Rainbow Electronics MAX1748 - Manual (Page 12)

MAX1748
Triple-Output TFT LCD DC-DC Converter
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
LR , is approximated by:
P LR
≅ (I
MAIN
✕ V MAIN / V IN ) 2 ✕ R L
where R L is the inductor series resistance.
Output Capacitor
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.
Input Capacitor
The input capacitor (C IN ) in boost designs reduces the
current peaks drawn from the input supply and reduces
noise injection. The value of C
IN is largely determined
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
IN as for C OUT . Table 1 lists suggested com-
ponent suppliers.
Integrator Capacitor
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 100k
Ω resistor
to GND.
Rectifier Diode
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
MAIN ).
Charge Pump
Efficiency Considerations
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:
Efficiency
≅ V
NEG / [V IN
✕ N];
for the negative charge pump
Efficiency ≅ V
POS / [V IN
✕ (N + 1)];
for the positive charge pump
where N is the number of charge-pump stages.
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
SUPPLIER PHONE FAX
INDUCTORS
Coilcraft 847-639-6400 847-639-1469
Coiltronics 561-241-7876 561-241-9339
Sumida USA 847-956-0666 847-956-0702
Toko 847-297-0070 847-699-1194
CAPACITORS
AVX 803-946-0690 803-626-3123
Kemet 408-986-0424 408-986-1442
Sanyo 619-661-6835 619-661-1055
Taiyo Yuden 408-573-4150 408-573-4159
DIODES
Central
Semiconductor
516-435-1110 516-435-1824
International
Rectifier
310-322-3331 310-322-3332
Motorola 602-303-5454 602-994-6430
Nihon 847-843-7500 847-843-2798
Zetex 516-543-7100 516-864-7630
Table 1. Component Suppliers
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