LP3385 datasheet(7/9 Pages) POWER

7 / 9 page

Preliminary Datasheet

LP3385

LP3385 – 00 Version 1.0 Datasheet

FEB.-2012

www.lowpowersemi.com

Page 7 of 9

PWM signal should be higher than the maximum

enable voltage of EN pin, in order to let the

dimming control perform correctly.

Cout

1uF

22uH

Cin

10uF

Vin

VFB=300mV

PWM

Vin

OVP

U80

LP3385

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.

Cout

1uF

22uH

Cin

10uF

Vin

90k

VDC

Vin

OVP

U77

LP3385

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

recomended 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%.

PWM

Vin

Cout

1uF

Cin

10uF

22uH

90k

Cout

0.1uF

100K

Vin

OVP

U78

LP3385

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