LP3302S datasheet(9/11 Pages) POWER

9 / 11 page

Preliminary Datasheet

LP3302S

LP3302S –02

Version 1.2 Datasheet

Feb.-2012

www.lowpowersemi.com

Page 9 of 11

Applications Information

LED Current Control

The LP3302S 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 2.2 to 22µH. Small size

and better efficiency are the major concerns for

portable device, such as LP3302S 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

For controlling the LED brightness, the LP3302S

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.

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 application circuit is shown in the

Figure 6. In this circuit, the output ripple depends

on the frequency of PWM signal. For smaller output

voltage

ripple (<100mV), the recommended

frequency of 2.8V PWM signal should be above

2kHz. To fix the frequency of PWM signal and

change the duty cycle of PWM signal can get

different

output

current.

According

the

application circuit of Figure 5, output current is

from 20.5mA to 5.5mA by adjusting the PWM duty

cycle from 10% to 90%.

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