LP3303 datasheet(6/7 Pages) POWER

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Preliminary Datasheet

LP3303

LP3303 –03 Ver. 1.3 Datasheet

Feb.-2009

Page 6 of 7

Applications Information

LED Current Control

The LP3303 regulates the LED current by setting

the current sense resistor (Rset) connecting to

feedback and ground. The internal feedback

reference voltage is 200mV. The LED current can

be set from following equation easily.

ILED(mA)=200mV/Rset

In order to have an accurate LED current, precision

resistors are preferred (1% is recommended). The

table for Rset selection is shown below.

Rset Resistor Value selection

ILED(mA)

RSET(Ω)

13.3

16.6

Inductor Selection

The recommended value of inductor for 2 to 8

WLEDs applications are 4.7 to 22µH. Small size

and better efficiency are the major concerns for

portable device, such as LP3303 used for mobile

phone. The inductor should have low core loss at

1.4MHz and low DCR for better efficiency. To

avoid inductor saturation current rating should be

considered.

Dimming control

a. Using a PWM Signal to CTRL Pin

For controlling the LED brightness, the LP3303 can

perform the dimming control by applying a PWM

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

range dimming frequency from 100Hz to 50KHz

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 CTRL pin, in order to let the

dimming control perform correctly.

Rset

LP3303

CTRL

VIN

OUT

10R

VIN

22uH

PWM

1uF

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 RSET 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.

10R

1uF

Rset

OFF

VIN

(0-2.8V)

1uF

LP3303

CTRL

VIN

OUT

22uH

90K

VDC

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%.

(0-100KHz)

LP3303

CTRL

VIN

OUT

VIN

1uF

PWM

22uH

0.1uF

1uF

Rset

OFF

100K

90K

10R

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