Rainbow-electronics MAX1748 Bedienungsanleitung PDF herunterladen (Seite 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

, is approximated by:

≅ (I

MAIN

/ V

)

where R

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 boost designs reduces the

current peaks drawn from the input supply and reduces

noise injection. The value of C

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

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 100kΩ 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

N];

for the negative charge pump

Efficiency ≅ V

POS

/ [V

(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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