MAX1778EUG+ Maxim Integrated Products, MAX1778EUG+ Datasheet - Page 29

• Manufacturer Part Number: MAX1778EUG+
• Description: IC DCDC CONV MULTI OUT 24TSSOP
• Manufacturer: Maxim Integrated Products

Specifications of MAX1778EUG+

• Applications: Converter, TFT, LCD
• Voltage - Input: 2.7 ~ 5.5 V
• Number Of Outputs: 5
• Voltage - Output: 2.7 ~ 13 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-TSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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
itive charge pump’s diode supply (Figure 4).
Adjust the positive output voltage by connecting a volt-
age divider from the output (V
Typical Operating Circuit). Adjust the negative output
voltage by connecting a voltage-divider from the output
(V
100kΩ 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
regulation (see Electrical Characteristics Table).
Calculate the remaining resistors with the following
equations:
where V
40V, and V
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:
NEG
) to FBN to REF. Select R4 and R6 in the 50kΩ to
V
CXN POS
REF
η
NEG
POS
(
= 1.25V. V
R3 = R4 [(V
η
may range from 0V to -40V.
≅
NEG
)
R5 = R6 |V
______________________________________________________________________________________
>
V
SUPD
≅
1 5
.
V
POS
[
V
V
SUPN
V
+
NEG
POS
POS
SUPD
NEG
V
Output Voltage Selection
may range from V
N
SUPP
POS
/ V
/ V
+
REF
N
) to FBP to GND (see
REF
V
SUPP
) - 1]
Flying Capacitor
|
SUPD
REF
(
N
remains in
is the pos-
1 -
Quad-Output TFT LCD DC-DC
)
]
SUPP
to
for the negative charge pump, where N is the stage
number in which the flying capacitor appears, and
V
(Figure 4). For example, the two-stage positive charge
pump in the typical application circuit (Figure 1) where
V
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.
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
Characteristics).
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).
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
for the positive charge pump and V
tive charge pump.
Adjust the linear-regulator output voltage by connecting
a voltage-divider from LDOOUT to FBL to GND (Figure
5). Select R8 in the 5kΩ to 50kΩ range. Calculate R7
with the following equation:
where V
1.25V to (V
Converters with Buffer
SUPP
SUPD
Low-Dropout Linear Regulator (MAX1778/
= V
is the positive charge pump’s diode supply
FBL
CHP
MAX1881/MAX1883/MAX1884 Only)
SUPD
SUPL
V
R7 = R8 [(V
CXN NEG
= 1.25V, and V
C
OUT
is typically f
(
- 300mV). FBL’s input bias current is
= 8V contains two flying capacitors.
≥
)
Charge-Pump Output Capacitor
Charge-Pump Rectifier Diodes
f
Charge-Pump Input Capacitor
LDOOUT
> 1 5
CHP RIPPLE
I
LOAD
. (
V
Output Voltage Selection
LDOOUT
OSC
V
SUPN
/ V
/2 (see Electrical
FBL
SUPN
N
)
) - 1]
may range from
for the nega-
SUPP
29