LP3311 datasheet(6/8 Pages) POWER

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Preliminary Datasheet

LP3311

LP3311 – 00 Version 1.0 Datasheet

Feb.-2011

www.lowpowersemi.com

Page 6 of 8

Applications Information

LED Current Control

The LP3311 regulates the LED current by setting

the current sense resistor (R1) connecting to

feedback

and

ground.

The

internal

feedback

reference voltage is 200mV or LP3311A is 300mV.

The LED current can be set from following equation

easily.

ILED=200mV/R1 (LP3311)

ILED=300mV/R1 (LP3311A)

In order to have an accurate LED current, precision

resistors are preferred (1% is recommended). The

table for R1 selection is shown below.

Inductor Selection

The recommended value of inductor for 2 to 10

WLEDs applications are 2.2 to 22µH. Small size

and better efficiency are the major concerns for

portable device, such as LP3311 used for mobile

phone. The inductor should have low core loss at

1.3MHz and low DCR for better efficiency. To

avoid inductor saturation current rating should be

considered.

Dimming control

a. Using a PWM Signal to EN Pin

For controlling the LED brightness, the LP3311 can

perform the dimming control by applying a PWM

signal to EN pin. The internal soft-start and wide

range dimming frequency from 100Hz to 100KHz

can

insignificantly

reduce

audio

noise

when

dimming. The average LED current is proportional

to the PWM signal duty cycle. The magnitude of the

PWM signal should be higher than the maximum

enable voltage of EN pin, in order to let the

dimming control perform correctly.

Cout

1uF

4.7uH

Cin

10uF

Vin

OVP

LP3311

PWM

b. Using a DC Voltage

Using a variable DC voltage to adjust the brightness

is a popular method in some applications. The

dimming control using a DC voltage circuit is

shown in Figure 4. According to the Superposition

Theorem, as the DC voltage increases, the voltage

contributed to VFB increases and the voltage drop

on R2 decreases, i.e. the LED current decreases. For

example, if the VDC range is from 0V to 2.8V, the

selection of resistors in Figure 4 sets dimming

control of LED current from 20mA to 0mA.

Cout

1uF

4.7uH

Cin

10uF

Vin

OVP

LP3311

90k

VDC

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