Rainbow Electronics MAX1885 User Manual
Page 29
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC-DC
Converters with Buffer
______________________________________________________________________________________
29
input voltage at higher output currents (see Typical
Operating Characteristics). So the maximum efficiency
may be approximated by:
for the positive charge pump, and:
for the negative charge pump, where V
SUPD
is the pos-
itive charge pump’s diode supply (Figure 4).
Output Voltage Selection
Adjust the positive output voltage by connecting a volt-
age divider from the output (V
POS
) to FBP to GND (see
Typical Operating Circuit). Adjust the negative output
voltage by connecting a voltage-divider from the output
(V
NEG
) to FBN to REF. Select R4 and R6 in the 50k
Ω to
100k
Ω range. Higher resistor values improve efficiency
at low output current but increase output voltage error
due to the feedback input bias current. For the negative
charge pump, higher resistor values also reduce the
load on the reference, which should not exceed 50µA
for greatest accuracy (including current through the
FLTSET resistors) to guarantee that V
REF
remains in
regulation (see Electrical Characteristics Table).
Calculate the remaining resistors with the following
equations:
R3 = R4 [(V
POS
/ V
REF
) - 1]
R5 = R6 |V
NEG
/ V
REF
|
where V
REF
= 1.25V. V
POS
may range from V
SUPP
to
40V, and V
NEG
may range from 0V to -40V.
Flying Capacitor
Increasing the flying capacitor (CX) value increases the
output current capability. Above a certain point,
increasing the capacitance has a negligible effect
because the output current capability becomes domi-
nated by the internal switch resistance and the diode
impedance. The flying capacitor’s voltage rating must
exceed the following:
for the positive charge pump, and:
for the negative charge pump, where N is the stage
number in which the flying capacitor appears, and
V
SUPD
is the positive charge pump’s diode supply
(Figure 4). For example, the two-stage positive charge
pump in the typical application circuit (Figure 1) where
V
SUPP
= V
SUPD
= 8V contains two flying capacitors.
The flying capacitor in the first stage (C4) requires a
voltage rating over 12V. The flying capacitor in the sec-
ond stage (C6) requires a voltage rating over 24V.
Charge-Pump Output Capacitor
Increasing the output capacitance or decreasing the
ESR reduces the output ripple voltage and the peak-to-
peak transient voltage. With ceramic capacitors, the
output voltage ripple is dominated by the capacitance
value. Use the following equation to approximate the
required capacitor value:
where f
CHP
is typically f
OSC
/2 (see Electrical
Characteristics).
Charge-Pump Input Capacitor
Use a bypass capacitor with a value equal to or greater
than the flying capacitor. Place the capacitor as close
to the IC as possible. Connect directly to power ground
(PGND).
Charge-Pump Rectifier Diodes
Use Schottky diodes with a current rating equal to or
greater than two times the average charge-pump input
current, and a voltage rating at least 1.5 times V
SUPP
for the positive charge pump and V
SUPN
for the nega-
tive charge pump.
Low-Dropout Linear Regulator (MAX1778/
MAX1881/MAX1883/MAX1884 Only)
Output Voltage Selection
Adjust the linear-regulator output voltage by connecting
a voltage-divider from LDOOUT to FBL to GND (Figure
5). Select R8 in the 5k
Ω to 50kΩ range. Calculate R7
with the following equation:
R7 = R8 [(V
LDOOUT
/ V
FBL
) - 1]
where V
FBL
= 1.25V, and V
LDOOUT
may range from
1.25V to (V
SUPL
- 300mV). FBL’s input bias current is
C
I
f
V
OUT
LOAD
CHP RIPPLE
≥
V
V
N
CXN NEG
SUPN
(
)
. (
)
> 1 5
V
V
V
N
CXN POS
SUPD
SUPP
(
)
.
(
)
>
+
[
]
1 5
1
-
η
NEG
V
V
N
NEG
SUPN
≅
η
POS
V
V
V
N
POS
SUPD
SUPP
≅
+