High-Efficiency, 26V Step-Up Converters

for Two to Six White LEDs

6

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Detailed Description

The MAX1561/MAX1599s’ high efficiency and small size

make them ideally suited to drive up to six series-con-

nected LEDs. The device operates as a boost DC-DC

converter that regulates output current rather than volt-

age. The MAX1561/MAX1599 provide even illumination

by sourcing the same output current through each LED,

eliminating the need for expensive factory calibration.

The fast 1MHz (500kHz for the MAX1599) internal oscil-

lator allows for a small inductor and small input and out-

put capacitors while minimizing input and output ripple.

The single analog control input allows easy adjustment

of LED brightness and on/off control. This allows either

simple logic-level on/off control, analog voltage control,

or PWM duty-cycle control of both brightness and shut-

down. In shutdown, supply current is reduced to a low

0.3µA. A soft-start gradually illuminates the LEDs, elimi-

nating the inrush current during startup.

Soft-Start

The MAX1561/MAX1599 attain soft-start by charging

rises above 1.25V, the internal MOSFET begins switch-

2.25V, the duty cycle is at its maximum. See the Typical

Operating Characteristics for an example of soft-start

operation.

Shutdown

less than 100mV for more than 8.2ms (16.4ms for the

MAX1599). In shutdown, supply current is reduced to

0.3µA by powering down the entire IC except for the

during shutdown, allowing the device to reinitiate soft-

start when it is enabled. Although the internal N-chan-

nel MOSFET does not switch in shutdown, there is still

a DC current path between the input and the LEDs

through the inductor and Schottky diode. The minimum

forward voltage of the LED array must exceed the max-

imum input voltage to ensure that the LEDs remain off

in shutdown. However, with two or more LEDs, the for-

ward voltage is large enough to keep leakage current

low, less than 1µA (typ). Typical shutdown timing char-

acteristics are shown in the Typical Operating

Characteristics.

Overvoltage Protection

above 27V. The protection circuitry stops the internal

to 0V. The device comes out of OVLO and into soft-

PIN

NAME

FUNCTION

1

OUT

drops below 25V, then the IC reenters soft-start. Connect a 0.1µF capacitor from OUT to PGND.

2

IN

Input Voltage Supply. Input voltage range is 2.6V to 5.5V. Connect a 2.2µF capacitor from IN to PGND.

3

CTRL

Brightness Control Input. LED brightness is controlled by the voltage applied to CTRL. Varying the voltage

from 0.24V to 1.62V adjusts the brightness from dim to bright, respectively. Any voltage above 1.62V does

not increase brightness. If CTRL is held below 100mV for more than 8.2ms (16.4ms for the MAX1599), the

IC shuts down.

4CS

Current-Sense Feedback Input. Connect a resistor from CS to GND to set the LED bias current. The

5

COMP

6

GND

Ground. Connect to PGND and the exposed pad directly under the IC.

7

PGND

Power Ground. Connect to GND and the exposed pad directly under the IC.

8

LX

Inductor Connection. This pin is high impedance during shutdown.

Exposed

Pad

EP

Ground. Connect directly to GND and PGND under the IC.

Pin Description