LP3302 Datasheet, PDF (11/13 Pages) Lowpower Semiconductor inc – 1.1MHz Fixed-Frequency PWM Operation

Preliminary Datasheet
Applications Information
LED Current Control
The LP3302 regulates the LED current by setting the
current sense resistor (R2) connecting to feedback
and ground. The internal feedback reference voltage is
0.25V. The LED current can be set from following
equation easily.
In order to have an accurate LED current, precision
resistors are preferred (1% is recommended). The
table for R2 selection is shown below.
LP3302
Inductor Selection
The recommended value of inductor for 2 to 6 WLEDs
applications are 4.7 to 22µH. Small size and better
efficiency are the major concerns for portable device,
such as LP3302 used for mobile phone. The inductor
should have low core loss at 1.1MHz and low DCR for
better efficiency. To avoid inductor saturation current
rating should be considered.
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.
Dimming control
a. Using a PWM Signal to EN Pin
To control the brightness of LED, the LP3302 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.
LP3302B6F-00 Version 1.0 Datasheet May.-2013 www.lowpowersemi.com
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