LP3311S Datasheet, PDF (5/7 Pages) Lowpower Semiconductor inc – 1MHz Fixed-Frequency PWM Operation

Preliminary Datasheet
LP3311S
Applications Information
LED Current Control
The LP3311S regulates the LED current by setting the
current sense resistor (R1) connecting to feedback
and ground. The internal feedback reference voltage is
200mV. The LED current can be set from following
equation easily.
ILED=200mV/R1
In order to have an accurate LED current, precision
resistors are preferred (1% is recommended). The
table for R1 selection is shown below.
Inductor Selection
The recommended value of inductor for 2 to 10
WLEDs applications are 4.7 to 22µH. Small size and
better efficiency are the major concerns for portable
device, such as LP3311S used for mobile phone. The
inductor should have low core loss at 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 LP3311S 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 1MHz 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.
Vin
10uF
PWM
4.7uH
Vin
SW
EN
OVP
GND
FB
LP3311S
1uF
10R
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.
Vin
4.7uH
10uF
1uF
Vin
SW
EN
OVP
GND
FB
LP3311S
VDC
5K
65K
10R
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
LP3311s – 02 Version 2.0 Datasheet May.-2013
www.lowpowersemi.com
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