Preliminary Datasheet

LP3390

LP3390 Version 1.0 Datasheet

NOV.-2013

www.lowpowersemi.com

Page 5 of 8

Application Information

LED Current Setting

The loop of Boost structure will keep the FB pin

voltage equal to the reference voltage VREF.

FB voltage is 200mV,Therefore, when RSET

connects FB pin and GND, the current flows from

VOUT through LED and RSET to GND will be

decided by the current on RSET, which is equal to

following equation:

Dimming Control

a.

Using a PWM Signal to EN Pin

For the brightness dimming control of the LP3390,

the IC provides typically 200mV feedback voltage

when the EN pin is pulled constantly high. However,

EN pin allows a PWM signal to reduce this regulation

voltage by changing the PWM duty cycle to achieve

LED brightness dimming control. The relationship

between the duty cycle and FB voltage can be

calculated as following equation:

Where

Duty = duty cycle of the PWM signal

200mV = internal reference voltage

The duty cycle of the PWM signal is used to cut the

internal 200mV reference voltage. An internal low

pass filter is used to filter the pulse signal. And then

the reference voltage can be made by connecting the

output of the filter to the error amplifier for the FB pin

voltage regulation.

However, the internal low pass filter 3db frequency is

500Hz. When the dimming frequency is lower then

500Hz, VA is also a PWM signal and the LED current

is controlled directly by this signal. When the

frequency is higher than 500Hz, PWM is filtered by

the internal low pass filter and the VA approach a DC

signal. And the LED current is a DC current which

elimate the audio noise. Two figures of PWM

Dimming from EN are shown in Typical Operating

Characteristics section and the PWM dimming

But there is an offset in error amplifier which will

cause the VA variation. In low PWM duty signal

situation, the filtered reference voltage is low and the

offset can cause bigger variation of the output

current. So the LP3390 is not recommend to be

dimming by the EN pin. For the LP3390, the

minimum duty vs frequency is listed in following

table:

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. The

output voltage can be calculated by the following

Equations.

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. 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