MAX1578-MAX1579 Datasheet, PDF(11/13 Page) Maxim Integrated Products – Complete Bias and White LED Power Supplies for Small TFT Displays

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MAX1578-MAX1579 Datasheet, PDF (11/13 Pages) Maxim Integrated Products – Complete Bias and White LED Power Supplies for Small TFT Displays

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Complete Bias and White LED Power Supplies

for Small TFT Displays

Component Selection

Use only ceramic capacitors with an X5R, X7R, or bet-

ter dielectric. See Table 1 for a list of recommended

components.

Capacitor Selection

Use low-ESR ceramic capacitors. Recommended val-

ues for the capacitors are shown in Table 1. To ensure

stability over a wide temperature range, ceramic

capacitors with an X5R or X7R dielectric are recom-

mended. Place these capacitors as close to the IC as

possible.

Inductor Selection

Recommended inductor values range from 10ÂµH to

47ÂµH. A 22ÂµH inductor optimizes the efficiency for most

applications while maintaining low 15mVP-P input ripple.

With input voltages near 5V, a larger value of inductance

can be more efficient. To prevent core saturation, ensure

that the inductor-saturation current rating exceeds the

peak inductor current for the application. Calculate the

peak inductor current with the following formula:

IPEAK

VOUT(MAX) Ã ILED(MAX)

0.8 Ã VIN(MIN)

VIN(MIN) Ã 0.8Âµs

2ÃL

Schottky Diode Selection

The MAX1578/MAX1579 require a high-speed rectifica-

tion diode (D1) for optimum performance. A Schottky

diode is recommended due to its fast recovery time

and low forward-voltage drop. Ensure that the diodeâs

average and peak current ratings exceed the average

output current and the peak inductor current, respec-

tively. In addition, the diodeâs reverse breakdown volt-

age must exceed VOUT. The RMS diode current is

calculated as:

IDIODE(RMS) = IOUT Ã IPEAK

PC Board Layout and Routing

Due to fast switching waveforms, careful PC board lay-

out is required. An evaluation kit (MAX1578EVKIT) is

available to speed design. When laying out a board,

minimize trace lengths between the IC and R1, the

inductor, the diode, the input capacitor, and the output

capacitor. Keep traces short, direct, and wide. Keep

noisy traces, such as the LX node trace, away from CS.

The IN bypass capacitor (CIN) should be placed as

close to the IC as possible. The transfer capacitors for

the charge pumps should be located as close as possi-

ble to the IC. PGND and GND should be connected

directly to the exposed paddle underneath the IC. The

ground connections of CIN and COUT should be as

close together as possible. The traces from IN to the

inductor and from the Schottky diode to the LEDs may

be longer. The MAX1579 evaluation kit contains a sam-

ple layout to speed designs.

Chip Information

TRANSISTOR COUNT: 3801

PROCESS: BiCMOS

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