LP3302 datasheet(11/13 Pages) POWER

11 / 13 page

Preliminary Datasheet

LP3302

LP3302B6F-00

Version 1.0 Datasheet

May.-2013

www.lowpowersemi.com

Page 11 of 13

Applications Information

LED Current Control

The LP3302 regulates the LED current by setting the

current sense resistor (R2) connecting to feedback

and ground. The internal feedback reference voltage is

0.25V. The LED current can be set from following

equation easily.

In order to have an accurate LED current, precision

resistors are preferred (1% is recommended). The

table for R2 selection is shown below.

Inductor Selection

The recommended value of inductor for 2 to 6 WLEDs

applications are 4.7 to 22µH. Small size and better

efficiency are the major concerns for portable device,

such as LP3302 used for mobile phone. The inductor

should have low core loss at 1.1MHz 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

To control the brightness of LED, the LP3302 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.

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.

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