LP3321 datasheet(6/9 Pages) POWER

6 / 9 page

Preliminary Datasheet

LP3321

LP3321 – 01 Vesion 1.1 Datasheet

Feb.-2011

www.lowpowersemi.com

Page 6 of 9

Applications Information

LED Current Control

The LP3321 regulates the LED current by setting

the current sense resistor (R1) connecting to

feedback

and

ground.

The

internal

feedback

reference voltage is 195mV or LP3321A is 300mV.

The LED current can be set from following equation

easily.

ILED=195mV/R1 (LP3321)

ILED=300mV/R1 (LP3321A)

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 4.7 to 22µH. Small size

and better efficiency are the major concerns for

portable device, such as LP3321 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 LP3321 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.

PWM

Cout

1uF

10uH

SS14

Cin

10uF

Vin

VFB=195mV

LP3321

Vin

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.

VDC

LP3321

Vin

Cout

1uF

Cin

10uF

10uH

90k

SS14

LP3321 Datasheet(PDF) 6 Page - Lowpower Semiconductor inc

LP3321 Datasheet(HTML) 6 Page - Lowpower Semiconductor inc

Html Pages

1 2 3 4 5 6 7 8 9

Datasheet Download

Link URL