MAX1748 Maxim, MAX1748 Datasheet - Page 12

• Manufacturer Part Number: MAX1748
• Description: Triple-Output TFT LCD DC-DC Converter
• Manufacturer: Maxim

The MAX1748’s fast current-limit circuitry allows the use
of soft-saturation inductors while still protecting the IC.
The inductor’s DC resistance significantly affects effi-
ciency. For best performance, select inductors with
resistance less than the internal N-channel FET resis-
tance. To minimize radiated noise in sensitive applica-
tions, use a shielded inductor.
The inductor should have as low a series resistance as
possible. For continuous inductor current, the power
loss in the inductor resistance, P
where R
A 10µF capacitor works well in most applications. The
equivalent series resistance (ESR) of the output filter
capacitor affects efficiency and output ripple. Output
voltage ripple is largely the product of the peak induc-
tor current and the output capacitor ESR. Use low-ESR
ceramic capacitors for best performance. Low-ESR,
surface-mount tantalum capacitors with higher capacity
may be used for load transients with high peak cur-
rents. Voltage ratings and temperature characteristics
should be considered.
The input capacitor (C
current peaks drawn from the input supply and reduces
noise injection. The value of C
by the source impedance of the input supply. High
source impedance requires high input capacitance,
particularly as the input voltage falls. Since step-up DC-
DC converters act as “constant-power” loads to their
input supply, input current rises as input voltage falls. A
good starting point is to use the same capacitance
value for C
ponent suppliers.
The MAX1748 contains an internal current integrator
that improves the DC load regulation but increases the
peak-to-peak transient voltage (see the load-transient
waveforms in the Typical Operating Characteristics).
For highly accurate DC load regulation, enable the cur-
rent integrator by connecting a 470pF capacitor to
INTG. To minimize the peak-to-peak transient voltage at
the expense of DC regulation, disable the integrator by
connecting INTG to REF and adding a 100kΩ resistor
to GND.
Triple-Output TFT LCD DC-DC Converter
12
______________________________________________________________________________________
L
is the inductor series resistance.
IN
P
LR
as for C
≅ (I
MAIN
OUT
IN
✕
) in boost designs reduces the
. Table 1 lists suggested com-
V
MAIN
IN
LR
/ V
Integrator Capacitor
is largely determined
, is approximated by:
IN
Output Capacitor
)
2
Input Capacitor
✕
R
L
Use a Schottky diode with an average current rating
equal to or greater than the peak inductor current, and
a voltage rating at least 1.5 times the main output volt-
age (V
The efficiency characteristics of the MAX1748 regulated
charge pumps are similar to a linear regulator. They are
dominated by quiescent current at low output currents
and by the input voltage at higher output currents (see
Typical Operating Characteristics). So the maximum
efficiency may be approximated by:
where N is the number of charge-pump stages.
Adjust the positive output voltage by connecting a volt-
age-divider from the output (V
Typical Operating Circuit). Adjust the negative output
Table 1. Component Suppliers
INDUCTORS
Coilcraft
Coiltronics
Sumida USA
Toko
CAPACITORS
AVX
Kemet
Sanyo
Taiyo Yuden
DIODES
Central
Semiconductor
International
Rectifier
Motorola
Nihon
Zetex
SUPPLIER
MAIN
Efficiency ≅ V
).
for the negative charge pump
Efficiency ≅ V
for the positive charge pump
847-639-6400
561-241-7876
847-956-0666
847-297-0070
803-946-0690
408-986-0424
619-661-6835
408-573-4150
516-435-1110
310-322-3331
602-303-5454
847-843-7500
516-543-7100
POS
PHONE
NEG
Efficiency Considerations
/ [V
Output Voltage Selection
POS
/ [V
IN
) to FBP to GND (see
✕
IN
(N + 1)];
✕
Charge Pump
N];
Rectifier Diode
847-639-1469
561-241-9339
847-956-0702
847-699-1194
803-626-3123
408-986-1442
619-661-1055
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