LP3311 Datasheet, PDF (6/8 Pages) Lowpower Semiconductor inc – 1.3MHzFixed-Frequency PWM Operation

Preliminary Datasheet
LP3311
Applications Information
LED Current Control
Vin
The LP3311 regulates the LED current by setting
4.7uH
the current sense resistor (R1) connecting to
Cin
10uF
Cout
1uF
feedback and ground. The internal feedback
reference voltage is 200mV or LP3311A is 300mV.
6 _{Vin OVP} 5
The LED current can be set from following equation
easily.
PWM
4 _EN
FB ³
ILED=200mV/R1 (LP3311)
ILED=300mV/R1 (LP3311A)
LP3311
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 2.2 to 22µH. Small size
and better efficiency are the major concerns for
portable device, such as LP3311 used for mobile
phone. The inductor should have low core loss at
1.3MHz 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 LP3311 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.
R1
10
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
Cin
10uF
4.7uH
U4
6 _{Vin OVP} 5
Cout
1uF
4 _EN
FB ³
LP3311
R3
R2
VDC
90k
5k
R1
10
LP3311 – 00 Version 1.0 Datasheet Feb.-2011
www.lowpowersemi.com
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