between the input and the LEDs through the inductor

and Schottky diode. To ensure the LEDs remain off

when the MAX1582 is in shutdown, the minimum input

forward voltage of the LED array must exceed the maxi-

mum input voltage. Typically, the leakage current

through the LEDs in shutdown is less than 1µA.

Soft-Start

above 1.25V, the internal MOSFET begins switching,

2.25V, the duty cycle is at its maximum.

Overvoltage Protection

tion circuitry stops the internal MOSFET from switching

below 25V.

Design Procedure

Adjusting LED Current

Adjusting the MAX1582’s output current changes the

brightness of the LEDs. The LED current is set by the

0 to +1.62V. To set the maximum current, calculate

therefore, a standard chip resistor is sufficient.

PWM Dimming Control

CTRL is also used as a digital input, allowing LED

brightness control with a logic-level PWM signal

applied directly to CTRL. The frequency range is from

200Hz to 200kHz, while 0% duty cycle corresponds to

zero current and 100% duty cycle corresponds to full

current. The error amplifier and compensation capaci-

tor form a lowpass filter, so PWM dimming results in DC

current to the LEDs without any additional RC filters

required.

Capacitor Selection

The exact values of input and output capacitors are not

critical. The typical value for the input capacitor is

2.2µF, and the typical value for the output capacitor is

0.1µF. Larger value capacitors can be used to reduce

input and output ripple, but at the expense of size and

higher cost.

Connect a 0.022µF capacitor from COMP to GND. The

time is found from:

Inductor Selection

Recommended inductor values range from 10µH to

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

ple. With input voltages near 5V, a larger value of induc-

tance may 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:

Schottky Diode Selection

The MAX1582’s high switching frequency demands a

high-speed rectification diode (D1) for optimum

efficiency. A Schottky diode is recommended due to its

fast recovery time and low forward-voltage drop.

Ensure that the diode’s average and peak current

rating exceeds the average output current and peak

inductor current. In addition, the diode’s reverse break-

current can be calculated from:

Applications Information

PC Board Layout

Due to fast-switching waveforms and high-current

paths, careful PC board layout is required. An evalua-

tion kit (MAX1582EVKIT) is available to speed design.

II

I

DIODE RMS

OUT

PEAK

()

≅×

1

I

VI

V

Vs

L

PEAK

OUT MAX

LED MAX

IN MIN

IN MIN

≅

×

×

+

×µ

×

.

.

()

()

()

()

1

08

08

2

tC

V

A

SS

COMP

.

=×

µ









125

5

R

I

SENSE

LED MAX

.

()

=

×

162

10

I

V

R

LED

CTRL

SENSE

=

×

10

High-Efficiency, 26V Step-Up Converter

for White LED Main and Subdisplay Backlighting

_______________________________________________________________________________________

7