LP3311S datasheet(5/7 Pages) POWER

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Preliminary Datasheet

LP3311S

LP3311s – 02 Version 2.0 Datasheet

May.-2013

www.lowpowersemi.com

Page 5 of 7

Applications Information

LED Current Control

The LP3311S regulates the LED current by setting the

current sense resistor (R1) connecting to feedback

and ground. The internal feedback reference voltage is

200mV. The LED current can be set from following

equation easily.

ILED=200mV/R1

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 LP3311S used for mobile phone. The

inductor should have low core loss at 1MHz 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 LP3311S can

perform the dimming control by applying a PWM signal

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

dimming

frequency

from

100Hz

1MHz

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.

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.

c. Using a Filtered PWM signal

Another common application is using a filtered PWM

signal as an adjustable DC voltage for LED dimming

control. A filtered PWM signal acts as the DC voltage

to regulate the output current. The recommended

1uF

4.7uH

10uF

Vin

GND

Vin

OVP

LP3311S

10R

PWM

65K

VDC

1uF

4.7uH

10uF

Vin

GND

Vin

OVP

LP3311S

10R

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