LP3302S Datasheet, PDF (9/11 Pages) Lowpower Semiconductor inc – 1.1MHz Fixed-Frequency PWM Operation

Preliminary Datasheet
LP3302S
Applications Information
LED Current Control
The LP3302S 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.
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.
Inductor Selection
The recommended value of inductor for 2 to 6
WLEDs applications are 2.2 to 22µH. Small size
and better efficiency are the major concerns for
portable device, such as LP3302S 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.
Dimming control
a. Using a PWM Signal to EN Pin
For controlling the LED brightness, the LP3302S
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.
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 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 to 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%.
LP3302S –02 Version 1.2 Datasheet Feb.-2012 www.lowpowersemi.com
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