V
V
POS
P
SWITCH
INPUT
DIODE
NEG
GOFF
SWITCH
INPUT
DIODE
=
+
Ч
=
+
+
Ч
−
−
−
−
5
2
5
2
n
V
V
V
V
NEG
GOFF
SWITCH
INPUT
DIODE
=
+
×
−
−
2
n
V
V
V
V
V
POS
P
SWITCH
SUPCP
INPUT
DIODE
=
+
Ч
−
−
2